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Published online before print January 25, 2002, 10.1148/radiol.2223011156
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(Radiology 2002;222:835-842.)
© RSNA, 2002

Technical Developments

Cardiac Function: MR Evaluation in One Breath Hold with Real-time True Fast Imaging with Steady-State Precession1

Vivian S. Lee, MD, PhD, Daniel Resnick, BS, Jeffrey M. Bundy, PhD, Orlando P. Simonetti, PhD, Peter Lee, MD and Jeffrey C. Weinreb, MD

1 From the Department of Radiology–MRI, New York University Medical Center, 530 First Ave, HCC Basement, New York, NY 10016 (V.S.L., D.R., P.L., J.C.W.); and Siemens Medical Systems, Chicago, Ill (J.M.B., O.P.S.). Received July 5, 2001; revision requested August 22; revision received September 26; accepted October 22. Address correspondence to V.S.L. (e-mail: lee@mri.med.nyu.edu).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results
 Discussion
 REFERENCES
 
In 12 healthy volunteers and eight patients with cardiac disease, cine magnetic resonance (MR) imaging in the heart was performed with real-time true fast imaging with steady-state precession (FISP), which permitted evaluation of the entire left ventricle in one breath hold (91 msec per frame, 13 frames per section position, nine short-axis section positions per breath hold). Contrast-to-noise ratios (CNRs) and left ventricular mass and function measurements with this technique were compared in all subjects with single-section true FISP imaging and, in the volunteers only, with segmented fast low-angle shot (FLASH) MR imaging. Myocardium-to-blood CNR was significantly higher for both true FISP sequences compared with the FLASH sequence. Measurements of resting left ventricular function with real-time true FISP imaging were comparable with those derived from a series of separate breath-hold single-section true FISP acquisitions.

© RSNA, 2002

Index terms: Heart, MR, 51.121412, 51.121416 • Heart, ventricles, 51.92 • Heart, volume, 51.92 • Magnetic resonance (MR), cine study, 51.121412, 51.121416 • Magnetic resonance (MR), pulse sequences, 51.121412, 51.121416 • Magnetic resonance (MR), volume measurement, 51.121412, 51.121416


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results
 Discussion
 REFERENCES
 
Cine gradient-echo magnetic resonance (MR) imaging is an established method to evaluate cardiac function and serves as a reference standard for new imaging techniques, such as three-dimensional echocardiography (1–9). One factor that has limited its widespread clinical use has been the need for patients to perform numerous breath holds to complete the study. Typically, seven to 10 separate cine gradient-echo acquisitions in the short-axis plane and two or three long-axis acquisitions are required. Imaging at each section position usually necessitates a 15–20-second breath hold with fast MR methods, such as segmented fast low-angle shot (FLASH) techniques (2,10,11). Thus as part of a routine protocol for evaluating left ventricular function, patients must perform at least 10 to 12 15–20-second breath holds.

Recent improvements in gradient performance have led to decreased minimum repetition times and have allowed successful implementation of an alternative approach to cine gradient-echo MR imaging: fast imaging with steady-state precession (FISP) (12–16). Unlike spoiled gradient-echo FLASH imaging, which relies on inflow enhancement for image contrast, steady-state gradient-echo true FISP sequences produce images with contrast that is dependent on the ratio of T2 to T1. Given the much higher ratio for blood than for myocardium, true FISP imaging offers the promise of higher contrast-to-noise ratios (CNRs) than can be achieved with FLASH sequences. Early results (17–19) have shown the feasibility of true FISP cine imaging of the heart with commercial MR units and have demonstrated that single-section segmented true FISP imaging provides superior image quality, despite shorter acquisition times, than single-section segmented FLASH imaging. When implemented with short repetition times (<3 msec) and short echo times (<1.5 msec), true FISP imaging can also be modified for real-time cardiac MR imaging. Real-time true FISP sequences can be configured to acquire cine images at a series of section positions sequentially, thereby enabling multisection cine imaging in one breath hold, albeit with lower spatial and temporal resolution than with existing single-section cine gradient-echo methods.

We developed a real-time true FISP imaging technique that acquires cine images at nine short-axis section positions from the left ventricular base to the apex in one breath hold. We hypothesize that this method can provide comparable measurements of left ventricular mass and function compared with single-section true FISP imaging. The purpose of this study was to evaluate this real-time true FISP imaging technique.


   Materials and Methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
 Discussion
 REFERENCES
 
After they gave their written informed consent, 12 healthy volunteers (10 men and two women; age range, 24–40 years) and eight consecutive patients with cardiac disease (six men and two women; age range, 53–76 years) underwent MR imaging at 1.5 T (Magnetom Symphony with Quantum gradients [maximum gradient amplitude, 30 mT/m; slew rate, 125 mT/m/sec]; Siemens Medical Systems, Erlangen, Germany) with use of a four-channel torso phased-array coil. All subjects underwent imaging according to a protocol approved by the institutional review board. The volunteers had no history of cardiac disease and had normal findings at physical examination within 1 year of the study. All patients were referred for clinical MR studies to evaluate myocardial viability on the basis of results at stress-rest radionuclide examinations that suggested discrepancies between fixed perfusion defects and wall motion abnormalities.

MR Imaging Protocol
The protocol was different for volunteers and patients. For all volunteers, the MR imaging protocol included (a) vertical and horizontal long-axis cine imaging in the left ventricle; (b) short-axis cine imaging from the apex to the base of the left ventricle with three cine gradient-echo techniques: breath-hold segmented cine FLASH imaging, single-section breath-hold segmented cine true FISP imaging, and real-time multisection cine true FISP imaging; and (c) breath-hold phase-contrast imaging for flow quantification through the ascending aorta. Subsequently, the same protocol was performed in eight patients except that, on the basis of the results of the volunteer studies and because of the demanding nature of the breath-hold protocol, only the single-section and real-time multisection true FISP sequences (and not the FLASH sequences) were performed for all the short-axis cine views. Sequence parameters are summarized in Table 1.


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  		TABLE 1. Imaging Parameters for Short-Axis Cine MR Acquisitions

 
For all subjects, MR-compatible electrocardiographic leads were placed on the back before imaging and were attached to the MR imaging unit for electrocardiographic gating. For all sequences, electrocardiographic triggering was used to synchronize imaging with the onset of systole, and all short-axis section positions were prescribed identically for the three cine techniques. The single-section electrocardiography-triggered true FISP sequence was performed with repetition time msec/echo time msec of 3.6/1.8, flip angle of 60°, matrix of 120 x 256, field of view of 244–281 x 325–375 mm, 15 lines per heartbeat, and temporal resolution of 54 msec. For complete coverage of the left ventricle, eight to 10 short-axis sections (8-mm thick with 2-mm gaps) were acquired. Acquisition time for each section was eight heartbeats, or 7–11 seconds depending on the subject’s heart rate. The same section positions were then imaged with a multisection real-time electrocardiography-triggered true FISP sequence with echo sharing: 2.6/1.3, flip angle of 50°, matrix of 62 x 128, field of view of 244–281 x 325–375 mm, and 70 lines per heartbeat. As a real-time acquisition, each cine frame required 182 msec of data acquisition. With echo sharing, however, frames could be reconstructed every 91 msec. For all subjects, nine short-axis section positions were imaged with real-time true FISP for 1.2 seconds at each section position after the electrocardiographic trigger to ensure imaging through at least one complete cardiac cycle. The electrocardiography-triggered multisection acquisition required imaging times of 20–30 seconds and was performed during one breath hold.

The volunteers also underwent imaging with a segmented cine FLASH sequence with echo sharing: 9.0/6.1, flip angle of 20°, matrix of 126 x 256, field of view of 244–281 x 325–375 mm, and effective temporal resolution of 45 msec. With cine FLASH imaging, acquisition time for each section was 14 heartbeats, or 12–20 seconds.

All acquisitions were performed during a breath hold at end expiration. The short-axis acquisitions were performed in the following order: (a) FLASH imaging in volunteers only followed by (b) single-section true FISP imaging and then (c) multisection real-time true FISP imaging in all subjects. For all patients with heart disease, oxygen (2 L via a nasal cannula) was provided to improve breath-holding capacity (20).

In all patients, an oblique axial breath-hold segmented two-dimensional phase-contrast acquisition was performed through the aortic root (31.8/3.4, flip angle of 30°, encoding velocity of 150 cm/sec, matrix of 96 x 256, rectangular field of view of 175 x 350 mm) (21). This acquisition was repeated, and the results were averaged for analysis.

All subjects tolerated the examination well. During the study, subject heart rates ranged from 55 to 90 beats per minute.

Image Analysis
Left ventricular myocardium and blood-pool signal-to-noise ratios and myocardium-blood CNRs for the three cine sequences were measured by two independent observers (D.R., P.L.), who manually defined regions of interest with the satellite console (Siemens Medical Systems). For measurements of left ventricular myocardial signal intensity, four circular regions (anterior, lateral, posterior, and septal walls) were defined on a midventricular short-axis image and subsequently averaged to obtain overall measurements of myocardial signal intensity. For blood-pool measurements, the largest circular regions of interest that contained only intraventricular blood were drawn on the same short-axis image. The size of regions of interest in the myocardium was approximately 30–50 mm2, while regions of interest in the blood pool were larger than 300 mm2. Noise was defined as the SD of signal intensity (SI) measured in air outside the body and measured in consistent locations for each subject, with regions of interest defined to be at least 300 mm2. The myocardium (m)-blood (b) CNR was defined as follows: (SIb - SIm)/noise. Normalized CNR values for each sequence were also computed to take into account differences in voxel size and measurement time. Normalized CNR was computed by dividing CNR by voxel size (in cubic millimeters) and the square root of frame time (in seconds), which was computed by multiplying repetition time by the number of phase-encoding steps per image (Table 1) (19). All measurements were made separately for an end-systolic and an end-diastolic frame, and the same midventricular section position was used for evaluation of all cine images. Results from the two readers were averaged for each subject for analysis.

Cardiac function analysis was performed for each short-axis image through the left ventricle acquired with each imaging technique. End-diastolic and end-systolic endocardial and epicardial borders were manually defined by three independent viewers (D.R., P.L., V.S.L.). Decisions about which sections to include in volume and mass calculations were made by projecting short-axis section positions onto long-axis systolic and diastolic images. Because short-axis sections were obtained with 10-mm intervals (8-mm-thick sections, 2-mm gaps), end-diastolic volumes could be computed by summing the cross-sectional areas contained by the endocardial borders of all short-axis sections of the left ventricle measured during end diastole. End-systolic volumes were computed in a similar fashion for all short-axis sections measured during end systole. Papillary muscles were excluded from endocardial borders; hence, they were not included in end-systolic and end-diastolic volume calculations. End-systolic and end-diastolic frames were defined to contain the smallest and largest blood-pool areas, respectively, and were selected consistently for the three acquisitions in a given subject. Stroke volume is defined as the difference between end-diastolic volume and end-systolic volume, while ejection fraction is stroke volume divided by end-diastolic volume. The areas contained between the epicardial and endocardial borders, including papillary muscles, were summed for all short-axis sections for left ventricular mass, by assuming a tissue density of 1.04 g/mL.

MR phase-contrast data were analyzed by one investigator (V.S.L.) by using commercially available software with the satellite console. Background-corrected forward systolic flow rate (in milliliters per second) through the aortic root during one heartbeat was computed for each phase-contrast acquisition with aortic regions of interest manually adjusted throughout the cardiac cycle. The forward flow rate was multiplied by the duration of systole, as defined by the duration of forward flow, to estimate left ventricular stroke volume. Results for the two phase-contrast acquisitions were averaged for analysis. None of the patients had a clinical history or MR evidence of aortic or mitral stenosis or insufficiency.

Data Analysis
In volunteers, averaged signal-to-noise ratio and CNR measurements, left ventricular volumes, stroke volume, left ventricular mass, and ejection fraction for the three imaging techniques were compared with one-way analysis of variance. When a statistically significant difference among the three methods was found, a two-tailed paired Student t test was performed for individual pairs of techniques, with the Bonferroni adjustment, where P < .01 was considered to indicate a significant difference.

In the patients, comparisons between the two true FISP imaging methods were made with a two-tailed paired Student t test. Left ventricular volumes and mass, stroke volume, and ejection fraction were compared between pairs of cine sequences with Bland-Altman analysis and plots (22). Agreement between stroke volume calculations with the cine techniques and phase-contrast measurements was also evaluated with Bland-Altman methods.


   Results
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results
Discussion
 REFERENCES
 
Representative end-diastolic and end-systolic short-axis images obtained with cine FLASH, single-section true FISP imaging, and multisection true FISP imaging are shown in Figure 1.



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  		Figure 1a. Horizontal long-axis MR images in a healthy volunteer. Cine FLASH images at (a) end diastole and (b) end systole. Single-section true FISP images at (c) end diastole and (d) end systole. (e, f) True FISP images in c and d, respectively, with short-axis section positions projected (oblique lines). These projections facilitated the selection of which sections to include in calculations of left ventricular volumes and masses. In e, the single line (arrow) perpendicular to the short-axis planes was used to define the vertical long-axis (two-chamber) view. Nine short-axis sections are used in e (end diastole). Seven short-axis sections are used in f (end systole) because the most basal and apical sections (lower and upper arrows, respectively) do not lie within the left ventricle. In-plane saturation contributes to the loss of signal intensity of the left ventricular blood pool in a and b. This is less apparent in c and d, especially at end systole (arrows in d) at blood-pool margins Midventricular short-axis MR images in the same volunteer. (g, h) Cine FLASH, (i, j) single-section true FISP, and (k, l) multisection true FISP MR images were obtained at end diastole (g, i, k) and end systole (h, j, l). In-plane saturation likely causes the relative loss of signal intensity in the blood pool (arrows in j and l) adjacent to the myocardium in g and h that is not as apparent in i-l despite their lower spatial resolution. This leads to decreased blood-pool volumes and higher ventricular masses with cine FLASH MR imaging, as was observed in all study subjects (Table 3).

 


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  		Figure 1b. Horizontal long-axis MR images in a healthy volunteer. Cine FLASH images at (a) end diastole and (b) end systole. Single-section true FISP images at (c) end diastole and (d) end systole. (e, f) True FISP images in c and d, respectively, with short-axis section positions projected (oblique lines). These projections facilitated the selection of which sections to include in calculations of left ventricular volumes and masses. In e, the single line (arrow) perpendicular to the short-axis planes was used to define the vertical long-axis (two-chamber) view. Nine short-axis sections are used in e (end diastole). Seven short-axis sections are used in f (end systole) because the most basal and apical sections (lower and upper arrows, respectively) do not lie within the left ventricle. In-plane saturation contributes to the loss of signal intensity of the left ventricular blood pool in a and b. This is less apparent in c and d, especially at end systole (arrows in d) at blood-pool margins Midventricular short-axis MR images in the same volunteer. (g, h) Cine FLASH, (i, j) single-section true FISP, and (k, l) multisection true FISP MR images were obtained at end diastole (g, i, k) and end systole (h, j, l). In-plane saturation likely causes the relative loss of signal intensity in the blood pool (arrows in j and l) adjacent to the myocardium in g and h that is not as apparent in i-l despite their lower spatial resolution. This leads to decreased blood-pool volumes and higher ventricular masses with cine FLASH MR imaging, as was observed in all study subjects (Table 3).

 


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  		Figure 1c. Horizontal long-axis MR images in a healthy volunteer. Cine FLASH images at (a) end diastole and (b) end systole. Single-section true FISP images at (c) end diastole and (d) end systole. (e, f) True FISP images in c and d, respectively, with short-axis section positions projected (oblique lines). These projections facilitated the selection of which sections to include in calculations of left ventricular volumes and masses. In e, the single line (arrow) perpendicular to the short-axis planes was used to define the vertical long-axis (two-chamber) view. Nine short-axis sections are used in e (end diastole). Seven short-axis sections are used in f (end systole) because the most basal and apical sections (lower and upper arrows, respectively) do not lie within the left ventricle. In-plane saturation contributes to the loss of signal intensity of the left ventricular blood pool in a and b. This is less apparent in c and d, especially at end systole (arrows in d) at blood-pool margins Midventricular short-axis MR images in the same volunteer. (g, h) Cine FLASH, (i, j) single-section true FISP, and (k, l) multisection true FISP MR images were obtained at end diastole (g, i, k) and end systole (h, j, l). In-plane saturation likely causes the relative loss of signal intensity in the blood pool (arrows in j and l) adjacent to the myocardium in g and h that is not as apparent in i-l despite their lower spatial resolution. This leads to decreased blood-pool volumes and higher ventricular masses with cine FLASH MR imaging, as was observed in all study subjects (Table 3).

 


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  		Figure 1d. Horizontal long-axis MR images in a healthy volunteer. Cine FLASH images at (a) end diastole and (b) end systole. Single-section true FISP images at (c) end diastole and (d) end systole. (e, f) True FISP images in c and d, respectively, with short-axis section positions projected (oblique lines). These projections facilitated the selection of which sections to include in calculations of left ventricular volumes and masses. In e, the single line (arrow) perpendicular to the short-axis planes was used to define the vertical long-axis (two-chamber) view. Nine short-axis sections are used in e (end diastole). Seven short-axis sections are used in f (end systole) because the most basal and apical sections (lower and upper arrows, respectively) do not lie within the left ventricle. In-plane saturation contributes to the loss of signal intensity of the left ventricular blood pool in a and b. This is less apparent in c and d, especially at end systole (arrows in d) at blood-pool margins Midventricular short-axis MR images in the same volunteer. (g, h) Cine FLASH, (i, j) single-section true FISP, and (k, l) multisection true FISP MR images were obtained at end diastole (g, i, k) and end systole (h, j, l). In-plane saturation likely causes the relative loss of signal intensity in the blood pool (arrows in j and l) adjacent to the myocardium in g and h that is not as apparent in i-l despite their lower spatial resolution. This leads to decreased blood-pool volumes and higher ventricular masses with cine FLASH MR imaging, as was observed in all study subjects (Table 3).

 


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  		Figure 1e. Horizontal long-axis MR images in a healthy volunteer. Cine FLASH images at (a) end diastole and (b) end systole. Single-section true FISP images at (c) end diastole and (d) end systole. (e, f) True FISP images in c and d, respectively, with short-axis section positions projected (oblique lines). These projections facilitated the selection of which sections to include in calculations of left ventricular volumes and masses. In e, the single line (arrow) perpendicular to the short-axis planes was used to define the vertical long-axis (two-chamber) view. Nine short-axis sections are used in e (end diastole). Seven short-axis sections are used in f (end systole) because the most basal and apical sections (lower and upper arrows, respectively) do not lie within the left ventricle. In-plane saturation contributes to the loss of signal intensity of the left ventricular blood pool in a and b. This is less apparent in c and d, especially at end systole (arrows in d) at blood-pool margins Midventricular short-axis MR images in the same volunteer. (g, h) Cine FLASH, (i, j) single-section true FISP, and (k, l) multisection true FISP MR images were obtained at end diastole (g, i, k) and end systole (h, j, l). In-plane saturation likely causes the relative loss of signal intensity in the blood pool (arrows in j and l) adjacent to the myocardium in g and h that is not as apparent in i-l despite their lower spatial resolution. This leads to decreased blood-pool volumes and higher ventricular masses with cine FLASH MR imaging, as was observed in all study subjects (Table 3).

 


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  		Figure 1f. Horizontal long-axis MR images in a healthy volunteer. Cine FLASH images at (a) end diastole and (b) end systole. Single-section true FISP images at (c) end diastole and (d) end systole. (e, f) True FISP images in c and d, respectively, with short-axis section positions projected (oblique lines). These projections facilitated the selection of which sections to include in calculations of left ventricular volumes and masses. In e, the single line (arrow) perpendicular to the short-axis planes was used to define the vertical long-axis (two-chamber) view. Nine short-axis sections are used in e (end diastole). Seven short-axis sections are used in f (end systole) because the most basal and apical sections (lower and upper arrows, respectively) do not lie within the left ventricle. In-plane saturation contributes to the loss of signal intensity of the left ventricular blood pool in a and b. This is less apparent in c and d, especially at end systole (arrows in d) at blood-pool margins Midventricular short-axis MR images in the same volunteer. (g, h) Cine FLASH, (i, j) single-section true FISP, and (k, l) multisection true FISP MR images were obtained at end diastole (g, i, k) and end systole (h, j, l). In-plane saturation likely causes the relative loss of signal intensity in the blood pool (arrows in j and l) adjacent to the myocardium in g and h that is not as apparent in i-l despite their lower spatial resolution. This leads to decreased blood-pool volumes and higher ventricular masses with cine FLASH MR imaging, as was observed in all study subjects (Table 3).

 


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  		Figure 1g. Horizontal long-axis MR images in a healthy volunteer. Cine FLASH images at (a) end diastole and (b) end systole. Single-section true FISP images at (c) end diastole and (d) end systole. (e, f) True FISP images in c and d, respectively, with short-axis section positions projected (oblique lines). These projections facilitated the selection of which sections to include in calculations of left ventricular volumes and masses. In e, the single line (arrow) perpendicular to the short-axis planes was used to define the vertical long-axis (two-chamber) view. Nine short-axis sections are used in e (end diastole). Seven short-axis sections are used in f (end systole) because the most basal and apical sections (lower and upper arrows, respectively) do not lie within the left ventricle. In-plane saturation contributes to the loss of signal intensity of the left ventricular blood pool in a and b. This is less apparent in c and d, especially at end systole (arrows in d) at blood-pool margins Midventricular short-axis MR images in the same volunteer. (g, h) Cine FLASH, (i, j) single-section true FISP, and (k, l) multisection true FISP MR images were obtained at end diastole (g, i, k) and end systole (h, j, l). In-plane saturation likely causes the relative loss of signal intensity in the blood pool (arrows in j and l) adjacent to the myocardium in g and h that is not as apparent in i-l despite their lower spatial resolution. This leads to decreased blood-pool volumes and higher ventricular masses with cine FLASH MR imaging, as was observed in all study subjects (Table 3).

 


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  		Figure 1h. Horizontal long-axis MR images in a healthy volunteer. Cine FLASH images at (a) end diastole and (b) end systole. Single-section true FISP images at (c) end diastole and (d) end systole. (e, f) True FISP images in c and d, respectively, with short-axis section positions projected (oblique lines). These projections facilitated the selection of which sections to include in calculations of left ventricular volumes and masses. In e, the single line (arrow) perpendicular to the short-axis planes was used to define the vertical long-axis (two-chamber) view. Nine short-axis sections are used in e (end diastole). Seven short-axis sections are used in f (end systole) because the most basal and apical sections (lower and upper arrows, respectively) do not lie within the left ventricle. In-plane saturation contributes to the loss of signal intensity of the left ventricular blood pool in a and b. This is less apparent in c and d, especially at end systole (arrows in d) at blood-pool margins Midventricular short-axis MR images in the same volunteer. (g, h) Cine FLASH, (i, j) single-section true FISP, and (k, l) multisection true FISP MR images were obtained at end diastole (g, i, k) and end systole (h, j, l). In-plane saturation likely causes the relative loss of signal intensity in the blood pool (arrows in j and l) adjacent to the myocardium in g and h that is not as apparent in i-l despite their lower spatial resolution. This leads to decreased blood-pool volumes and higher ventricular masses with cine FLASH MR imaging, as was observed in all study subjects (Table 3).

 


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  		Figure 1i. Horizontal long-axis MR images in a healthy volunteer. Cine FLASH images at (a) end diastole and (b) end systole. Single-section true FISP images at (c) end diastole and (d) end systole. (e, f) True FISP images in c and d, respectively, with short-axis section positions projected (oblique lines). These projections facilitated the selection of which sections to include in calculations of left ventricular volumes and masses. In e, the single line (arrow) perpendicular to the short-axis planes was used to define the vertical long-axis (two-chamber) view. Nine short-axis sections are used in e (end diastole). Seven short-axis sections are used in f (end systole) because the most basal and apical sections (lower and upper arrows, respectively) do not lie within the left ventricle. In-plane saturation contributes to the loss of signal intensity of the left ventricular blood pool in a and b. This is less apparent in c and d, especially at end systole (arrows in d) at blood-pool margins Midventricular short-axis MR images in the same volunteer. (g, h) Cine FLASH, (i, j) single-section true FISP, and (k, l) multisection true FISP MR images were obtained at end diastole (g, i, k) and end systole (h, j, l). In-plane saturation likely causes the relative loss of signal intensity in the blood pool (arrows in j and l) adjacent to the myocardium in g and h that is not as apparent in i-l despite their lower spatial resolution. This leads to decreased blood-pool volumes and higher ventricular masses with cine FLASH MR imaging, as was observed in all study subjects (Table 3).

 


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  		Figure 1j. Horizontal long-axis MR images in a healthy volunteer. Cine FLASH images at (a) end diastole and (b) end systole. Single-section true FISP images at (c) end diastole and (d) end systole. (e, f) True FISP images in c and d, respectively, with short-axis section positions projected (oblique lines). These projections facilitated the selection of which sections to include in calculations of left ventricular volumes and masses. In e, the single line (arrow) perpendicular to the short-axis planes was used to define the vertical long-axis (two-chamber) view. Nine short-axis sections are used in e (end diastole). Seven short-axis sections are used in f (end systole) because the most basal and apical sections (lower and upper arrows, respectively) do not lie within the left ventricle. In-plane saturation contributes to the loss of signal intensity of the left ventricular blood pool in a and b. This is less apparent in c and d, especially at end systole (arrows in d) at blood-pool margins Midventricular short-axis MR images in the same volunteer. (g, h) Cine FLASH, (i, j) single-section true FISP, and (k, l) multisection true FISP MR images were obtained at end diastole (g, i, k) and end systole (h, j, l). In-plane saturation likely causes the relative loss of signal intensity in the blood pool (arrows in j and l) adjacent to the myocardium in g and h that is not as apparent in i-l despite their lower spatial resolution. This leads to decreased blood-pool volumes and higher ventricular masses with cine FLASH MR imaging, as was observed in all study subjects (Table 3).

 


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  		Figure 1k. Horizontal long-axis MR images in a healthy volunteer. Cine FLASH images at (a) end diastole and (b) end systole. Single-section true FISP images at (c) end diastole and (d) end systole. (e, f) True FISP images in c and d, respectively, with short-axis section positions projected (oblique lines). These projections facilitated the selection of which sections to include in calculations of left ventricular volumes and masses. In e, the single line (arrow) perpendicular to the short-axis planes was used to define the vertical long-axis (two-chamber) view. Nine short-axis sections are used in e (end diastole). Seven short-axis sections are used in f (end systole) because the most basal and apical sections (lower and upper arrows, respectively) do not lie within the left ventricle. In-plane saturation contributes to the loss of signal intensity of the left ventricular blood pool in a and b. This is less apparent in c and d, especially at end systole (arrows in d) at blood-pool margins Midventricular short-axis MR images in the same volunteer. (g, h) Cine FLASH, (i, j) single-section true FISP, and (k, l) multisection true FISP MR images were obtained at end diastole (g, i, k) and end systole (h, j, l). In-plane saturation likely causes the relative loss of signal intensity in the blood pool (arrows in j and l) adjacent to the myocardium in g and h that is not as apparent in i-l despite their lower spatial resolution. This leads to decreased blood-pool volumes and higher ventricular masses with cine FLASH MR imaging, as was observed in all study subjects (Table 3).

 


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  		Figure 1l. Horizontal long-axis MR images in a healthy volunteer. Cine FLASH images at (a) end diastole and (b) end systole. Single-section true FISP images at (c) end diastole and (d) end systole. (e, f) True FISP images in c and d, respectively, with short-axis section positions projected (oblique lines). These projections facilitated the selection of which sections to include in calculations of left ventricular volumes and masses. In e, the single line (arrow) perpendicular to the short-axis planes was used to define the vertical long-axis (two-chamber) view. Nine short-axis sections are used in e (end diastole). Seven short-axis sections are used in f (end systole) because the most basal and apical sections (lower and upper arrows, respectively) do not lie within the left ventricle. In-plane saturation contributes to the loss of signal intensity of the left ventricular blood pool in a and b. This is less apparent in c and d, especially at end systole (arrows in d) at blood-pool margins Midventricular short-axis MR images in the same volunteer. (g, h) Cine FLASH, (i, j) single-section true FISP, and (k, l) multisection true FISP MR images were obtained at end diastole (g, i, k) and end systole (h, j, l). In-plane saturation likely causes the relative loss of signal intensity in the blood pool (arrows in j and l) adjacent to the myocardium in g and h that is not as apparent in i-l despite their lower spatial resolution. This leads to decreased blood-pool volumes and higher ventricular masses with cine FLASH MR imaging, as was observed in all study subjects (Table 3).

 
Averaged blood signal-to-noise ratio and myocardium-blood CNR values for the three techniques are summarized in Table 2. For the volunteers, myocardium-blood CNR was significantly higher (P < .01) for single-section true FISP imaging (52 ± 13 [mean ± SD]) than for cine FLASH imaging (41 ± 9). Furthermore, the CNR for multisection true FISP imaging (80 ± 27) was significantly higher than that for the other two sequences (P < .01). The same results were seen in the patients: multisection true FISP imaging resulted in significantly higher myocardium-blood CNR than did single-section true FISP imaging (87 ± 16 vs 45 ± 6, respectively; P < .01). For normalized myocardium-blood CNR, which, by correcting for voxel size and acquisition times, provides a better assessment of CNR inherent to the sequences (19), both FISP sequences resulted in significantly higher (P < .01) values than did FLASH imaging. The multisection true FISP imaging technique provided lower normalized CNR measurements than did single-section true FISP imaging (P < .02); the mean normalized myocardium-blood CNR for single-section true FISP imaging was 1.5 times higher than that for real-time multisection true FISP imaging in volunteers but was only 1.2 times higher in the patients.


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  		TABLE 2. Signal-to-Noise Ratios and CNRs for Myocardium and Blood with Three MR Imaging Techniques

 
A summary of left ventricular functional and anatomic measurements made with the three imaging sequences is given in Table 3. Left ventricular measurements were compared for the three imaging sequences in the 12 volunteers. The average left ventricular mass was higher with FLASH imaging than with either FISP imaging method. For example, left ventricular mass averaged 24 g ± 11 higher with FLASH imaging than with single-section true FISP imaging. Conversely, ventricular volumes were lower with FLASH imaging than with FISP imaging; differences averaged 11 mL ± 11 for end-diastolic volume and 14 mL ± 8 for end-systolic volume. Stroke volume values were only slightly different between FLASH imaging and single-section FISP imaging (4 mL ± 8 less with FISP imaging than with FLASH imaging).


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  		TABLE 3. Comparison of Left Ventricular Function Values with Three Cine MR Imaging Techniques

 
When we compared left ventricular mass measurements with the single-section and real-time multisection FISP imaging methods for all 20 subjects, the mean difference was negligible (0 g ± 8). The multisection true FISP sequence provided slightly higher estimates of end-diastolic volume and end-systolic volume compared with single-section true FISP imaging (mean difference, 4 mL ± 9 and 4 mL ± 15, respectively); stroke volume values were comparable (mean difference, 0 mL ± 14).

We found that the ejection fraction measurements among the 12 volunteers were higher with FLASH imaging than with single-section true FISP imaging by an average of 9% ± 5, with the true FISP imaging values ranging from 48% to 76%. Ejection fraction measurements with single-section and multisection FISP imaging were remarkably similar for all subjects. Differences in ejection fraction averaged less than 2% (1.6% ± 7.0) for both volunteers and patients (Fig 2). The range of cardiac function for the patients was wide; ejection fractions measured with single-section true FISP imaging ranged from 27% to 78%.



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  		Figure 2. Bland-Altman plot for ejection fraction measurements with multisection true FISP compared with single-section true FISP MR imaging. Differences are plotted against the average of the two ejection fraction measurements. Values for mean difference (1.6%) and mean ± 2 SDs (range, -11.2% to 14%) are also plotted. Despite the lower temporal and spatial resolutions with multisection true FISP compared with single-section true FISP imaging, measurements of ejection fraction with the two methods were remarkably similar. There was no appreciable underestimation of ejection fraction with the real-time sequence.

 
With use of an independent measurement technique—MR phase-contrast measurements of total forward flow in the ascending aorta—stroke volume measurements in this study varied from 51 to 106 mL. We compared phase-contrast values with stroke volume measurements (end-diastolic volume minus end-systolic volume) with the three cine MR imaging sequences. On average, cine FLASH imaging estimates of stroke volume exceeded phase-contrast values by 8 mL ± 11, while both single-section and multisection true FISP imaging values exceeded phase-contrast values by less (5 mL ± 14 and 4 mL ± 12, respectively); differences between techniques did not achieve statistical significance (P > .1).


   Discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results
 Discussion
REFERENCES
 
The temporal and spatial resolutions of cine gradient-echo MR imaging methods are constrained by the total acquisition time. With breath-hold techniques, acquisition times are limited by patient comfort and capability and should be less than 20 seconds for most patients. When implemented with segmented k-space methods (10), spoiled gradient-echo imaging with FLASH imaging can provide cine images of the heart with good spatial and temporal resolutions within acquisition times of about 20 seconds. With FLASH sequences, myocardium-blood contrast relies primarily on inflow enhancement; therefore, these methods require a minimum repetition time of about 8 msec to allow time for inflow. Standard FLASH sequences therefore do not benefit from the shorter repetition times possible with improved gradient hardware because with less time for inflow, blood signal becomes saturated and myocardium-blood contrast is diminished or lost.

Instead, with shorter repetition times, it becomes advantageous to consider alternative imaging techniques that exploit differences in blood and myocardial relaxation times, such as true FISP imaging. In this study, we found the CNR of single-section FISP imaging exceeded that of single-section FLASH imaging despite substantially shorter acquisition times (7–11 vs 12–20 seconds). Results in this study agree with those in two recent studies (18,19) with single-section true FISP MR imaging. These groups reported an average increase in CNR of 2.0–2.2 times with single-section true FISP compared with FLASH imaging in 18 patients and five volunteers (19) and an increase in CNR of 1.5 in a separate study of 10 patients and 10 volunteers (18).

In this study with cine FLASH imaging at a repetition time of 9 msec in 12 volunteers, we observed in-plane saturation of left ventricular blood-pool signal intensity, particularly on long-axis images and near blood-myocardium interfaces that was not apparent on true FISP images (Fig 1). We believe that saturation effects on FLASH images explain the discrepancies we observed between left ventricular measurements with FLASH and FISP imaging, namely, the significantly higher mass values and lower volumes with FLASH imaging. This observation has been reported previously (18,23) and has been shown to affect implementation of automated image segmentation algorithms for calculation of left ventricular function parameters (18).

With true FISP imaging, to extend beyond implementation as a single-section cine technique, spatial resolution can be traded for faster imaging. As a real-time imaging technique, data for each relatively low-resolution true FISP image (4.2 x 2.7-mm in-plane resolution in the current implementation) are collected in approximately 182 msec; with echo sharing, an effective temporal resolution of 91 msec can be obtained. We have shown that a real-time multisection true FISP imaging technique can be performed with a commercial 1.5-T MR unit to provide cine MR images of nine short-axis sections in the left ventricle in one breath hold. We measured an improved myocardium-blood CNR with multisection real-time true FISP compared with conventional single-section FLASH imaging. As a tool for measuring left ventricular mass and resting function in one breath hold, the multisection true FISP imaging method provided results closely comparable with those computed from a series of separate single-section true FISP acquisitions in volunteers and eight patients with a wide range of cardiac function (ejection fractions, 27%–78%).

Real-time true FISP MR imaging has limitations of relatively low spatial and temporal resolutions. Setser et al (24) performed a theoretic analysis to consider the minimum spatial and temporal resolution requirements necessary for accurate real-time ventricular function MR measurements. For the range of temporal resolutions achieved in this study (45–91 msec) with FLASH and FISP imaging methods, their predicted volume errors throughout the cardiac cycle would be in the range of 5%–10%. On the basis of their analysis of a segmented echo-planar pulse sequence with in-plane resolution of 2.7 x 3.7 mm, they also predicted that left ventricular volumes should deviate by approximately 10%–15% from those computed with images with 1 x 1-mm resolution. In this study, the real-time true FISP sequence provided a typical in-plane resolution of 4.2 x 2.7 mm and an effective temporal resolution of 91 msec. The multisection method we tested resulted in left ventricular mass and function measurements comparable with those obtained with a single-section true FISP sequence with temporal resolution of 54 msec and spatial resolution of 2.2 x 1.4 mm. However, this study was limited to the measurement of resting left ventricular function in a sample of patients with heart rates that ranged from 55 to 90 beats per minute. It is likely that at higher heart rates, such as in conditions of pharmacologically induced stress, accuracy of measurements, particularly end-systolic volume and consequently ejection fraction, will diminish unless further improvements in temporal resolution can be achieved. More efficient parallel acquisition methods (25,26) may enable further reductions in real-time true FISP acquisition times.

Recently, real-time FLASH imaging methods have been described in which echo-planar read-out schemes are used to achieve cine imaging of the heart with temporal resolution of 62 msec or less with pixel areas of approximately 9 mm2 (27–30). Results for measurements of left ventricular volumes and contractility have been reported to be comparable with those obtained with conventional FLASH imaging methods (27,29,30). Direct comparisons between these approaches and real-time FISP imaging are needed.

In this study, we performed the real-time true FISP sequence with prospective electrocardiographic gating, although gating is not necessary for real-time methods. In fact, one possible advantage of real-time MR imaging sequences is the potential to evaluate disease in patients with cardiac arrhythmias, which have previously been difficult to image with MR methods. We chose to use electrocardiographic gating to ensure that images at each section position were obtained at consistent times after the R wave (eg, 91, 182, 273 msec). This enabled us to achieve a more accurate estimate of end-diastolic and end-systolic ventricular volumes based on the summation of cross-sectional areas of all multiple short-axis sections at a fixed time in the cardiac cycle.

There are several limitations of this study. First, as is common with MR validation studies, reference methods to confirm the accuracy of MR data are needed. Results of this study demonstrated lower estimates of end-systolic volume and higher estimates of ventricular mass with FLASH compared with FISP sequences and are in agreement with recent reports (18,23). For estimates of stroke volume, we were able to compare FISP and FLASH stroke volume values against an independent estimate—MR phase-contrast measurements—of forward flow in the ascending aorta (21). With this approach, we found better agreement between phase-contrast measurements and FISP imaging values than between phase-contrast and FLASH imaging values, and we observed comparable stroke volume estimates with multisection real-time FISP and single-section FISP imaging. For measurements of left ventricular volumes and mass, however, we are unable to prove definitively that FISP imaging measurements are more accurate than those with FLASH imaging.

Other applications of real-time true FISP imaging that remain to be explored include the use of navigational devices to control orientation of the imaging plane so that fluoroscopic cardiac MR images can be obtained that are similar to echocardiographic images (31). With the real-time true FISP sequence, imaging in patients with arrhythmias should be possible without the degradation in image quality seen with FLASH and single-section FISP imaging, where data for each cine frame are collected during multiple heartbeats. Finally, cine imaging is often useful clinically for applications beyond measurement of left ventricular mass and ejection fraction. Further clinical studies are warranted to evaluate true FISP imaging in the assessment of valvular disease or regional contractility and motion.

In conclusion, compared with cine FLASH imaging, true FISP imaging provides a technique for faster cine MR imaging in the heart with improved myocardium-blood CNR. When multisection true FISP is implemented as a real-time acquisition, it can provide estimates of resting left ventricular function in volunteers and patients that are comparable with those obtained with single-section true FISP methods, despite lower temporal and spatial resolutions. Real-time true FISP MR imaging is a promising method for evaluating cardiac function in one breath hold.


   FOOTNOTES
 
Abbreviations: CNR = contrast-to-noise ratio, FISP = fast imaging with steady-state precession, FLASH = fast low-angle shot

Author contributions: Guarantor of integrity of entire study, V.S.L.; study concepts and design, V.S.L.; literature research, V.S.L., J.M.B., O.P.S.; clinical studies, V.S.L., J.C.W.; experimental studies, V.S.L., D.R., P.L.; data acquisition and analysis/interpretation, V.S.L., D.R., P.L.; statistical analysis, V.S.L.; manuscript preparation and definition of intellectual content, V.S.L.; manuscript editing, revision/review, and final version approval, all authors.


   REFERENCES
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 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results
 Discussion
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