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Published online before print February 21, 2008, 10.1148/radiol.2471070132
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(Radiology 2008;247:106-114.)
© RSNA, 2008

Experimental Studies

Respiratory Motion and Cardiac Arrhythmia Effects on Diagnostic Accuracy of Myocardial Delayed-enhanced MR Imaging in Canines1

Burkhard Sievers, MD, Wolfgang G. Rehwald, PhD, Timothy S. E. Albert, MD, Manesh R. Patel, MD, Michele A. Parker, MS, Raymond J. Kim, MD, and Robert M. Judd, PhD

1 From the Duke Cardiovascular Magnetic Resonance Center, Duke Clinic, Room 4229, Trent Drive, Durham, NC 27710. Received January 1, 2007; revision requested March 16; revision received June 6; accepted June 27; final version accepted September 7. R.M.J. supported by R01-HL63268 and K02-HL04394. R.J.K. supported by R01-HL64726. R.J.K. and R.M.J. have a U.S. patent, which is owned by Northwestern University, on delayed-enhancement MR imaging. Address correspondence to R.M.J. (e-mail: robert.judd{at}duke.edu).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 IMPLICATION FOR PATIENT CARE
 References
 
Purpose: To prospectively compare in canines the diagnostic accuracy for myocardial infarction (MI) of standard delayed-enhancement (DE) magnetic resonance (MR) imaging versus that of subsecond DE MR imaging with and without breath holding and/or cardiac arrhythmia, with histologic findings or absence of surgical creation of MI as the reference standard.

Materials and Methods: This study was approved by the Institutional Animal Care and Use Committee; 21 canines were imaged with one standard and two subsecond DE MR techniques in four conditions: condition 1, breath holding and steady gating; 2, non–breath holding and steady gating; 3, breath holding and irregular heart rhythm; and 4, non–breath holding and irregular heart rhythm. Images were randomized and scored for diagnostic accuracy, image quality, and observer confidence. Sensitivity, specificity, and diagnostic accuracy for MI detection were calculated for each technique and clinical condition separately. The {chi}2, paired t, and McNemar tests were used for comparisons.

Results: Fifteen dogs had MIs. Among conditions 2–4, differences were not significant (P > .05); data were pooled and referred to as group B. Condition 1 was group A. Accuracy, image quality, and observer confidence, respectively, for standard DE MR imaging were 96%, 3.7 ± 0.8, and 2.7 ± 0.6 in group A but only 74%, 2.4 ± 0.8, and 1.8 ± 0.7 in group B (P ≤ .004 for each). Corresponding scores for subsecond techniques were unaffected by respiratory motion and/or arrhythmia. Subsecond techniques had higher accuracy (82% and 86% vs 74%), better image quality (3.9 ± 0.7 and 3.2 ± 0.8 vs 2.4 ± 0.8), and greater confidence (2.4 ± 0.7 and 2.1 ± 0.7 vs 1.8 ± 0.7) (P ≤ .0002 for each) than standard DE MR imaging. In group A, standard performed better than subsecond DE MR imaging.

Conclusion: Standard DE MR imaging is appropriate for MI detection with breath holding and regular heart rhythm, while subsecond techniques are appropriate with an irregular heart rhythm and when breath holding is not possible.

© RSNA, 2008


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 IMPLICATION FOR PATIENT CARE
 References
 
During the past 5 years, the detection of myocardial infarction by using delayed-enhancement (DE) magnetic resonance (MR) imaging with a segmented inversion-recovery gradient-echo pulse sequence (1) (standard DE MR imaging) has been the subject of more than 40 studies in humans published in major journals (2). Concurrent with this increasing number of research publications, the use of standard DE MR imaging in routine clinical practice has also increased and, compared with research studies, generally includes a more heterogeneous patient population. As a consequence, imaging of patients in whom respiratory motion and arrhythmia cannot be avoided is becoming increasingly common. While standard DE MR imaging has clear advantages in terms of image quality for patients who can hold their breath and have a steady heart rhythm, it is also recognized that image quality is considerably poorer when these conditions cannot be met. To date and to our knowledge, however, the effects of respiratory motion and arrhythmia on diagnostic accuracy, image quality, and observer confidence at standard DE MR imaging have not been systematically studied.

Recently, fast (subsecond) DE MR imaging techniques (3) for infarct detection have become practical and may potentially overcome the limitations of standard DE MR imaging. These new techniques can acquire snapshot images in a few hundred milliseconds, which effectively eliminates the effects of heart motion as well as the need to image during six to 10 consecutive cardiac cycles. The diagnostic accuracy of subsecond inversion-recovery DE MR imaging techniques (subsecond DE MR imaging) for the detection of myocardial infarction during uncorrected respiratory and cardiac motion, however, has not been systematically studied to our knowledge.

Thus, the purpose of our study was to prospectively compare in canines the diagnostic accuracy for myocardial infarction of segmented inversion-recovery DE MR imaging (standard DE MR imaging) versus that of subsecond DE MR imaging with and without breath holding and/or cardiac arrhythmia, by using histologic findings or absence of surgical creation of myocardial infarction as the reference standard.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 IMPLICATION FOR PATIENT CARE
 References
 
One author (W.G.R.) is an employee of Siemens Medical Solutions (Chicago, Ill). Other authors had control of inclusion of any data and information that might present a conflict.

Animals and Reference Standard
The experiments were designed to allow us to simulate the effects of uncorrected respiration and/or artificial irregular cardiac gating in the same animal under controlled conditions. Twenty-one canines of both sexes and with weights of 23–32 kg were studied with approval of our Institutional Animal Care and Use Committee. Seventeen animals underwent thoracotomy and isolation of the left anterior descending coronary artery distal to its first major diagonal branch; thoracotomy and artery isolation were performed by using sterile techniques and procedures (4). The left anterior descending coronary artery was then occluded for 90 minutes, followed by permanent reperfusion. The chest was then closed, and the animals were allowed to recover. Four animals did not undergo surgery. The reference standard for the presence or absence of myocardial infarction was knowledge of whether the animal underwent surgery and/or direct histologic examination of the tissue by using established staining techniques (4,5) (triphenyltetrazolium chloride). Histologic examination was performed in two animals that underwent surgery but had no hyperenhancement at DE MR imaging. Histologic examination was performed by two operators (R.M.J. and W.G.R., with 13 and 8 years of experience, respectively) with extensive experience in identifying myocardial infarcts on postmortem tissue samples.

MR Imaging
MR imaging was performed with the use of anesthesia (isoflurane)(4). The dogs were placed supine in a 1.5-T clinical imager (Sonata; Siemens, Erlangen, Germany), and an eight-element phased-array receiver coil was placed on the chest for imaging. All images were electrocardiographically gated. The baseline heart rate was recorded in all dogs. Images were acquired in the same views for all imaging techniques and included multiple contiguous short-axis views that covered the left ventricle. Each animal was imaged with the standard DE MR imaging technique and two subsecond DE MR imaging techniques under four conditions: (a) condition 1, breath holding and regular heart rhythm (steady gating); (b) condition 2, non–breath holding and regular heart rhythm; (c) condition 3, breath holding and irregular heart rhythm (arrhythmia simulation); and (d) condition 4, non–breath holding and irregular heart rhythm. All images were acquired in a random order with respect to imaging technique and condition. Thus, each animal underwent 12 distinct acquisitions.

A total of 28 complete studies were performed. Five animals were imaged twice and one was imaged three times to ensure infarct imaging in the acute and chronic stages. All studies in animals with infarction were performed between 4 days and 4 months after surgery (≥1 month apart for serial imaging).

Standard DE MR imaging.—Segmented inversion-recovery gradient-recalled echo (GRE) DE MR imaging (1) was performed in the usual fashion (5,6) after intravenous administration of 0.2 mmol gadoversetamide (OptiMARK; Mallinckrodt, St Louis, Mo) per kilogram of body weight. Data acquisition was performed in a segmented fashion for approximately 240 msec per heartbeat during mid-diastole to take advantage of the fact that the heart is relatively motionless during this time (Table 1). Images were acquired every second heartbeat to allow the magnetization to return to baseline. The inversion time (typically 240–340 msec) was set to null signal from normal myocardium and was adjusted during the examination (6).


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  		Table 1. Pulse Sequence Parameters for Imaging Techniques

 
Subsecond DE MR imaging.—Similar to standard DE MR imaging, an inversion-recovery prepulse was used to accentuate the visibility of infarcted regions for both the subsecond steady-state free precession sequence and the spoiled GRE (hereafter, subsecond GRE) sequence (Table 1). The inversion time (typically 240–340 msec) was set to null signal from normal myocardium and was adjusted over time. Parallel image acquisition (7) (integrated parallel acquisition technique, acceleration factor of two) was used to speed up imaging. Importantly, the entire data set for one image was acquired in a single continuous stream (single shot). This technique provides infarct imaging with complete left ventricular coverage in less than 30 seconds. Images were acquired in a random order to minimize the effect of postcontrast imaging time.

Simulated respiratory motion and cardiac arrhythmia.—Non–breath-hold image acquisition was performed with the respirator turned on (18 breaths per minute). The arrhythmia model was specifically designed to ensure that at least 50% of the data were acquired in diastole, to remove bias among imaging techniques. For arrhythmia simulation, low-level software commands were sent to the physiologic control unit of the imager to cause interference with the electrocardiographic signal at random time points and simulate arrhythmia. Specifically, for standard DE MR imaging, the duration between the R wave and the beginning of the image acquisition was increased every cardiac cycle. An incremental trigger time delay (50-msec increments) was chosen and was added to the next cardiac cycle length. Thus, data were acquired at different time points in each cardiac cycle to simulate an irregular heart rhythm. The image acquisition was triggered on the R wave. For subsecond DE MR imaging, an artificial electrocardiogram (cycle length, 400 msec) was activated on the imager console. The real electrocardiogram was turned off during this time. The R-R interval of the artificial electrocardiogram was always different from the true R-R interval (typically 600–750 msec). The sequence was triggered to the artificial R wave, which occurred at different time points within the cardiac cycle.

Diagnostic Accuracy
All images were placed in random order and scored visually by consensus of two experienced observers (T.S.E.A. and B.S., with 4 and 5 years of experience in cardiovascular MR imaging, respectively) masked to animal identity, presence of infarction, and acquisition technique. Standard and subsecond DE MR images were read independently during separate sessions. Analysis was performed for all three imaging sequences and the four conditions separately.

Hyperenhancement was scored with a 17-segment model (8) by using a five-point scale (a score of 0 indicated no hyperenhancement; a score of 1, 1%–25% hyperenhancement; a score of 2, 26%–50% hyperenhancement; a score of 3, 51%–75% hyperenhancement; and a score of 4, 76%–100% hyperenhancement). Infarct size as a percentage of left ventricular myocardium was calculated by summing the regional scores from standard DE MR images (each weighted by the midpoint of the range of hyperenhancement) and dividing this number by the total number of regions.

Image Quality and Observer Confidence
Image quality (1 = very poor and not analyzable, 2 = poor, 3 = acceptable, 4 = good, 5 = very good) and the degree of observer confidence with regard to the presence or absence of infarction (1 = low confidence, 2 = some confidence, 3 = high confidence) were scored separately on the basis of the 17-segment model by consensus of the two experienced observers mentioned above. The image quality was deemed not analyzable when normal myocardium could not be differentiated from the blood pool because of extensive cardiac and/or respiratory motion artifacts. In the presence of image artifacts in which infarcted tissue could still be identified, the image quality was scored as poor. The image quality was deemed very good when a clear differentiation between infarcted (hyperenhanced) myocardium, normal (black) myocardium, and blood pool (gray) was possible and when no artifacts were present.

Quantitative Analysis
Quantitative analysis was performed on images that were acquired during breath holding and steady gating (condition 1) and during free breathing and arrhythmia (condition 4) to evaluate potential mechanisms for differences in the results between the three techniques. We chose conditions 1 and 4 because we assumed that differences between these techniques were the most pronounced. Ten dogs with myocardial infarction and hyperenhancement with all three techniques were randomly selected. A single short-axis image with the largest hyperenhanced region was selected for each animal. Regions of interest (average size, 20 mm2) were placed within the infarct, in noninfarcted (remote) myocardium, and in the center of the left ventricular cavity, and signal intensities were measured. Additionally, a region of interest was placed outside the body (near the sternum) to measure the standard deviation of background noise. Regions of interest were placed by an experienced observer (B.S., with 5 years of experience) who was masked to clinical information and imaging technique.

Signal-to-noise ratios (SNRs) and contrast-to-noise ratios (CNRs) were calculated as follows: SNR = mean SIinf/(1.43 x SDbn) and CNR = (mean SIinf – mean SIrm)/(1.43 x SDbn), where SIinf is the signal intensity of the infarct, SDbn is the standard deviation of background noise, and SIrm is the signal intensity of the remote myocardium. The correction factor of 1.43 accounts for the underestimation of noise that occurs when noise is measured from magnitude images (9) after adjustment for an eight-element coil array (10). Signal intensity ratios were calculated as follows: SI ratio = mean SIinf/mean SIrm. This method of measuring noise has limitations in the setting of parallel imaging (11). Thus, SNR and CNR values from the subsecond DE MR images should be considered only approximations.

Statistical Analysis
Continuous data are expressed as means ± standard deviations. Sensitivity, specificity, and diagnostic accuracy (12) for infarct detection with standard DE MR imaging was calculated for each clinical condition separately. Comparisons of sensitivity, specificity, and diagnostic accuracy between the three clinical conditions that included non–breath holding and/or arrhythmia were made by using the McNemar test with a Bonferroni correction. Linear mixed-effects techniques were used to assess image quality scores, confidence scores, and infarct size for standard DE MR imaging; a Bonferroni adjustment was used for the pairwise comparisons between the three clinical conditions that included non–breath holding and/or arrhythmia.

Mixed-effects models were used to compare sensitivity, specificity, and diagnostic accuracy, as well as image quality and confidence scores between the two groups for each technique to take into account the multiple acquisitions for combined clinical conditions. For the comparisons of sensitivity, specificity, and diagnostic accuracy between techniques, the McNemar test was used. Differences in image quality scores, confidence scores, and infarct size between techniques were assessed with paired t tests. Statistical analyses were performed by using software (S-Plus 6 for Windows, 2002; Insightful, Seattle, Wash). All statistical test results are two tailed, and P < .05 was regarded as indicating a significant difference.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 ADVANCES IN KNOWLEDGE
 IMPLICATION FOR PATIENT CARE
 References
 
Two animals underwent left anterior descending coronary artery occlusion but had no evidence of myocardial infarction at direct histologic examination of the tissue. Thus, of the 21 animals, 15 had myocardial infarction and six did not (Fig 1). For animals with infarction, infarct size at standard DE MR imaging ranged from 0% to 20.8% of left ventricular mass (mean, 6.6% ± 4.6), which confirmed that a wide range of infarct sizes was considered. During imaging, the mean heart rate was 90.5 min–1 ± 7.8 (range, 76–108 min–1). A total of 5712 myocardial segment scores were analyzed (28 acquisitions times 17 segments times three imaging techniques times four conditions).


Figure 1
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  		Figure 1: Flow diagram of animals eligible for participation in the study. Two dogs that underwent surgery with left anterior descending coronary artery occlusion followed by permanent reperfusion with no hyperenhancement at standard DE MR imaging and no infarction at histologic examination (triphenyltetrazolium chloride [TTC] staining) were assigned to the control group.

 
Among conditions 2, 3, and 4, differences in diagnostic accuracy, image quality, and observer confidence were not statistically significant (P > .05). Accordingly, the data for these three groups (conditions 2–4) were pooled for further analysis and are hereafter referred to as group B (Table 2); condition 1 (breath holding and regular heart rhythm) is hereafter referred to as group A (Table 2).


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  		Table 2. Sensitivity and Specificity according to Imaging Technique in Groups A and B

 
Diagnostic Accuracy
For standard DE MR imaging, diagnostic accuracy was significantly higher for group A than group B (P = .004) (Figs 2, 3). For both the subsecond steady-state free precession and the subsecond GRE sequences, there were no significant differences between group A and group B (P = .33 and P > .99, respectively) for diagnostic accuracy (Figs 4–7). The reason for these results was that image quality was largely unaffected by the uncorrected respiratory and cardiac motion in group B (Figs 4, 6).


Figure 2
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  		Figure 2: Standard DE MR images (short-axis view; repetition time msec/echo time msec, 8.4/3.8; flip angle, 25°) in group A (left) (breath holding and steady gating) and group B (right) (non–breath holding and arrhythmia). Infarct (arrow) on group A image can be diagnosed with high confidence (score of 3); diagnostic accuracy is very high. Group B image, however, has very poor image quality (score of 1); infarct cannot be detected with confidence (score of 1), and diagnostic accuracy is low.

 

Figure 3A
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  		Figure 3a: Bar graphs of (a) diagnostic accuracy, (b) image quality, and (c) confidence level for infarct detection with standard DE MR images show significantly higher values for group A than for group B. Error bars = standard deviation.

 

Figure 3B
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  		Figure 3b: Bar graphs of (a) diagnostic accuracy, (b) image quality, and (c) confidence level for infarct detection with standard DE MR images show significantly higher values for group A than for group B. Error bars = standard deviation.

 

Figure 3C
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  		Figure 3c: Bar graphs of (a) diagnostic accuracy, (b) image quality, and (c) confidence level for infarct detection with standard DE MR images show significantly higher values for group A than for group B. Error bars = standard deviation.

 

Figure 4
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  		Figure 4: Standard DE MR and subsecond steady-state free precession (SSFP) (1.6/1.1; flip angle, 50°) images (short-axis view). Image quality for standard DE MR imaging is good (score of 4) for group A but very poor for group B, in which infarct (arrow) cannot be detected with confidence (score of 1). For subsecond steady-state free precession sequence, however, image quality is not affected by uncorrected respiratory and cardiac motion (group B), and infarct (arrow) can be diagnosed with high confidence (score of 3).

 

Figure 5A
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  		Figure 5a: Bar graphs of (a) diagnostic accuracy, (b) image quality, and (c) confidence level for subsecond steady-state free precession (gray bars) compared with standard (white bars) DE MR imaging. For group A, diagnostic accuracy and confidence level were lower for subsecond steady-state free precession images, whereas image quality was not significantly different compared with that of standard DE MR images. For group B, diagnostic accuracy, image quality, and confidence level for infarct detection remained unaffected by respiratory and cardiac motion. Error bars = standard deviation.

 

Figure 5B
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  		Figure 5b: Bar graphs of (a) diagnostic accuracy, (b) image quality, and (c) confidence level for subsecond steady-state free precession (gray bars) compared with standard (white bars) DE MR imaging. For group A, diagnostic accuracy and confidence level were lower for subsecond steady-state free precession images, whereas image quality was not significantly different compared with that of standard DE MR images. For group B, diagnostic accuracy, image quality, and confidence level for infarct detection remained unaffected by respiratory and cardiac motion. Error bars = standard deviation.

 

Figure 5C
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  		Figure 5c: Bar graphs of (a) diagnostic accuracy, (b) image quality, and (c) confidence level for subsecond steady-state free precession (gray bars) compared with standard (white bars) DE MR imaging. For group A, diagnostic accuracy and confidence level were lower for subsecond steady-state free precession images, whereas image quality was not significantly different compared with that of standard DE MR images. For group B, diagnostic accuracy, image quality, and confidence level for infarct detection remained unaffected by respiratory and cardiac motion. Error bars = standard deviation.

 

Figure 6
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  		Figure 6: DE MR images (short-axis view) demonstrate all three DE techniques (standard DE MR, subsecond steady-state free precession [SSFP], and subsecond GRE [1.8/1.5; flip angle, 8°] imaging). In group A, for standard DE MR imaging, image quality is very good (score of 5); infarct (arrows) can be diagnosed with high confidence (score of 3). In group B, image quality is very poor (score of 1) because of motion artifacts, which makes correct interpretation of this image impossible. However, infarct can be diagnosed with high confidence (score of 3) on subsecond steady-state free precession and subsecond GRE images, despite the presence of uncorrected respiratory and cardiac motion (group B). Note that for both subsecond DE MR imaging techniques, image quality between group A and group B is very similar.

 

Figure 7A
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  		Figure 7a: Bar graphs of (a) diagnostic accuracy, (b) image quality, and (c) confidence level for subsecond GRE (black bars) compared with standard (white bars) DE MR imaging. For group A, diagnostic accuracy, image quality, and confidence level were higher for standard DE MR imaging, whereas subsecond GRE imaging performed better in group B. Gray bars = subsecond steady-state free precession values, error bars = standard deviation.

 

Figure 7B
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  		Figure 7b: Bar graphs of (a) diagnostic accuracy, (b) image quality, and (c) confidence level for subsecond GRE (black bars) compared with standard (white bars) DE MR imaging. For group A, diagnostic accuracy, image quality, and confidence level were higher for standard DE MR imaging, whereas subsecond GRE imaging performed better in group B. Gray bars = subsecond steady-state free precession values, error bars = standard deviation.

 

Figure 7C
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  		Figure 7c: Bar graphs of (a) diagnostic accuracy, (b) image quality, and (c) confidence level for subsecond GRE (black bars) compared with standard (white bars) DE MR imaging. For group A, diagnostic accuracy, image quality, and confidence level were higher for standard DE MR imaging, whereas subsecond GRE imaging performed better in group B. Gray bars = subsecond steady-state free precession values, error bars = standard deviation.

 
In group A, diagnostic accuracy was higher for standard DE MR imaging than for both the subsecond steady-state free precession sequence and the subsecond GRE sequence (P = .08 and P = .18, respectively) (Figs 4, 6). In group B, conversely, diagnostic accuracy was higher for the subsecond steady-state free precession and subsecond GRE techniques than for standard DE MR imaging (P = .09 and P = .03, respectively) (Figs 5, 7).

Image Quality and Observer Confidence
Overall, the results for both image quality and observer confidence followed the same patterns observed for diagnostic accuracy. Specifically, for standard DE MR imaging, both image quality and observer confidence were significantly higher in group A than in group B (P < .0001) (Fig 3). In addition, for subsecond DE MR imaging, both measures were similar in groups A and B (P ≥ .07) (Fig 5, 7). Importantly, subsecond DE MR imaging performed better than standard DE MR imaging in group B (P ≤ .0002). The opposite was generally the case in group A (P ≤ .0005) (Fig 5, 7), except that the image quality for the subsecond steady-state free precession technique was similar to that for standard DE MR imaging (P = .32).

Quantitative Analysis
During breath holding and steady gating, SNR and CNR were significantly higher for standard DE MR imaging than for both subsecond DE MR imaging techniques (Table 3). However, during free breathing and arrhythmia, both SNR and CNR were generally lower for standard DE MR imaging (Table 3).


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  		Table 3. SNRs and CNRs for Breath Holding and Steady Gating and for Free Breathing and Arrhythmia with DE MR Imaging Techniques

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
ADVANCES IN KNOWLEDGE
 IMPLICATION FOR PATIENT CARE
 References
 
The main findings of our study are as follows: (a) diagnostic accuracy, image quality, and confidence level are strongly affected by the choice of MR imaging pulse sequence, the regularity of the heart rate, and the use of breath holding; (b) in the setting of a steady heart rate and an ability to hold breath, segmented inversion-recovery pulse sequences are superior to subsecond (single-shot) DE MR imaging techniques; and (c) in the setting of incorrect cardiac gating and/or an inability to hold breath, subsecond DE MR imaging techniques are preferred.

Subsecond DE MR imaging enables the acquisition of images of the entire heart in a few hundred milliseconds rather than in a segmented manner during several heartbeats, as with standard DE MR imaging. The fast acquisition time is most likely the reason subsecond DE MR imaging is insensitive to uncorrected respiratory and cardiac motion (group B). However, the rapid image acquisition limits the total number of k-space lines. In our study, the spatial resolution of subsecond DE MR imaging was lower (2.3 times) than that of standard DE MR imaging, which resulted in increased partial volume effects and poorer image quality. This may explain why the diagnostic accuracy of subsecond DE MR imaging in group A was lower than that of the standard technique (86% vs 96%).

The finding that diagnostic accuracy, image quality, and observer confidence for both subsecond DE MR imaging techniques remained unchanged regardless of whether breath holding and steady cardiac gating were present indicates that spatial resolution was unaffected by uncorrected respiratory and cardiac motion. Our finding that both subsecond DE MR imaging techniques (subsecond steady-state free precession and subsecond GRE) performed better in group B than standard DE MR imaging emphasizes that the insensitivity to uncorrected respiratory and cardiac motion is a consequence of the fast image acquisition time rather than being related to the data readout technique (steady-state free precession vs GRE).

One potential way to overcome the limitation of standard DE MR imaging in patients who are unable to hold their breath (eg, acutely ill patients or patients with advanced heart failure and severely reduced cardiac function) might be to decrease the breath-hold duration—for example, to only three heartbeats instead of approximately 10. This compromise between improved temporal and spatial resolution, however, would still require repetitive breath holds, and the image quality still might not be as high as that with subsecond DE MR imaging. Other approaches, such as navigator-echo–gated three-dimensional gradient-echo sequences, have been used for infarct imaging during free breathing (13). Despite encouraging results, however, the approaches nevertheless require more time for setup and image acquisition than subsecond DE MR imaging. A technique that is fast and easy to use might be favorable, especially in sick patients where cooperation and imaging time are limited. Kellman et al (14) proposed a motion-corrected free-breathing single-shot phase-sensitive inversion-recovery steady-state free precession technique with parallel imaging that showed excellent agreement of measured CNR and infarct size compared with those at standard DE MR imaging. This technique is comparable to our subsecond steady-state free precession technique and thus may also prove robust against uncorrected cardiac motion.

Although the majority of patients have a regular heart rhythm, it is not uncommon for patients with cardiovascular disease who are referred for cardiovascular MR imaging examination to have an irregular heart rhythm, most commonly frequent ectopic beats or atrial fibrillation. Atrial fibrillation, for example, occurs in 1% of the overall population, and its prevalence increases with age. For patients 65 years and older with cardiovascular disease, atrial fibrillation is present in 10%–15% (15–17). In the United States, atrial fibrillation is the reason for hospital admission for almost 500 000 people per year (18).

Therapeutic strategies have changed dramatically in the past decade, and invasive procedures such as catheter ablation for various kinds of arrhythmia are now being widely used for selected patient groups (19,20). Cardiovascular MR imaging has been proved to be useful for preablation and follow-up studies in these patients (21–23). Thus, in addition to more acutely ill patients, more patients with arrhythmia may be referred for a comprehensive MR imaging examination, which further increases the need for practical approaches to imaging in the setting of uncorrected motion.

The model of cardiac arrhythmia used in our study was specifically designed to simulate arrhythmia in patients. Arrhythmia in a clinical setting is often complex, however, and it was impractical to attempt to account for every possible scenario. Even if it were argued that our arrhythmia model more closely represented poor cardiac gating than arrhythmia per se, both conditions would nevertheless be expected to result in similar problems related to poor synchronization of the acquisition. While it seems likely that different forms of arrhythmia would affect image quality differently, it also seems likely that the overall conclusion that subsecond DE MR imaging techniques perform better in the presence of uncorrected motion would remain unchanged.

Practical applications: The choice of DE MR imaging technique may be based on several factors in addition to the ability to hold breath and the presence of arrhythmia, including the acuity of illness, logistic issues at the MR imaging center, the technical knowledge level of the MR imaging operators, and the clinical indication for the study. Nevertheless, the results of our study suggest that a practical approach to routine cardiac MR imaging would be to image all patients with a regular heart rate and the ability to hold breath by using traditional segmented inversion-recovery pulse sequence and to image all others with subsecond DE MR imaging techniques.


   ADVANCES IN KNOWLEDGE
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
IMPLICATION FOR PATIENT CARE
 References
 


   IMPLICATION FOR PATIENT CARE
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 IMPLICATION FOR PATIENT CARE
References
 


   FOOTNOTES
 

Abbreviations: CNR = contrast-to-noise ratio • DE = delayed enhancement • GRE = gradient-recalled echo • SNR = signal-to-noise ratio

See Materials and Methods for pertinent disclosures.

Author contributions: Guarantors of integrity of entire study, all authors; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; manuscript final version approval, all authors; literature research, B.S., W.G.R., T.S.E.A., M.R.P., R.J.K., R.M.J.; experimental studies, all authors; statistical analysis, B.S., W.G.R., M.A.P., R.J.K., R.M.J.; and manuscript editing, all authors


   References
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 IMPLICATION FOR PATIENT CARE
 References
 

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