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Published online before print January 5, 2007, 10.1148/radiol.2423060299
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(Radiology 2007;242:735-742.)
© RSNA, 2007

Cardiac Imaging

Irreversible Myocardial Injury: Assessment with Cardiovascular Delayed-Enhancement MR Imaging and Comparison of 1.5 and 3.0 T—Initial Experience1

Adrian S. H. Cheng, MBBS, MRCP, Matthew D. Robson, PhD, Stefan Neubauer, MD, FRCP and Joseph B. Selvanayagam, MBBS, FRACP, DPhil

1 From the University of Oxford Centre for Clinical Magnetic Resonance Research and Department of Cardiovascular Medicine, University of Oxford, John Radcliffe Hospital, Headley Way, Headington, Oxford OX3 9DU, England. Received February 16, 2006; revision requested April 20; revision received May 15; final version accepted July 7. Supported by grants from the British Heart Foundation and Medical Research Council. A.S.H.C. supported by a grant from the Oxfordshire Health Services Research Committee. J.B.S. supported by an intermediate research fellowship from the British Heart Foundation. Address correspondence to J.B.S. (e-mail: joseph.selvanayagam{at}cardiov.ox.ac.uk).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
Purpose: To prospectively compare visualization and quantification of irreversible myocardial injury in patients with chronic myocardial infarction at 1.5- and 3.0-T magnetic resonance (MR) imaging.

Materials and Methods: The institutional research ethics committee approved the study. Participants gave written informed consent. Sixteen male patients (mean age, 66 years ± 13 [standard deviation]) with myocardial infarction were imaged with the same sequence by the same operator at 1.5 and 3.0 T. After cine imaging, a bolus of gadodiamide was administered. Short-axis images of entire left ventricle (LV) were acquired with a breath-hold T1-weighted segmented inversion-recovery turbo fast low-angle shot (FLASH) sequence. Agreement for myocardial hyperenhancement (HE) mass between field strengths was assessed with Bland-Altman method; agreement for detection and transmural extent of HE was assessed with {kappa} statistics. Intra- and interobserver reproducibility of mass and transmural extent of HE were assessed at 1.5 and 3.0 T.

Results: Bland-Altman analysis revealed no systematic bias (mean difference, 0.2 g; 95% confidence interval: –0.7 g, 1.2 g) and acceptable limits of agreement (–3.3 to 3.8 g) between field strengths for HE mass. HE mass measurements were strongly correlated (R2 = 0.99); there was no significant difference in measurements at 1.5 and 3.0 T (28.1 g ± 15.7 [22.6% ± 10.9 of LV mass] vs 27.8 g ± 15.7 [22.3% ± 10.7 of LV mass], respectively; P = .599). For all segments, there was a high degree of agreement for HE detection ({kappa} = 0.90) and transmural grade ({kappa} = 0.79) between field strengths. Intra- and interobserver variability were low between both field strengths. Initial inversion time selected to null the signal of normal myocardium at 3.0 T was 57 msec ± 20 longer than at 1.5 T (P < .01).

Conclusion: By using the same turbo FLASH MR pulse sequence, there was strong agreement in mass and transmural extent of myocardial HE between 1.5 and 3.0 T.

© RSNA, 2007


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
Results of studies have demonstrated the effectiveness of a segmented inversion-recovery fast gradient-echo magnetic resonance (MR) sequence to help differentiate irreversibly injured myocardium from normal myocardium, with signal intensity (SI) differences of nearly 500% obtained with 1.5-T systems (1). This technique of delayed-enhancement (DE) MR imaging has been shown in animal and human studies to depict the presence, location, and extent of acute and chronic irreversible myocardial injury in those with coronary disease (2–5). DE MR imaging allows assessment of the transmural extent of irreversible injury in vivo with high spatial resolution and is superior to single photon emission computed tomography (SPECT) for identification of subendocardial myocardial infarction (6–8). Furthermore, DE MR imaging permits quantification of even small areas of myocardial necrosis due to native coronary disease and percutaneous and surgical revascularization (9–11).

Three-tesla MR systems that have been approved for clinical use are expected to provide an increased signal-to-noise ratio that may improve applications, such as perfusion imaging and spectroscopy, that are currently limited by low temporal and spatial resolution at 1.5 T (12–18). Assessment of myocardial viability plays an important role in the clinical interpretation of the results derived from such applications. For example, a current strategy to improve the accuracy of cardiac MR imaging for depiction of coronary disease incorporates rest and stress cardiac MR perfusion results with DE MR imaging findings (19).

We hypothesized that there would be strong agreement in the mass and transmural grade of myocardial hyperenhancement (HE) between 1.5 and 3.0 T. Hence, the purpose of our study was to prospectively compare the visualization and quantification of irreversible myocardial injury in patients with confirmed chronic myocardial infarction at 1.5- and 3.0-T MR imaging.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
Study Population
Sixteen consecutive male patients (mean age, 66 years ± 13 [standard deviation]) with evidence of myocardial infarction at least 1 month old (mean age of infarction, 2.4 years ± 1.6) were recruited prospectively. A sample size of 16 patients was chosen to give 80% power to detect a moderate correlation. In all patients, the infarction was confirmed with an elevated troponin level together with clinical history and/or previous imaging results. The only exclusion criteria were the standard contraindications to MR imaging (incompatible metallic implants, such as pacemakers, defibrillators, and cerebral aneurysm clips, ocular metallic deposits, and severe claustrophobia). Means of participant characteristics were as follows: height, 1.77 m ± 0.07; weight, 86 kg ± 15; and heart rate, 65 beats per minute ± 18. The institutional research ethics committee approved our study. All participants gave written informed consent.

Cardiac MR Imaging Protocol
For each patient, images were acquired by the same operator (A.S.H.C. or J.B.S.) at both 1.5 (Sonata; Siemens Medical Solutions, Erlangen, Germany) and 3.0 T (Trio; Siemens Medical Solutions) in random order, such that eight patients underwent imaging at 1.5 T first, followed by imaging at 3.0 T an hour later, and vice versa for the other eight patients.

Images were acquired with the patient in the supine position by using anterior phased-array surface coils and either posterior phased-array surface coils (at 3.0 T) or two elements of the integrated spine coil (at 1.5 T). A standard electrocardiographic lead set (Siemens) was used; four leads were positioned over the anterior chest wall. From standard pilot images, short-axis cine images covering the entire left ventricle (LV) were acquired by using a retrospectively electrocardiographically gated steady-state free precession sequence (Table 1) to aid delineation of the subendocardial border by matching these images with contrast material–enhanced images. At 3.0 T, the protocol included a postacquisition delay of 3 seconds and a steady-state free precession frequency pilot (single-shot acquisition per cardiac cycle with a trigger delay of 350 msec; section thickness, 7 mm; repetition time, 3.1 msec; frequency offsets from –200 to 200 Hz in 40-Hz increments; breath-hold time, 11 cardiac cycles) to select the optimal frequency offset.


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  		Table 1. Parameters of Steady-State Free Precession MR Sequence Used at Each Field Strength

 
A bolus of 0.1 mmol per kilogram of body weight of a gadolinium-based contrast agent (gadodiamide, Omniscan; GE Healthcare, Amersham, England) followed by a 10-mL saline flush were administered through an intravenous cannula inserted into the antecubital fossa. Six minutes later, a series of segmented steady-state free precession images at a midventricular short-axis location was acquired after an inversion pulse, which followed the approach of Scheffler and Hennig (20), with increasing increments in inversion time (TI) equal to the temporal resolution (voxel size, 1.8 x 3.6 x 8 mm; flip angle, 50°; repetition time, 25.6 msec at 1.5 T and 26.7 msec at 3.0 T). The TI that best nulled the signal of normal myocardium was determined with qualitative visual assessment of these images by two operators (A.S.H.C., 2 years of experience in cardiac MR; J.B.S., 6 years of experience in cardiac MR) working in agreement. Electrocardiographically gated images (typically 8–10) were acquired in long- and short-axis planes identical to those of the cine images by using a breath-hold T1-weighted segmented inversion-recovery turbo fast low-angle shot (FLASH) sequence (Table 2). We used a higher flip angle in the 3.0-T sequence, which followed the preliminary work by Regenfus et al (21). Each short-axis image was reviewed prior to acquiring the next image, and TI was adjusted to maintain optimum nulling of the signal of normal myocardium by two operators (A.S.H.C., J.B.S.) working together in agreement.


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  		Table 2. Parameters of Turbo FLASH DE MR Sequence Used at Each Field Strength

 
Image acquisition was triggered to the same number of cardiac cycles at each field strength for each patient. Wherever feasible, our preference was for image acquisition to be triggered to every other cardiac cycle to allow adequate longitudinal relaxation between successive inversion pulses (22). The total duration of each imaging examination was 30 minutes.

Imaging at both field strengths was well tolerated by all patients, and all images acquired were of sufficient quality for analysis, despite sternal wires in two (13%) of 16 patients, coronary stents in eight (50%) patients, and atrial fibrillation in one (6%) patient. In eight (50%) patients, we were able to trigger image acquisition to every other cardiac cycle. The remaining eight patients could not hold their breath long enough to enable triggering of image acquisition to every other cardiac cycle, and in these patients, image acquisition was triggered to every cardiac cycle. No problems with electrocardiographic gating were encountered.

Image Analysis
By using software (Argus, version 25A; Siemens Medical Solutions), LV end-diastolic and end-systolic volume, ejection fraction, and mass were calculated for each patient in the standard way by manually tracing the endocardial and epicardial contours on end-diastolic and end-systolic images at 1.5 T (A.S.H.C.).

By using computer software (Matlab, version 6.5; MathWorks, Natick, Mass), the mass of myocardial HE in the LV on each image was quantified by an experienced observer (J.B.S.), who was blinded to patient information and field strength and who was separate from the observer who calculated the volume and mass. Hyperenhanced pixels were defined as those with SI greater than 2 standard deviations above the mean SI in a remote, nonenhanced myocardial region on the same image (3). On each short-axis image, computer-assisted planimetry was used to delineate the area of HE, which was multiplied by the section thickness and the myocardial density of 1.05 g/mL to obtain the infarct mass. The infarct mass also was expressed as a percentage of LV mass by dividing the infarct mass by the total LV mass measured at 1.5 T. Image intensity settings were kept constant for each patient.

Furthermore, to assess agreement between both field strengths for detection and transmural grade of HE, quantitative analysis (J.B.S.) as per Kim et al (23) and blinded to patient information and field strength was performed by using the 17-segment model recommended by the American Heart Association (24) and the following scoring system: no HE, grade 0; 1%–25% HE, grade 1; 26%–50% HE, grade 2; 51%–75% HE, grade 3; and more than 75% HE, grade 4 (5,23). Because the most apical segment can be affected by partial volume effects, it was excluded from the analysis.

To assess inter- and intraobserver reproducibility of measurements, two observers (A.S.H.C., J.B.S.) blinded to the previous results independently quantified the mass and transmural extent of HE 6 months later with a random selection of 50% of the images.

For each cardiac MR examination, the short-axis image showing the largest hyperenhanced region of myocardium was selected by an observer (A.S.H.C.) blinded to patient information and field strength. On each selected image, the observer measured SIs in the myocardial region of elevated SI (hyperenhanced myocardium) and in a remote myocardial region (nonenhanced myocardium), as well as the standard deviation of the noise in a circular region outside the body and anterior to the chest wall. The percentage SI elevation in the myocardium was calculated as follows: (mean SIHM – mean SINM)/(mean SINM) · 100. The image contrast-to-noise ratio was calculated as follows: (mean SIHM – mean SINM)/(1.5 · standard deviation of noise), where HM is hyperenhanced myocardium and NM is nonenhanced myocardium.

Statistical Analysis
Data analysis was performed with software (SPSS, version 13.0 for Windows; SPSS, Chicago, Ill). All results are expressed as mean ± standard deviation. The Shapiro-Wilk test of normality and z scores for skewness and kurtosis were used to assess whether the data for mass of myocardial HE and for TI conformed to the normal distribution. Agreement for mass of HE between field strengths was assessed by using the method proposed by Bland and Altman (25) and correlation coefficients, and agreement for detection and transmural extent of HE was assessed by means of {kappa} statistics. The {kappa} categories were as follows: less than 0.21, poor agreement; 0.21–0.40, fair agreement; 0.41–0.60, moderate agreement; 0.61–0.80, substantial agreement; more than 0.80, almost perfect agreement (26). Intra- and interobserver reproducibility of measurements of mass and transmural extent of HE were assessed by using the Bland-Altman method and {kappa} statistics. Paired t tests were used to compare the effect of field strength on measured mass of HE, initial and mean TI, percentage SI elevation, and contrast-to-noise ratio and to assess whether the order of imaging affected measurement of mass of HE or initial TI. Multilevel models were fitted to the data to allow for the correlation between repeated measurements across myocardial segments. Statistical tests were two tailed, and a P value of less than .05 was considered to indicate a statistically significant difference.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
LV Volume, Ejection Fraction, and Mass
Cine MR image analysis of the LV at 1.5 T revealed moderately impaired ejection fraction and dilated ventricles, which is in keeping with a previous diagnosis of myocardial infarction. Ejection fraction was 37% ± 14, end-diastolic volume was 249 mL ± 62, end-systolic volume was 161 mL ± 67, stroke volume was 88 mL ± 28, and mass was 141 g ± 41.

Mass of Myocardial HE
The data for measured mass of myocardial HE conformed to the normal distribution. The mass of HE ranged from 2 g (1% of LV mass) to 57 g (40% of LV mass). There was a strong correlation between the measurements (Fig 1), and Bland-Altman analysis (Fig 2) revealed no systematic bias (mean difference, 0.2 g; 95% confidence interval: –0.7 g, 1.2 g) and acceptable limits of agreement (–3.3 to 3.8 g) between both field strengths for mass of HE. The difference between mass of HE at 1.5 and at 3.0 T was not statistically significant (28.1 g ± 15.7 [22.6% ± 10.9 of LV mass] vs 27.8 g ± 15.7 [22.3% ± 10.7 of LV mass], respectively; P = .599). Further analysis confirmed that the difference between mass of HE measured at 1.5 T and that at 3.0 T did not reach significance, irrespective of the order of imaging. In patients who underwent 1.5-T imaging first, the mass of HE was 31.7 g ± 15.3 (23.4% ± 12.6 of LV mass) at 1.5 T versus 31.7 g ± 14.1 (23.4% ± 12.0 of LV mass) at 3.0 T (P > .99). In patients who underwent 3.0-T imaging first, the mass of HE was 23.4 g ± 16.2 (20.8% ± 7.3 of LV mass) at 1.5 T versus 22.8 g ± 17.3 (20.1% ± 8.5 of LV mass) at 3.0 T (P = .462).


Figure 1
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  		Figure 1: Graph shows strong correlation between measurements of mass of myocardial HE at both field strengths for 16 patients (P < .01).

 

Figure 2
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  		Figure 2: Graph of Bland-Altman comparison of mass of HE at both field strengths. Solid line = mean difference. Dashed lines = upper and lower limits of agreement. There was no systematic bias and acceptable limits of agreement for mass of HE between field strengths.

 
Detection and Transmural Extent of Myocardial HE
Myocardial HE was detected on images in all patients at both field strengths. When all myocardial segments were considered, there was almost perfect agreement for the detection of HE between both field strengths ({kappa} = 0.90), with exact agreement in 243 (94.9%) of 256 myocardial segments. Of a total of 256 myocardial segments in all 16 patients, 127 (49.6%) were graded as showing some degree of HE at 1.5 T, and 128 (50.0%) were graded as showing some degree of HE at 3.0 T (Fig 3).


Figure 3A
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  		Figure 3a: DE MR images (breath-hold T1-weighted segmented inversion-recovery turbo FLASH sequence: voxel size, 2.4 x 1.4 x 8 mm; field of view, 350 x 262.5 mm; section thickness, 8 mm; bandwidth, 140 Hz/pixel) in 62-year-old patient with occlusion of midportion of left anterior descending artery and anteroseptal myocardial infarction that occurred 3 months prior. These consecutive midventricular short-axis sections reveal HE (arrow) in corresponding coronary artery territory at (a, b) 1.5 T (repetition time, 10.9 msec; echo time, 4.3 msec; flip angle, 25°) and (c, d) 3.0 T (repetition time, 9.8 msec; echo time, 4.8 msec; flip angle, 30°).

 

Figure 3B
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  		Figure 3b: DE MR images (breath-hold T1-weighted segmented inversion-recovery turbo FLASH sequence: voxel size, 2.4 x 1.4 x 8 mm; field of view, 350 x 262.5 mm; section thickness, 8 mm; bandwidth, 140 Hz/pixel) in 62-year-old patient with occlusion of midportion of left anterior descending artery and anteroseptal myocardial infarction that occurred 3 months prior. These consecutive midventricular short-axis sections reveal HE (arrow) in corresponding coronary artery territory at (a, b) 1.5 T (repetition time, 10.9 msec; echo time, 4.3 msec; flip angle, 25°) and (c, d) 3.0 T (repetition time, 9.8 msec; echo time, 4.8 msec; flip angle, 30°).

 

Figure 3C
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  		Figure 3c: DE MR images (breath-hold T1-weighted segmented inversion-recovery turbo FLASH sequence: voxel size, 2.4 x 1.4 x 8 mm; field of view, 350 x 262.5 mm; section thickness, 8 mm; bandwidth, 140 Hz/pixel) in 62-year-old patient with occlusion of midportion of left anterior descending artery and anteroseptal myocardial infarction that occurred 3 months prior. These consecutive midventricular short-axis sections reveal HE (arrow) in corresponding coronary artery territory at (a, b) 1.5 T (repetition time, 10.9 msec; echo time, 4.3 msec; flip angle, 25°) and (c, d) 3.0 T (repetition time, 9.8 msec; echo time, 4.8 msec; flip angle, 30°).

 

Figure 3D
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  		Figure 3d: DE MR images (breath-hold T1-weighted segmented inversion-recovery turbo FLASH sequence: voxel size, 2.4 x 1.4 x 8 mm; field of view, 350 x 262.5 mm; section thickness, 8 mm; bandwidth, 140 Hz/pixel) in 62-year-old patient with occlusion of midportion of left anterior descending artery and anteroseptal myocardial infarction that occurred 3 months prior. These consecutive midventricular short-axis sections reveal HE (arrow) in corresponding coronary artery territory at (a, b) 1.5 T (repetition time, 10.9 msec; echo time, 4.3 msec; flip angle, 25°) and (c, d) 3.0 T (repetition time, 9.8 msec; echo time, 4.8 msec; flip angle, 30°).

 
When all myocardial segments were considered, there was substantial agreement for transmural grade of HE between both field strengths ({kappa} = 0.79), with exact agreement in 221 (86.3%) of 256 myocardial segments (Fig 4). These associations remained strong and highly significant (both, P < .01) after adjusting for the correlation between multiple segments in multilevel models.


Figure 4
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  		Figure 4: Graph of distribution of transmural grade of myocardial HE in all 256 myocardial segments at 1.5 T and 3.0 T.

 
Reproducibility of Mass and Transmural Extent Measurements
Intra- and interobserver coefficients of variability for measurements of mass of HE were good for both field strengths (intraobserver: 3.6% at 1.5 T, 3.7% at 3.0 T; interobserver: 5.0% at 1.5 T and 4.3% at 3.0 T). Bland-Altman analysis revealed no systematic bias and acceptable limits of agreement for intra- and interobserver measurements of mass of HE at both field strengths (Table 3). For transmural grade of HE, there was substantial intraobserver agreement ({kappa} = 0.78 and 0.74 at 1.5 T and 3.0 T, respectively) and interobserver agreement ({kappa} = 0.70 and 0.67 at 1.5 T and 3.0 T, respectively).


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  		Table 3. Intra- and Interobserver Reproducibility of Measurements of Mass of Myocardial HE at Both Field Strengths

 
Inversion Time
The data for initial and mean TI conformed to the normal distribution. The initial TI selected by the operator to null the signal of normal myocardium was 57 msec ± 20 longer at 3.0 T than at 1.5 T (357 msec ± 13 vs 300 msec ± 18, respectively; P < .01) (Fig 5). Further analysis was performed to elucidate whether the order of imaging affected the initial TI selected by the operator. The initial TI selected in patients who underwent 1.5-T imaging first was 51 msec ± 15 longer at 3.0 T than at 1.5 T (353 msec ± 10 vs 302 msec ± 17, respectively; P < .01), while the initial TI selected in patients who underwent 3.0-T imaging first was 64 msec ± 24 longer at 3.0 T than at 1.5 T (361 msec ± 16 vs 297 msec ± 20, respectively; P < .01). The mean TI used during imaging was 54 msec ± 26 longer at 3.0 T than at 1.5 T (371 msec ± 15 vs 316 msec ± 20, respectively; P < .01).


Figure 5
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  		Figure 5: Graph of initial inversion time (TI) used for DE MR imaging at each field strength shows significantly longer TI is needed at 3.0 T (P < .01) than at 1.5 T. {blacksquare} = Initial TI used at each examination, ms = millisecond.

 
Percentage SI Elevation and Contrast-to-Noise Ratio
There was no significant difference in percentage SI elevation between 1.5 and 3.0 T (479% ± 297 vs 487% ± 263, respectively; P = .897) or contrast-to-noise ratio between 1.5 and 3.0 T (33 ± 10 vs 34 ± 23, respectively; P = .852).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
ADVANCES IN KNOWLEDGE
 References
 
We found strong agreement in measurements of both mass and transmural extent of myocardial HE between 3.0 and 1.5 T when using the same segmented inversion-recovery turbo FLASH pulse sequence in patients with previous myocardial infarction.

There are two critical elements in the clinical performance of the DE technique at 1.5 T (27). First, its ability to help quantify small areas of myocardial necrosis (with a resolution almost 30-fold greater than that at SPECT) allows visualization of microinfarcts that cannot be depicted with other imaging techniques (7). Second, by providing direct depiction of both nonviable and viable myocardium, DE MR imaging gives information about the transmural extent of irreversible injury, which makes it a powerful tool for the prediction of myocardial viability (23). In our study, we assessed both of these elements at 3.0 T and compared them with the already validated 1.5-T sequence. We found that both the measurement of volume (with computer-assisted planimetry) and transmural grade of HE were virtually identical at 1.5 and 3.0 T, despite sternal wires and coronary stents in a substantial proportion of patients and despite theoretic concerns of reliable R-wave triggering (amplified magnetohydrodynamic effects) at 3.0 T.

Furthermore, our study results demonstrated good intra- and interobserver agreement for measurements of mass and transmural extent of HE at both field strengths. This indicates that DE MR imaging with a turbo FLASH pulse sequence is a reproducible technique at either field strength. The mass of HE quantified in our study cohort was over a wide range, representing very small (2 g; 1% of LV mass) to large (57 g; 40% of LV mass) areas of myocardial injury. Some of the variability in measurements of mass of HE can be accounted for by intra- and interobserver variability, although observer variability was 5% or less at both field strengths. There was less agreement for transmural grade of HE than for detection of HE. The principal reasons were the inherently greater variability in scoring transmural grade from 0 to 4 when compared with that of the binary decision as to whether HE is present or absent and some variability in registration of segments between 1.5- and 3.0-T imaging.

Injured myocardium demonstrates greater shortening of the T1 after contrast material administration than does normal myocardium. Inherent to the higher field strength at 3.0 T, myocardial T1 is longer than that at 1.5 T. Results of our study demonstrate that optimal TI to null the signal of normal myocardium needs to be longer at 3.0 T at the same time after contrast material injection. This is to be expected because the TI chosen to provide maximal SI difference between infarcted and normal myocardium is directly related to myocardial T1.

The percentage SI elevation and contrast-to-noise ratio obtained in our study at 1.5 T compared favorably with those reported by Simonetti et al (1) in their work in canine hearts. However, we found no significant differences in the percentage SI elevation or contrast-to-noise ratio between both field strengths, which is consistent with the findings of Hackenbroch et al (28). Results of other studies demonstrate higher percentage SI elevation at 3.0 T than we found (1107% [21] and 1230% [28]), which might be explained by the use of double the contrast agent dose (0.2 mmol/kg) in these studies when compared with ours (0.1 mmol/kg). Theoretically, one might expect that the signal-to-noise ratio would double when the static magnetic field strength is doubled and that the increase in T1 would make the effects of contrast agents more pronounced. There may be several reasons for our findings. In practice, the increased T1 may have decreased our signal-to-noise ratio because of saturation effects, which counteracted the signal-to-noise ratio increase caused by the higher field strength. Second, different radiofrequency coil configurations were used with each system. Third, the sequence parameters at 1.5 T have been thoroughly optimized and one would expect 3.0-T data to improve with optimization, and contrast agent dose, repetition time, and flip angle are likely to play an important role.

Our study had limitations. The number of patients was small, and, to keep the patient cohort uniform, we excluded patients with acute myocardial infarction or nonischemic cardiomyopathy. Hence, we could not assess the phenomena of microvascular obstruction (8) (zone of hypoenhancement surrounded by hyperenhanced myocardium on images) or myocardial fibrosis seen in dilated or hypertrophic cardiomyopathy (29,30). Nevertheless, we suspect 3.0-T DE MR imaging to be similar to 1.5 T for these indications, provided the TI is appropriately lengthened.

Residual contrast material at acquisition of the second set of DE images in each patient may have affected the TI and/or mass of HE measured. However, in healthy human volunteers, Weinmann et al (31) demonstrated that the plasma concentration of gadolinium-based contrast material declines according to a biexponential function, with a distribution phase with a mean half-life of 12 minutes ± 7.8, and concluded that its efficiency to enhance SI will last up to only about 1 hour after intravenous injection of 0.1 mmol/kg. Accordingly, in our cardiac MR protocol, the time interval between acquisition of the sets of images with each system was 1 hour for each patient, the operator adjusted TI during each examination to maintain optimum nulling of the signal of the normal myocardium, and the randomization of the order of imaging was perfectly balanced to avoid bias. Furthermore, our data indicate that the order of imaging had no influence on the initial TI selected by the operator or the mass of HE measured.

The results of our study demonstrate that DE MR imaging at 3.0 T is a reproducible technique that can be applied for the identification of irreversible injury and myocardial viability. We envisage that DE MR imaging can be included as part of the comprehensive cardiac MR protocol at 3.0 T with applications such as myocardial perfusion and spectroscopy that are likely to be improved by the increased signal-to-noise ratio associated with the higher field strength.


   ADVANCES IN KNOWLEDGE
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
References
 


   FOOTNOTES
 

Abbreviations: DE = delayed enhancement • FLASH = fast low-angle shot • HE = hyperenhancement • LV = left ventricle • SI = signal intensity • TI = inversion time

Authors stated no financial relationship to disclose.

Author contributions: Guarantors of integrity of entire study, A.S.H.C., S.N., J.B.S.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; approval of final version of submitted manuscript, all authors; literature research, A.S.H.C., M.D.R., J.B.S.; clinical studies, A.S.H.C., J.B.S.; statistical analysis, A.S.H.C., M.D.R., J.B.S.; and manuscript editing, all authors


   References
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 

  1. Simonetti OP, Kim RJ, Fieno DS, et al. An improved MR imaging technique for the visualization of myocardial infarction. Radiology 2001;218:215–223.[Abstract/Free Full Text]
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