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Published online before print July 26, 2002, 10.1148/radiol.2243010984
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(Radiology 2002;224:896-904.)
© RSNA, 2002

Technical Developments

Thorax: Low-Dose Contrast-enhanced Three-dimensional MR Angiography with Subsecond Temporal Resolution—Initial Results1

J. Paul Finn, MD, Visveshwar Baskaran, BA, James C. Carr, MD, Richard M. McCarthy, MD, F. Scott Pereles, MD, Randall Kroeker, PhD and Gerhard A. Laub, PhD

1 From the Department of Radiology, Northwestern University Medical School, 448 E Ontario St, Suite 700, Chicago, IL 60611 (J.P.F., V.B., J.C.C., R.M.M., F.S.P.); and Siemens Medical Systems, Chicago, Ill (R.K., G.L.). From the 2000 RSNA scientific assembly. Received May 31, 2001; revision requested July 12; final revision received February 18, 2002; accepted March 14. Address correspondence to J.P.F. (e-mail: pfinn@northwestern.edu).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and Methods
Results
 Discussion
 REFERENCES
 
The purpose of the study was to implement a three-dimensional (3D) magnetic resonance (MR) angiographic technique with acquisition times on the order of 800 msec with use of a spoiled gradient-echo pulse sequence (repetition time, 1.60 msec; echo time, 0.65 msec) and bolus intravenous injection of contrast material doses as small as 6 mL. High-spatial-resolution conventional MR angiography performed with 30 mL of gadopentetate dimeglumine was the reference standard. As implemented, subsecond 3D MR angiography allowed temporal sampling that was rapid enough to depict short-lived processes, as illustrated in patients with shunts and dissections. With small contrast material doses and subsecond frame rates, it is also possible to measure pulmonary arteriovenous circulation times with this 3D MR angiographic technique.

© RSNA, 2002

Index terms: Magnetic resonance (MR), rapid imaging • Magnetic resonance (MR), vascular studies


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and Methods
Results
 Discussion
 REFERENCES
 
Magnetic resonance (MR) angiography is becoming widely accepted in the diagnosis of vascular diseases (1–10). When used for evaluation of arterial stenoses, aneurysm, thrombosis, or occlusion, MR angiography is a robust and accurate technique. When used in the thorax, however, it has several general and application-specific limitations. Typical acquisition times are on the order of 20–40 seconds, and the dynamic changes seen in intra- or extracardiac shunts, aortic dissections, and leaks may not be depicted. Conventional MR angiography requires matching of data acquisition and bolus arrival time on the basis of either real-time (11–15) or prior (16) information. Conventional MR angiography is sensitive to cardiac and respiratory motion artifacts (17–21), and wide acquisition windows are used, usually in association with sustained infusion of a double dose of gadolinium-based chelate. The result is simultaneous enhancement of structures in the right and left sides of the heart and necessitates interrogation of partition data or multiplanar reconstructions to unravel overlapping vessels (22,23). The purpose of this study was to evaluate a three-dimensional (3D) MR angiographic technique in the thorax with acquisition windows as narrow as 700 msec and with intravenous bolus injection of contrast material doses as small as 6 mL.


   Materials and Methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
 Discussion
 REFERENCES
 
Patients
The study population comprised 24 consecutive adult patients (12 men and 12 women; mean age, 49 years; age range, 18–83 years). Five of the patients had congenital heart disease (atrial septal defect, aortic atresia, patent ductus arteriosus, repaired coarctation, and dextrocardia and pulmonary atresia); three, pulmonary hypertension and Eisenmenger physiology; two, aortic dissections; five, aortic aneurysms; two, aortic stenoses; 10, negative final diagnoses. The subsecond 3D MR angiographic technique, which was developed and evaluated with informed consent in compliance with a protocol approved by the institutional review board, was subsequently incorporated into routine clinical practice. The patients included in this study population were referred for further clinical assessment of known or suspected aortic aneurysm, dissection, or congenital heart disease. These are conditions for which imaging of dynamic enhancement patterns may provide clinically relevant information. Institutional review board approval was obtained for the entire study, and informed consent was not required for retrospective review of patient images and records.

Imaging Technique
All studies were performed with a 1.5-T MR system (Magnetom Sonata; Siemens Medical Systems, Iselin, NJ) with maximum gradient amplitude of 40 mT/m and peak slew rate of 200 mT/m · msec-1. A phased-array body coil with four fast receiver channels was positioned over the chest.

Initial survey MR images of the entire thorax were acquired in all patients with one-heartbeat non–breath-hold cardiac-gated true fast imaging with steady-state precession, or FISP, imaging (3.2/1.6 [repetition time msec/echo time msec]) in the transverse and coronal planes. Subsecond MR angiography was implemented with a 3D spoiled gradient-echo pulse sequence (1.60/0.65) (Fig 1) for a 256 x 256 matrix and minimum field of view of 330 mm. The flip angle was 15°–20°, depending on the patient’s specific absorption rate. Other parameters are listed in Table 1. Asymmetric k-space sampling was incorporated in all dimensions to minimize the echo time and the acquisition time for each 3D data set. In the frequency-encoding direction, 192 k-space samples were acquired in a readout time of 512 µsec (64 points before the echo and 128 points after), which corresponds to a k-space sampling frequency of 375 kHz. A 33% readout oversampling was incorporated to remove wrap-around artifacts in the readout direction, which generated 256 sample points during 512 µsec for each of four channels. A partial Fourier scheme of 80% was used in the in-plane phase-encoding direction, with zero filling to 100%, before Fourier transformation. In the section-select direction, 62.5% of the full k-space vector was explicitly acquired, with zero padding of the remainder. Therefore, the k-space matrix preserved full in-plane resolution, while the through-plane resolution was 1.6 times less than the interpolated resolution.



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  		Figure 1. Timing diagram for the subsecond 3D MR angiographic sequence. With 1.60/0.65 (repetition time [TR] msec/echo time [TE] msec) and radio-frequency spoiling, transverse coherences are destroyed, and T1 weighting is isolated on the resulting images. Asymmetric sampling is used in frequency to shorten the echo time and with both in-plane (Np) and through-plane (Ns) encoding steps to shorten the acquisition. G = gradient, {alpha} = {alpha} pulse, ADC = analog digital converter.

 

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  		TABLE 1. Imaging Parameters for MR Angiography

 
The contrast material protocol involved injection of 6–10 mL of gadopentetate dimeglumine (Magnevist; Berlex Laboratories, Wayne, NJ) at a rate of 6 mL/sec followed by 20 mL of saline at a rate of 6 mL/sec. Because we observed very high vascular signal intensity with a 10-mL bolus in the patients seen early in our research, we prescribed a 1-second 6-mL bolus for routine clinical studies and maintained this dosage because of the positive results. Diagnoses in the eight patients who had the higher dose included the following: patent ductus arteriosis (n = 1), atrial septal defect (n = 1), Stanford type B aortic dissection (n = 2), and no major abnormality detected (n = 4). Oblique sagittal (left anterior oblique) and coronal acquisitions were performed. Interpolated partition thickness was on the order of 10 mm, which precludes useful off-axis maximum intensity projection (MIP) reconstruction but allows interrogation of partitions. Therefore, the processed data were quasiprojectional and were viewed on axis as acquired in oblique sagittal and coronal planes.

Patients were instructed to hold their breath in deep inspiration, and image acquisition was started simultaneously with the injection of contrast material. Measurements were repeated at intervals of 0.8 second (left anterior oblique) or 1.1 seconds (coronal) for a total duration of 20–25 seconds. With fixed repetition time, number of partitions, and pixel dimensions, the maximum speed of the 3D acquisition depends on the available asymmetry in the in-plane field of view. In the current study, eight partitions were acquired in all cases. In the sagittal or left anterior oblique orientations, anteroposterior phase encoding enabled the number of lines acquired to be reduced by 40%–50% while preserving spatial resolution. In patients with narrow anteroposterior diameters, the temporal resolution was maximized by minimizing the anteroposterior field of view. In the coronal plane, asymmetric phase-frequency fields of view were generally not applicable, and preservation of the in-plane resolution invoked a cost in imaging time. Therefore, there was some variation in subject-to-subject temporal resolution, depending on the amount of usable asymmetry in the field of view.

Our goal was to sample in a heartbeat or less in most cases and in less than two heartbeats in all cases. Magnitude subtraction, in the image domain, of the first (unenhanced) data set from all subsequent data sets was performed online, as was on-axis MIP reconstruction. Therefore, the subtracted images, the unsubtracted images, and MIPs of each phase were available for immediate viewing. The MIPs were time-stamped and were used in lieu of a timing acquisition for subsequent conventional 3D MR angiography.

Conventional 3D MR angiography was performed immediately after the subsecond 3D MR angiography. Parameters for conventional and subsecond MR angiography are summarized in Table 1. For conventional 3D MR angiography, as for subsecond 3D MR angiography, asymmetric k-space sampling was used in all dimensions, with factor 2 zero filling in the partitions dimension. The protocol involved injection of 30 mL of gadopentetate dimeglumine at a rate of 2 mL/sec followed by 20 mL of saline at a rate of 2 mL/sec. Breath holding and the start of data acquisition were timed to coincide with the arrival of the contrast material bolus.

Image Analysis
With the subsecond MR angiographic MIPs in 21 of the 24 patients, dynamic region-of-interest curves were generated by one operator (V.B.) for the main pulmonary artery (mean region-of-interest area, 0.87 mm2; range, 0.6–1.0 mm2), inferior pulmonary veins (mean region-of-interest area, 0.52 mm2; range, 0.3–0.6 mm2), and aorta (mean region-of-interest area, 0.85 mm2; range, 0.6–1.0 mm2). In the remaining three patients, a priori information about transit times was not available, and data acquisition was not sufficiently prolonged to define a clear aortic peak; therefore, these data were not included. Pulmonary arteriovenous transit time was estimated from the difference in time of appearance of contrast material in the pulmonary artery and pulmonary veins (onset-to-onset time) and the difference in time of peak signal intensity in the pulmonary artery and veins (peak-to-peak time) (24–26). The shape of the curves in the pulmonary artery and aorta was evaluated for (a) clear separation of phases in the right and left sides of the heart, and (b) the presence or absence of an abnormally early peak or second peak, which would suggest shunt formation.

Both the subsecond and conventional MR angiograms were evaluated subjectively for visualization, without overlap, of the right side of the heart, pulmonary arteries, pulmonary veins, left atrium, ascending aorta, and descending aorta. Visualization was scored on a scale from 0 to 4: 0, not seen; 1, poorly seen with severe overlap; 2, moderately well seen with overlap; 3, well seen with mild overlap; 4, well seen with no overlap. The ascending aorta was also specifically evaluated for the presence of pulsation artifact, a known problem with conventional MR angiographic techniques (18–21). Images were scored on a scale from 1 to 3: 1, no artifact; 2, mild artifact unlikely to be confused with dissection; 3, severe artifact that could mimic dissection. Two experienced observers (F.S.P., J.C.C., with a mean experience of 4 years), blinded to patient identity and diagnosis, evaluated the images independently. In the case of the subsecond MR angiograms, only the MIPs were evaluated. In the case of the conventional MR angiograms, both the MIPs and partition images were evaluated. If abnormalities were present, their location and extent were noted on all images evaluated. Specifically, the size and location of aortic aneurysms and the location and characteristics of abnormalities in vascular filling patterns were recorded.

Comparison was available with conventional MR angiography (n = 20), conventional angiography (n = 5), computed tomography (CT) (n = 4), surgery (n = 4), relevant clinical data (such as the presence of dyspnea and cyanosis), and blood gas analysis (n = 5). For purposes of diagnosis, conventional angiography and conventional MR angiography were regarded as the reference standards, if performed.

Statistical Evaluation
Pulmonary arteriovenous peak-to-peak and onset-to-onset times for patients without (n = 18) and those with (n = 3) pulmonary hypertension or heart failure were compared by means of the Student two-sample t test. Repeated measures analysis of variance was used to evaluate the subjective ratings for each reader separately and the averaged ratings for both readers. Mean scores were compared for the individual and averaged data with a Bonferroni correction to adjust for multiple pairwise comparisons. All statistical tests were two tailed, and differences with a P value of less than .05 were regarded as statistically significant.


   Results
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
Discussion
 REFERENCES
 
Dynamic subsecond MR images routinely allowed visualization of distinct enhancement phases in the right and left sides of the heart (Fig 2a, 2c). The pulmonary arterial, pulmonary venous, and systemic arterial phases of vascular enhancement were well defined (Fig 2b). Unequivocal right-to-left shunt formation was shown on the dynamic MR images in three patients, including one with a patent ductus arteriosus, one with an atrial septal defect, and one with intrapulmonary shunt formation, which complicated pulmonary atresia. Simultaneous enhancement of the pulmonary artery and descending aorta was visualized in the patient with ductus arteriosus, with later filling of the left side of the heart and ascending aorta (Fig 3). In the patient with the atrial septal defect, a catheter was passed easily through the septum at angiography (Fig 4a). Subsecond MR angiography demonstrated right-to-left shunt formation, as manifested in simultaneous filling of the right and left atria and a double peak in aortic enhancement (Fig 4b, 4c). In the patient with aortic atresia, a patent surgical shunt was shown to extend from the left ventricle to the distal descending aorta, with retrograde filling of the proximal descending aorta. An unexpected coarctation was also shown in this patient, who was unable to cooperate with either breath holding or supplementary pulse sequences and conventional MR angiography. Therefore, the total examination time in this patient was only several minutes, with a total contrast material dose of 6 mL. In another patient with a surgical shunt created to treat coarctation, subsecond MR angiograms showed a patent shunt from the ascending to the descending aorta; this finding correlated fully with those at conventional MR angiography.



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  		Figure 2a. (a) Subsecond MR angiograms (1.60/0.65), left anterior oblique projection, in a healthy 32-year-old woman. Six frames are shown from a 24-frame series. Contrast material is shown sequentially in the pulmonary artery (PA), pulmonary parenchyma, pulmonary veins (arrows), left atrium (LA), and aorta (A). The frame time for this study was 750 msec, after injection of 6 mL of gadolinium-based contrast agent at a rate of 6 mL/sec. (b) Time-intensity curves show clear separation of the curves for the right and left sides of the heart. {diamondsuit} = pulmonary artery, {blacksquare} = pulmonary vein, {blacktriangleup} = ascending aorta. Time 0 corresponds to the start of the contrast material injection into the antecubital vein. (c) Subsecond MR angiograms (1.60/0.65), coronal projection, after a second injection of 6 mL of gadolinium-based contrast agent at a rate of 6 mL/sec. Six frames are shown from a 24-frame series. Contrast material is shown sequentially in the pulmonary artery (PA), pulmonary parenchyma, pulmonary veins (arrows), left atrium (LA), and aorta (A).

 


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  		Figure 2b. (a) Subsecond MR angiograms (1.60/0.65), left anterior oblique projection, in a healthy 32-year-old woman. Six frames are shown from a 24-frame series. Contrast material is shown sequentially in the pulmonary artery (PA), pulmonary parenchyma, pulmonary veins (arrows), left atrium (LA), and aorta (A). The frame time for this study was 750 msec, after injection of 6 mL of gadolinium-based contrast agent at a rate of 6 mL/sec. (b) Time-intensity curves show clear separation of the curves for the right and left sides of the heart. {diamondsuit} = pulmonary artery, {blacksquare} = pulmonary vein, {blacktriangleup} = ascending aorta. Time 0 corresponds to the start of the contrast material injection into the antecubital vein. (c) Subsecond MR angiograms (1.60/0.65), coronal projection, after a second injection of 6 mL of gadolinium-based contrast agent at a rate of 6 mL/sec. Six frames are shown from a 24-frame series. Contrast material is shown sequentially in the pulmonary artery (PA), pulmonary parenchyma, pulmonary veins (arrows), left atrium (LA), and aorta (A).

 


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  		Figure 2c. (a) Subsecond MR angiograms (1.60/0.65), left anterior oblique projection, in a healthy 32-year-old woman. Six frames are shown from a 24-frame series. Contrast material is shown sequentially in the pulmonary artery (PA), pulmonary parenchyma, pulmonary veins (arrows), left atrium (LA), and aorta (A). The frame time for this study was 750 msec, after injection of 6 mL of gadolinium-based contrast agent at a rate of 6 mL/sec. (b) Time-intensity curves show clear separation of the curves for the right and left sides of the heart. {diamondsuit} = pulmonary artery, {blacksquare} = pulmonary vein, {blacktriangleup} = ascending aorta. Time 0 corresponds to the start of the contrast material injection into the antecubital vein. (c) Subsecond MR angiograms (1.60/0.65), coronal projection, after a second injection of 6 mL of gadolinium-based contrast agent at a rate of 6 mL/sec. Six frames are shown from a 24-frame series. Contrast material is shown sequentially in the pulmonary artery (PA), pulmonary parenchyma, pulmonary veins (arrows), left atrium (LA), and aorta (A).

 


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  		Figure 3a. (a) Subsecond MR angiograms (1.60/0.65), left anterior oblique projection, in a 37-year-old female patient with a patent ductus arteriosus and Eisenmenger syndrome. Six frames are shown from a 24-frame series. Contrast material is hyperintense simultaneously in the main pulmonary artery (PA) and distal arch (DA); these findings confirm a right-to-left shunt, with later filling of the pulmonary veins (arrows), left atrium (LA), and ascending aorta (AA). The frame time for this study was 900 msec. (b) Time-intensity curves show complete overlap of the curves for the pulmonary artery ({diamondsuit}) and proximal descending aorta (x), with later filling of the ascending aorta ({blacktriangleup}). {blacksquare} = pulmonary vein. Time zero corresponds to the start of the contrast material injection into the antecubital vein. (c) Subsecond MR angiograms (1.60/0.65), coronal projection, show the same sequence of enhancement. Six frames are shown from a 24-frame series. A separate injection of 10 mL of contrast material was given at a rate of 6 mL/sec. Note the prominent proximal pulmonary arteries (PA) and narrowed distal branches; these findings are consistent with pulmonary hypertension. AA = ascending aorta, LA = left atrium, arrows = pulmonary veins.

 


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  		Figure 3b. (a) Subsecond MR angiograms (1.60/0.65), left anterior oblique projection, in a 37-year-old female patient with a patent ductus arteriosus and Eisenmenger syndrome. Six frames are shown from a 24-frame series. Contrast material is hyperintense simultaneously in the main pulmonary artery (PA) and distal arch (DA); these findings confirm a right-to-left shunt, with later filling of the pulmonary veins (arrows), left atrium (LA), and ascending aorta (AA). The frame time for this study was 900 msec. (b) Time-intensity curves show complete overlap of the curves for the pulmonary artery ({diamondsuit}) and proximal descending aorta (x), with later filling of the ascending aorta ({blacktriangleup}). {blacksquare} = pulmonary vein. Time zero corresponds to the start of the contrast material injection into the antecubital vein. (c) Subsecond MR angiograms (1.60/0.65), coronal projection, show the same sequence of enhancement. Six frames are shown from a 24-frame series. A separate injection of 10 mL of contrast material was given at a rate of 6 mL/sec. Note the prominent proximal pulmonary arteries (PA) and narrowed distal branches; these findings are consistent with pulmonary hypertension. AA = ascending aorta, LA = left atrium, arrows = pulmonary veins.

 


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  		Figure 3c. (a) Subsecond MR angiograms (1.60/0.65), left anterior oblique projection, in a 37-year-old female patient with a patent ductus arteriosus and Eisenmenger syndrome. Six frames are shown from a 24-frame series. Contrast material is hyperintense simultaneously in the main pulmonary artery (PA) and distal arch (DA); these findings confirm a right-to-left shunt, with later filling of the pulmonary veins (arrows), left atrium (LA), and ascending aorta (AA). The frame time for this study was 900 msec. (b) Time-intensity curves show complete overlap of the curves for the pulmonary artery ({diamondsuit}) and proximal descending aorta (x), with later filling of the ascending aorta ({blacktriangleup}). {blacksquare} = pulmonary vein. Time zero corresponds to the start of the contrast material injection into the antecubital vein. (c) Subsecond MR angiograms (1.60/0.65), coronal projection, show the same sequence of enhancement. Six frames are shown from a 24-frame series. A separate injection of 10 mL of contrast material was given at a rate of 6 mL/sec. Note the prominent proximal pulmonary arteries (PA) and narrowed distal branches; these findings are consistent with pulmonary hypertension. AA = ascending aorta, LA = left atrium, arrows = pulmonary veins.

 


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  		Figure 4a. (a) Digital subtraction angiogram, right anterior oblique projection, in a 79-year-old female patient with an atrial septal defect. The catheter has been introduced cephalad from the inferior vena cava, it crosses the atrial septal defect, and its tip (arrow) lies in the left atrium. After the injection, the contrast material is hyperintense in the left side of the heart and aorta. (b) Subsecond MR angiograms (1.60/0.65), left anterior oblique projection. Six frames are shown from a 24-frame series. The contrast material is hyperintense simultaneously in both the right (RA) and left (LA) atria; this finding confirms the presence of a right-to-left shunt. Shortly thereafter, the main pulmonary artery (PA) and ascending aorta (AA) are enhanced simultaneously. Filling of the pulmonary veins occurs later (arrows), with a subsequent second peak in the ascending aorta. The frame time for this study was 900 msec after injection of 6 mL of gadolinium-based contrast agent at a rate of 6 mL/sec. (c) Time-intensity curves show complete overlap of the pulmonary arterial ({diamondsuit}) peak and the first peak of the ascending aorta ({blacktriangleup}); this finding confirms the presence of a right-to-left shunt. Subsequently, delayed filling of the pulmonary veins is followed by the second aortic peak. {blacksquare} = pulmonary vein. Time 0 corresponds to the start of the contrast material injection into the antecubital vein.

 


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  		Figure 4b. (a) Digital subtraction angiogram, right anterior oblique projection, in a 79-year-old female patient with an atrial septal defect. The catheter has been introduced cephalad from the inferior vena cava, it crosses the atrial septal defect, and its tip (arrow) lies in the left atrium. After the injection, the contrast material is hyperintense in the left side of the heart and aorta. (b) Subsecond MR angiograms (1.60/0.65), left anterior oblique projection. Six frames are shown from a 24-frame series. The contrast material is hyperintense simultaneously in both the right (RA) and left (LA) atria; this finding confirms the presence of a right-to-left shunt. Shortly thereafter, the main pulmonary artery (PA) and ascending aorta (AA) are enhanced simultaneously. Filling of the pulmonary veins occurs later (arrows), with a subsequent second peak in the ascending aorta. The frame time for this study was 900 msec after injection of 6 mL of gadolinium-based contrast agent at a rate of 6 mL/sec. (c) Time-intensity curves show complete overlap of the pulmonary arterial ({diamondsuit}) peak and the first peak of the ascending aorta ({blacktriangleup}); this finding confirms the presence of a right-to-left shunt. Subsequently, delayed filling of the pulmonary veins is followed by the second aortic peak. {blacksquare} = pulmonary vein. Time 0 corresponds to the start of the contrast material injection into the antecubital vein.

 


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  		Figure 4c. (a) Digital subtraction angiogram, right anterior oblique projection, in a 79-year-old female patient with an atrial septal defect. The catheter has been introduced cephalad from the inferior vena cava, it crosses the atrial septal defect, and its tip (arrow) lies in the left atrium. After the injection, the contrast material is hyperintense in the left side of the heart and aorta. (b) Subsecond MR angiograms (1.60/0.65), left anterior oblique projection. Six frames are shown from a 24-frame series. The contrast material is hyperintense simultaneously in both the right (RA) and left (LA) atria; this finding confirms the presence of a right-to-left shunt. Shortly thereafter, the main pulmonary artery (PA) and ascending aorta (AA) are enhanced simultaneously. Filling of the pulmonary veins occurs later (arrows), with a subsequent second peak in the ascending aorta. The frame time for this study was 900 msec after injection of 6 mL of gadolinium-based contrast agent at a rate of 6 mL/sec. (c) Time-intensity curves show complete overlap of the pulmonary arterial ({diamondsuit}) peak and the first peak of the ascending aorta ({blacktriangleup}); this finding confirms the presence of a right-to-left shunt. Subsequently, delayed filling of the pulmonary veins is followed by the second aortic peak. {blacksquare} = pulmonary vein. Time 0 corresponds to the start of the contrast material injection into the antecubital vein.

 
Average pulmonary arteriovenous peak-to-peak and onset-to-onset transit times for patients with and those without pulmonary hypertension or heart failure are summarized in Table 2. Transit times in the patients with a congenital shunt were significantly longer than in those without a shunt, as measured with both peak-to-peak and onset-to-onset parameters.


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  		TABLE 2. Summary of Transit Times for Patients without and Those with Pulmonary Hypertension or Heart Failure

 
The ascending aorta was visualized in all patients, free of vessel overlap and pulsation artifact. The partition images from the conventional 3D data set frequently showed pulsation artifact in the ascending aorta (Fig 5b), and overlap of structures in the right side of the heart was typical on MIPs. On partition images, it was possible to visualize vessel overlap, but pulsation artifact was present in 75% (18 of 24 image data sets) of averaged data. Subjective pulsatility ratings for conventional partition images were significantly greater (more artifact) than ratings for either conventional MIPs or subsecond MIPs for individual readers and averaged data for both readers. Subsecond MIPs and conventional MIPs were comparable in pulsatility ratings for individual and averaged data.



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  		Figure 5a. (a) True fast imaging with steady-state precession cine MR image (3.2/1.6, section thickness of 5 mm, acquisition time of 20 seconds, contrast material dose of 30 mL of gadopentetate dimeglumine at a rate of 2 mL/sec), left anterior oblique projection, in a 55-year-old male patient, depicts an ascending aortic aneurysm. A large smooth aneurysm extends from the aortic outlet (Ao) and tapers toward the arch (Ar), without evidence of dissection or thrombus. (b) Conventional partition MR angiograms (2.52/0.88), left anterior oblique projection, confirm the extent and form of the aneurysm. Note the pulsation artifact in the ascending aorta on several of the images and the persistent enhancement of the pulmonary arteries (arrows) and veins (arrowheads) that can obscure detail on MIPs (not shown). (c) Subsecond MR angiograms (1.60/0.65), left anterior oblique projection, are six frames from a 24-frame series. Contrast material is shown sequentially in the right side of the heart, main pulmonary artery (PA), pulmonary parenchyma, pulmonary veins (arrows), left atrium (LA), and aorta (A). The appearance, size, and extent of the ascending aortic aneurysm are similar to those shown in a and b. There is, however, extended volume coverage relative to b and clear separation of phases in the right and left sides of the heart relative to a. The frame time for this study was 900 msec.

 


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  		Figure 5b. (a) True fast imaging with steady-state precession cine MR image (3.2/1.6, section thickness of 5 mm, acquisition time of 20 seconds, contrast material dose of 30 mL of gadopentetate dimeglumine at a rate of 2 mL/sec), left anterior oblique projection, in a 55-year-old male patient, depicts an ascending aortic aneurysm. A large smooth aneurysm extends from the aortic outlet (Ao) and tapers toward the arch (Ar), without evidence of dissection or thrombus. (b) Conventional partition MR angiograms (2.52/0.88), left anterior oblique projection, confirm the extent and form of the aneurysm. Note the pulsation artifact in the ascending aorta on several of the images and the persistent enhancement of the pulmonary arteries (arrows) and veins (arrowheads) that can obscure detail on MIPs (not shown). (c) Subsecond MR angiograms (1.60/0.65), left anterior oblique projection, are six frames from a 24-frame series. Contrast material is shown sequentially in the right side of the heart, main pulmonary artery (PA), pulmonary parenchyma, pulmonary veins (arrows), left atrium (LA), and aorta (A). The appearance, size, and extent of the ascending aortic aneurysm are similar to those shown in a and b. There is, however, extended volume coverage relative to b and clear separation of phases in the right and left sides of the heart relative to a. The frame time for this study was 900 msec.

 


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  		Figure 5c. (a) True fast imaging with steady-state precession cine MR image (3.2/1.6, section thickness of 5 mm, acquisition time of 20 seconds, contrast material dose of 30 mL of gadopentetate dimeglumine at a rate of 2 mL/sec), left anterior oblique projection, in a 55-year-old male patient, depicts an ascending aortic aneurysm. A large smooth aneurysm extends from the aortic outlet (Ao) and tapers toward the arch (Ar), without evidence of dissection or thrombus. (b) Conventional partition MR angiograms (2.52/0.88), left anterior oblique projection, confirm the extent and form of the aneurysm. Note the pulsation artifact in the ascending aorta on several of the images and the persistent enhancement of the pulmonary arteries (arrows) and veins (arrowheads) that can obscure detail on MIPs (not shown). (c) Subsecond MR angiograms (1.60/0.65), left anterior oblique projection, are six frames from a 24-frame series. Contrast material is shown sequentially in the right side of the heart, main pulmonary artery (PA), pulmonary parenchyma, pulmonary veins (arrows), left atrium (LA), and aorta (A). The appearance, size, and extent of the ascending aortic aneurysm are similar to those shown in a and b. There is, however, extended volume coverage relative to b and clear separation of phases in the right and left sides of the heart relative to a. The frame time for this study was 900 msec.

 
Analysis of variance of visualization scores (carried out independently for the right side of the heart, pulmonary arteries, pulmonary veins, left atrium, ascending aorta, and descending aorta) showed that subsecond MR angiographic scores were equal to or greater than conventional partition scores, for both individual readers and averaged scores. In all structures except the descending aorta, conventional MIPs had significantly lower scores than those for either subsecond MR angiograms or conventional partition images for individual readers and averaged scores (Fig 6). In the descending aorta, ratings for subsecond MR angiograms, conventional partition MR images, and conventional MIPs were not significantly different.



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  		Figure 6. Bar graph depicts visualization ratings for anatomic structures on conventional partition MR images (black bars), conventional MIPs (gray bars), and subsecond MR angiograms (white bars). Higher scores imply easier visualization. RA = right atrium or ventricle, PA = pulmonary artery, PV = pulmonary vein, LA = left atrium or ventricle, AA = ascending aorta, DA = descending aorta. Conventional partition and subsecond MR images were not significantly different from each other, whereas both were significantly better than conventional MIPs (P < .05). This difference is explained by overlap of complex 3D anatomic structures on conventional 3D MIPs.

 
In four patients with aortic aneurysms, the dimensions of the aorta as measured on dynamic subsecond MR angiograms and conventional MR angiograms (MIPs and partition MR images) were the same (Fig 5). In two patients with aortic dissection, low-dose dynamic MR angiograms clearly showed the temporal filling patterns of true and false lumina.

In the healthy subjects, there was full agreement between findings on subsecond 3D MR angiograms and those on comparison images.


   Discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
 Discussion
REFERENCES
 
The results indicate that, with small contrast material doses and subsecond frame rates, it is possible to detect intra- and extracardiac shunt formation, to measure circulation times, and to evaluate dynamic vascular anatomy in projection format. As implemented, subsecond 3D MR angiography allowed temporal sampling that was rapid enough to depict short-lived processes, as illustrated in the patients with shunts and dissections.

When compared with conventional 3D MR angiography, subsecond 3D MR angiography has limited through-plane resolution and lower in-plane resolution, and the MIPs cannot usefully be viewed off axis. With conventional 3D MR angiography, however, vessel overlap can be troublesome on MIPs, and it is often necessary to review the partition MR images or multiplanar reformations (22,23). With compact bolus injection and sufficiently rapid image acquisition, temporal editing of the phases of enhancement in the aorta and right side of the heart is automatic, providing through-plane resolution on the basis of bolus integrity. Because the data acquisition window spans no more than one cardiac cycle, periodic ghosts due to cardiac and aortic pulsation are unlikely to occur. This is not the case with conventional 3D MR angiography, which spans 20–40 cardiac cycles and is vulnerable to pulsatility ghosts, which can mimic dissection (18–21). In the current study, some degree of pulsation artifact occurred in 75% (18 of 24 image data sets) of conventional 3D MR angiograms, whereas on subsecond MR images, pulsatility ratings greater than 1 were found on fewer than 10% (two of 24 image data sets) of the images.

In previous studies, the potential for nonenhanced MR imaging to help evaluate the thoracic aorta in disorders such as aneurysms and dissection has been addressed (27–30). The increasing popularity of MR angiography makes it tempting to use it to supplement more traditional nonenhanced methods with MR angiography. However, this supplementation comes at a cost when larger contrast material doses are used. In this context, the ability to supplement nonenhanced MR imaging with dynamic MR angiographic information, with use of very small contrast material doses, provides an attractive option, particularly with the advent of multidose vials of gadolinium-based contrast material.

Other methods for temporally resolved MR angiography have been described. Time-resolved imaging of contrast kinetics, or TRICKS (31), is a technique for time-resolved 3D MR angiography whereby low-spatial-frequency data are sampled more often than are high-spatial-frequency data. The latter are shared among neighboring frames to generate a full k-space data set for each frame. This technique represents a form of temporal interpolation whereby a sliding time window is incremented much less than its width, which enables rapid updating of 3D data sets. Frame durations as small as several seconds have been reported with TRICKS (31), but the overall acquisition window is several times greater. The subsecond 3D MR angiographic technique used in this study differs from TRICKS in that, although the through-plane resolution is lower, no temporal interpolation is used, and each subsecond data set is fully independent of its neighbors. These differences are achieved by using a very short repetition time and sacrificing spatial resolution in the partition-encoding direction to produce quasiprojection angiograms. Also, image processing with this subsecond MR angiographic technique is rapid and simple, such that the data are available for immediate viewing. With TRICKS, very large sets of data are generated, and image processing is computationally intensive (32).

Two-dimensional projection MR angiography with snapshot fast low-angle shot, or FLASH, has been described for evaluation of dynamic vascular changes (33) with temporal resolution of 1 second or less. The limitation with two-dimensional projection imaging has been poor signal-to-noise ratio (33), which can be offset to some degree with correlation analysis (34). This method involves consecutive injections of multiple contrast material boluses and algorithms for time-series analysis. Besides the simplicity of the image processing, the subsecond 3D MR angiographic technique differs from two-dimensional projection MR angiography in that averaging of the partition data increases the overall signal-to-noise ratio on the image. Furthermore, the 3D data are amenable to processing with the MIP algorithm (35), which further increases the contrast-to-noise ratio relative to a simple integration, as in the case of two-dimensional projection.

Another intriguing aspect of rapid MR angiography is the potential to measure circulation times (36). When a small volume of contrast material is introduced rapidly into the right side of the heart, time-intensity curves for the pulmonary veins approximate the impulse response or frequency function of lung transit times. The importance of this observation is at least twofold: (a) it may be possible to detect prolonged circulation times in patients with heart failure or pulmonary hypertension, as has recently been shown with electron-beam CT (36), and (b) the impulse response can be used to predict, by means of convolution, the response to arbitrary infusion waveforms (37) (if subsequent conventional MR angiography is planned). The former point is exemplified by the prolonged transit times in the patients with Eisenmenger physiology. Although simultaneous enhancement of the pulmonary and systemic arteries was obvious, subsequent enhancement of the pulmonary veins and left atrium occurred several seconds later than they did in the patients without shunts. If such a minimally invasive method proves reliable for measurement of circulation times, it may have a role in the assessment of heart failure and in the monitoring of results of therapy. Supplementary information is available from the same data set concerning lung parenchymal enhancement, the condition of the pulmonary veins, and the size of the left atrium. The clinical importance of these findings remains to be evaluated.

Limitations of the 3D subsecond MR angiographic technique include the relative lack of through-plane spatial resolution (compared with conventional MR angiography) and the requirement for high-performance imaging hardware. The former can be at least partly offset with the temporal filtering effect already mentioned. For ease of online image postprocessing, we used magnitude subtraction in the image domain, although there may be advantages to the use of complex subtraction in the k-space domain (38). There is no reason why either approach cannot be used successfully. A further disadvantage of the dynamic subsecond data acquisition is the requirement for rapid bolus injection into a good vein. At a minimum, a 20-gauge cannula secured in an antecubital vein is required to inject at a flow rate of 6 mL/sec. Slower infusion rates will tend to offset the advantage of high-temporal-resolution acquisition. In the current study, we did not find venous access to be a problem, and we routinely used 18-gauge cannulas.

In summary, rapid subsecond 3D MR angiography can depict dynamic processes in the heart and great vessels in circumstances where conventional MR angiography lacks the required temporal resolution. Additional advantages of the technique include speed, relative insensitivity to motion and pulsation artifact, and the requirement for only a small dose of contrast material.


   FOOTNOTES
 
Abbreviations: MIP = maximum intensity projection, 3D = three-dimensional

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


   REFERENCES
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Results
 Discussion
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