T2-weighted MR imaging of the chest: Comparison of electrocardiograph-triggered conventional and turbo spin-echo and nontriggered turbo spin-echo sequences

In 22 patients with a diverse range of thoracic abnormalities, T2-weighted magnetic resonance (MR) images of the chest were obtained with electrocardiograph (ECG)-triggered turbo spin-echo (TSE), ECG-triggered conventional spin-echo (CSE), and nontriggered TSE sequences, and the images were compared. A 5-point rating scale was used by three radiologists experienced in MR imaging of the chest to Independently evaluate the images for (a) freedom from ghosting, (b) clarity of heart wall and cardiac chambers, (c) clarity of mediastinal structures, (d) conspicuity of abnormalities, and (e) overall image quality. Evaluations were analyzed with statistical methods. For freedom from ghosting, clarity of heart wall and cardiac chambers, clarity of mediastinal structures, and overall image quality, the ECG-triggered TSE images were rated higher than the TSE images, which. In turn, were rated higher than the ECG-triggered CSE images at the P=.05 level of significance. No significant differences were seen between the pulse sequences in the conspicuity of abnormalities, although some differences were observed in individual cases. Our results suggest that ECG-triggered TSE imaging provides improved, time-efficient T2-weighted images of the chest.     

Automated identification of left ventricular borders from spin-echo magnetic resonance images. Experimental and clinical feasibility studies

Gated cardiac magnetic resonance imaging (MRI) permits detailed evaluation of cardiac anatomy, including the calculation of left ventricular volume and mass. Current methods of deriving this information, however, require manual tracing of boundaries in several images; such manual methods are tedious, time consuming, and subjective. The purpose of this study is to apply a new computerized method to automatically identify endocardial and epicardial borders in MRIs. The authors obtained serial, short-axis, spin-echo MRIs of 13 excised animal hearts. Also obtained were selected short-axis, spin-echo ventricular images of 11 normal human volunteers. A method of automated edge detection based on graph-searching principles was applied to the ex vivo and in vivo images. Endocardial and epicardial areas were used to compute left ventricular mass and were compared with the anatomic left ventricular mass for the images of excised hearts. The endocardial and epicardial areas calculated from computer-derived borders were compared with areas from observer tracing. There was very close correspondence between computer-derived and observer tracings for excised hearts (r = 0.97 for endocardium, r = 0.99 for epicardium) and in vivo scans (r = 0.92 for endocardium, r = 0.90 for epicardium). There also was a close correspondence between computer-generated and actual left ventricular mass in the excised hearts (r = 0.99). These data suggest the feasibility of automated edge detection in MRIs. Although further validation is needed, this method may prove useful in clinical MRI.     

Multicenter trial of automated border detection in cardiac MR imaging

The purpose of the present study was to evaluate the robustness of a method of automated border detection in cardiac magnetic resonance (MR) imaging. Thirty-seven short-axis spin-echo cardiac images were acquired from three medical centers, each with its own image-acquisition protocol. Endo- and epicardial borders and areas were derived from these images with a graph-searching-based method of edge detection. Computer results were compared with observer-traced borders. The method accurately defined myocardial borders in 36 of 37 images (97%), with excellent agreement between computer- and observer-derived endocardial and epicardial areas (correlation coefficients,.94-.99). The algorithm worked equally well for data from all three centers, despite differences in image-acquisition protocols, MR systems, and field strengths. These data suggest that a method of computer-assisted edge detection based on graphsearching principles yields endocardial and epicardial areas that correlate well with those derived by an independent observer.     

Single-shot versus interleaved echo-planar MR -imaging: Application to visualization of cardiac valve leaflets

Visualization of the cardiac valves with standard magnetic resonance (MR) imaging is not adequate because of long acquisition times. Echo-planar imaging (PI) can, however, be performed with a temporal resolution (30–50 msec) comparable to that of echocardiography. The authors evaluated the feasibility of real-time imaging of cardiac valve motion with ultrafast MR techniques. Eight healthy volunteers and three patients with mitral stenosis and re- gurgitation were studied with a 1.5-T whole-body im- ager. Two different EPI sequences were assessed: a standard single-shot gradient-echo EPI (GEPI) SCquence and a fast imaging technique based on multiple-shot EPI with interleaved k-space acquisition (IGEPI). Fat-suppressed images with an in-plane resolution of 3.7 × 3.7 mm were obtained equally spaced through the cardiac cycle. Half-k-space acquisition was used. Morphologic evaluation was superior with IGEPI, owing to the better intracavitary signal homogeneity (P ≤ 0.01). and the mitral valve leaflets were easier to identify on systolic images. IGEPI provided adequate valve visibility in all three patients.     

Cardiac through-plane motion: comparison of long- and short-axis MR images of the left ventricle

Left ventricular (LV) long-axis shortening produces cardiac motion through fixed short-axis sections, complicating accurate quantification of myocardial and wall-motion parameters with MRI. Therefore, LV long-axis length and shortening was studied in both long- and short-axis end-diastolic (ED) and end-systolic (ES) MRI scans. A group of 38 male volunteers underwent gradient-echo cine-MRI: single-slice long-axis and multi-slice short-axis. The LV dimensions were directly measured in the long-axis images using epicardial and endocardial contours. The position of the apex and the base (aortic valve) were assessed in the sections of the short-axis scan. The LV dimensions as measured on both long- and short-axis scans showed a correlation of r = 0.62 for ED and r = 0.69 for ES. Relative LV shortening between long- and short-axis scans showed a poor correlation (r = 0.22). These results suggest that short-axis MRI is not sufficient to accurately assess through-plane motion. Therefore, long-axis images are needed for optimal quantification of myocardial and wall-motion parameters. Correspondence to: N. Matheijssen     

Fast spin-echo MR imaging of the abdomen: Contrast optimization and artifact reduction

The effects of various fast spin-echo (FSE) magnetic resonance (MR) imaging parameters and artifact reduction techniques on FSE image contrast and quality were studied. The authors performed 139 abdominal MR examinations, comparing standard FSE images (echo train length [ETL] = 8, echo space [E-space] = 17 msec, bandwidth = ±16-kHz) with FSE images with an ETL of 16 (n = 22) or FSE images with a ±32-kHz bandwidth and an E-space of 11-14 msec (n = 22). FSE artifact reduction techniques were evaluated with spectral fat saturation (n = 40) or with a new flow compensation FSE sequence (n = 55). Images of liver lesions were reviewed qualitatively and with contrast-to-noise ratio (C/N) measurements. Decreasing the time of echo train sampling produced superior image quality, with increased anatomic sharpness, less image artifact, and improved liver-lesion C/N. Images obtained with an ETL of 16 showed more image blurring and a 23% decrease in relative contrast and 28% decrease in relative C/N for liver tumors. Increasing the bandwidth reduced E-space, producing a 12% decrease in background noise. Artifact reduction with fat saturation or flow compensation produced images with less ghosting artifact and superior overall image quality, with 39% and 20% increases in liver-tumor C/N, respectively. FSE image quality and contrast in the depiction of hepatic disease can be optimized with careful selection of imaging parameters and the use of artifact reduction techniques.     

Cardiac masses: signal intensity features on spin-echo, gradient-echo, gadolinium-enhanced spin-echo, and TurboFLASH images.

Fifteen patients with cardiac or paracardiac masses underwent magnetic resonance (MR) imaging with spin-echo (n = 15), cine gradient-echo (n = 15), gadopentetate dimeglumine-enhanced spin-echo (n = 15), and TurboFLASH (fast low-angle shot) (n = 7) sequences. All masses had either histologic confirmation (n = 11) or confirmation with other imaging modalities (n = 4). Myxomas (n = 6) were characterized by an intermediate signal intensity (SI) on spin-echo (n = 6) and low SI on cine gradient-echo (n = 6) images and moderately high-SI contrast enhancement (n = 5). Percent contrast enhancement for myxomas was 57% +/- 11%. Thrombus (n = 4) had intermediate (n = 3) or high (n = 1) SI on spin-echo images and low (n = 2) or intermediate (n = 2) SI on gradient-echo images and did not show substantial contrast enhancement. Postcontrast first-pass TurboFLASH images were useful by clearly demonstrating the nonenhancing mass lesion in a high-SI blood pool. Other cardiac and paracardiac tumors (n = 5) showed variable pre- and postcontrast spin-echo SI; however, no other tumor showed low SI on cine gradient-echo images.     

Arrhythmogenic right ventricular dysplasia: ex vivo and in vivo fat detection with black-blood MR imaging

PURPOSE: To assess electrocardiographically gated spin-echo (SE) and double inversion-recovery fast SE magnetic resonance (MR) imaging in the depiction of intramyocardial fat in cadaveric heart specimens and patients with arrhythmogenic right ventricular dysplasia (ARVD). MATERIALS AND METHODS: A phantom was used to determine the effective in-plane spatial resolution of SE and fast SE MR imaging protocols. Two cadavers with proved ARVD were imaged with identical sequences with spectrally selected fat suppression. Contrast-to-noise ratios (CNRs) of intramyocardial fat in the right ventricle (RV) were compared by using analysis of variance and Student t test with Bonferroni correction. Eleven patients with ARVD and 10 control subjects underwent fast SE MR imaging. Two blinded readers semiquantitatively evaluated images for fat conspicuity and image quality. RESULTS: Fast SE MR imaging achieved better spatial resolution but lower CNR than that of gated SE imaging. CNRs in cadaveric specimens were higher for double R-R than for single R-R fast SE sequences for all section thicknesses (P <.0001). Absolute CNR values were higher for fat-suppressed fast SE sequences than for those without fat suppression. Cadaveric specimens demonstrated fatty infiltration from epicardium toward endocardium of the RV free wall. Intramyocardial fat was detected in eight of 11 (73%) patients with ARVD and in no control subjects (P <.001). CONCLUSION: Intramyocardial fat detection in ARVD was better with fast SE MR imaging alone and combined with fat suppression than was gated SE MR imaging. When fast SE imaging is applied in vivo, however, breath-holding constraints limit the spatial resolution for RV fat detection.     

Automated pixel-by-pixel mapping of dynamic contrast-enhanced MR images for evaluation of osteosarcoma response to chemotherapy: Preliminary results

An automated technique for pixel-by-pixel computer mapping of tumor necrosis was developed to improve the accuracy and applicability of dynamic contrast agent–enhanced magnetic resonance (MR) imaging in assessing the response of osteosarcoma to preoperative chemotherapy. The technique was evaluated in nine osteosarcoma patients who underwent MR imaging at diagnosis (n = 7) and after preoperative chemotherapy (n = 9). Sequential FLASH (fast low-angle shot) images of the tumor were obtained in one plane every 15 seconds before and for 3 minutes after gadopentetate dimeglumine injection. A region of interest was selected that included the entire tumor area. Slopes representing percent increase in signal intensity over baseline values per minute were calculated automatically for each pixel and displayed as grayscale maps of the tumor. Matching histologic maps of each tumor were obtained. Visual region-by-region comparison of the pixel and histologic maps showed a high degree of correlation and the ability of the MR imaging technique to help identify small foci of residual viable tumor.     

Experimental validation of an automated edge-detection method for a simultaneous determination of the endocardial and epicardial borders in short-axis cardiac MR images: Application in normal volunteers

The goal of this study was to put together several techniques of image segmentation to provide a reliable assessment of the left ventricular mass with short-axis cardiac MR images. No initial manual input was required for this process based on region growing, gradient detection, and adaptive thresholding. A comparison between actual mass and automatic assessment was implemented with 9 minipigs that underwent spin-echo MR imaging. Fifteen normal volunteers were studied with a fast-gradient-echo sequence. The automatic segmentation was then controlled by three trained observers. Actual mass and automatic segmentation were strongly correlated (r = .97 with P < .01). For normal volunteers, the standard error of estimation of the automatic assessment (12 g) compared well with the average myocardial mass (120 ± 30 g) and the interobserver reproducibility of the manual assessment (9 g). These results allow the application of this method to the quantification of the left ventricular function and mass in clinical practice.     

Effects of physiologic waveform variability in triggered MR imaging: Theoretical analysis

One of the assumptions inherent in most forms of triggered magnetic resonance (MR) imaging is that the pulsatile waveform (be it cardiac, respiratory, or some other) is purely periodic. In reality, the periodicity condition is rarely met. Physiologic waveform variability may lead to image artifacts and errors in velocity or volume flow rate estimates. The authors analyze the effects of physiologic waveform variability in triggered MR imaging. They propose that this variability be treated as a modulation of the underlying motion waveform. This report concentrates on amplitude modulation of the velocity waveform, which results in amplitude and phase modulation of the transverse magnetization. Established Fourier and modulation theory and the recently described principles of (k, t)-space were used to derive the appearance of physiologic waveform variability artifacts in triggered MR images and to predict errors in time-averaged and instantaneous velocity estimates that may result from such motion effects, including effects such as ghost overlap. Simulations and experimental results are provided to confirm the theory.     

Fast,interactive algorithm for segmentation of a series of related images: Application to volumetric analysis of MR images of the heart

Magnetic resonance (MR) cine images of the beating heart have excellent spatial and temporal resolution. Extracting the boundaries of the heart from MR images for volumetric measurements is of considerable interest; however, since the number of images involved is large, tracing the boundaries by hand is tedious and prohibitively time consuming. The authors have developed an interactive method of boundary detection that uses the correlation between the cardiac boundaries on temporally or spatially adjacent images to increase the speed of the process and reproducibility of the measurement. A simulated cine MR study of a phantom (total of 155 images) and cardiac cine studies of two patients (192 images each) were analyzed by two independent observers. Analysis of the phantom data was completed in 5.6 minutes (2.16 seconds per image) by observer 1 and 6.3 minutes (2.4 seconds per image) by observer 2. The percent measurement errors for 31 phantom volumes (30-120 mL) were 0.96% and 0.83% for observers 1 and 2, respectively. The observers analyzed the patient studies in 14-23 minutes (4.4-7.2 seconds per image), with interobserver variabilities of 5.8% and 3.7% for the two patients, respectively. The authors conclude that their flexible, semiautomatic, interactive algorithm allows rapid and reproducible detection of structural boundaries.     

Improved reproducibility in measuring LV volumes and mass using multicoil breath-hold cine MR imaging

There is a generally recognized need for improvement in quality of fast cardiac MR images. Consequently, breath-hold cine MR images were obtained with multiple surface coils connected to phased array receivers, and C/N, intra-observer and inter-observer variabilities for LV volumes and mass were evaluated. Two sets of short-axis images of the LV, one with multiple surface coils and another with a body coil, were acquired in eight subjects with a fast cine MR sequence using k-space segmentation (TR/TE=7/2.2 msec, temporal resolution=56 msec). C/N with multicoil imaging was 32.2 ± 7.6 (mean ± SD), significantly higher than that with a body coil (11.0 ± 3.3, P < .01). The mean percentage differences in intra-observer and inter-observer measurements with multicoil imaging were significantly better than those with a body coil. In conclusion, multicoil imaging provides significant gain in C/N on breath-hold cine MRI of the heart. In addition, intra-observer and inter-observer reproducibilities are improved with multicoil imaging.     

MR imaging with remote control: feasibility study in cardiovascular disease

The institutional review board approved this HIPAA-compliant study and waived informed consent. The purpose was to retrospectively evaluate remote control magnetic resonance (MR) imaging in complex cardiovascular procedures, whereby operational expertise was made available locally from a remote location. Thirty patients underwent cardiac (12 patients) and/or vascular (30 patients) 1.5-T MR imaging with a remote operator by using a personal computer. All patient studies were compared with 30 control studies obtained with conventional local imaging. Cardiac cine, myocardial delayed enhancement, and MR angiograms were assessed for overall image quality and motion artifact. MR angiograms were evaluated for vascular definition. Image quality was excellent in 90% (38 of 42) of remote images versus 60% (25 of 42) of control group images (P < .01). Scores for motion artifact were not significantly different (P = .11). Interactive MR imaging was successfully implemented with remote control in complex cardiovascular cases; diagnostic quality of images was superior to that of images obtained locally.     

Segmentation of small structures in MR images: Semiautomated tissue hydration measurement

Segmentation of small anatomic structures In noisy magnetic resonance (MR) images is inherently challenging because the edge Information is contained in the same high-frequency image component as the noise. The authors overcame this obstacle in the analysis of the sural nerve in the ankle by processing images to reduce noise and extracting edges with an edge detection algorithm less sensitive to noise. Anatomic accuracy of the segmentation was confirmed by a neuroradiologist. A nerve hydration coefficient was determined from the signal intensity of the nerve in these segmented images. These semiautomated measurements of hydration agreed closely with those obtained with a previously described manual method (n=44, P=.76). Each image in the study was analyzed identically, with no modification of the computer algorithm parameters. The data suggest that this robust method may be useful in a multicenter evaluation of diabetes treatment protocols.     

Magnetic resonance imaging of athlete's heart: myocardial mass, left ventricular function, and cross-sectional area of the coronary arteries

To evaluate left ventricular myocardial mass and function as well as ostial coronary artery cross-sectional area in endurance athletes, an athlete group of 12 highly trained rowers and a control group of 12 sedentary healthy subjects underwent MR examination. An ECG-gated breath-hold cine gradient-echo sequence was used to calculate myocardial mass, end-diastolic and end-systolic volumes, stroke volume, and cardiac output, all related to body surface area, as well as ejection fraction. A 3D fat-saturated ECG- and respiratory-triggered navigator echo sequence was used to evaluate coronary arteries: left main (LM), left anterior descending (LAD), left circumflex (LCx), and right coronary artery (RCA). Cross-sectional area was calculated and divided for body surface area. Myocardial mass was found significantly larger in athlete group than in control group (p = 0.0078), the same being for end-diastolic volume (p = 0.0078), stroke volume (p = 0.0055), LM (p = 0.0066) and LAD (p = 0.0129). No significant difference was found for all the remaining parameters. Significant correlation with myocardial mass was found for LM (p < 0.001) and LAD (p = 0.0340), not for LCx and RCA. Magnetic resonance imaging is a useful tool in evaluating the myocardial hypertrophy and function of athlete's heart. Magnetic resonance angiography is a valuable noninvasive method to visualize the correlated cross-sectional area increase of the left coronary artery system. Received: 25 March 1999; Revised: 31 August 1999; Accepted: 1 September 1999     

The Significance of Perfusion Defect at Myocardial Perfusion MR Imaging in a Cat Model of Acute Reperfused Myocardial Infarction

Objective

To determine whether the size of a perfusion defect seen at myocardial perfusion MR imaging represents the extent of irreversibly damaged myocardium in acute reperfused myocardial infarction.

Materials and Methods

In nine cats, reperfused myocardial infarction was induced by occlusion of the left anterior descending coronary artery for 90 minutes and subsequent reperfusion for 90 minutes. At single-slice myocardial perfusion MR imaging at the midventricular level using a turbo-FLASH sequence, 60 short-axis images were sequentially obtained with every heart beat after bolus injection of gadomer-17. The size of the perfusion defect was measured and compared with both the corresponding unstained area seen at triphenyl tetrazolium chloride (TTC) staining and the hyperenhanced area seen at gadophrin-2-enhanced MR imaging performed in the same cat six hours after myocardial perfusion MR imaging.

Results

The sizes of perfusion defects seen at gadomer-17-enhanced perfusion MR imaging, unstained areas at TTC staining, and hyperenhanced areas at gadophrin-2-enhanced MR imaging were 20.4 ± 4.3%, 29.0 ± 9.7%, and 30.7 ± 10.6% of the left ventricular myocardium, respectively. The perfusion defects seen at myocardial perfusion MR imaging were significantly smaller than the unstained areas at TTC staining and hyperenhanced areas at gadophrin-2-enhanced MR imaging (p < .01). The sizes of both the perfusion defect at myocardial perfusion MR imaging and the hyperenhanced area at gadophrin-2-enhanced MR imaging correlated well with the sizes of unstained areas at TTC staining (r = .64, p = .062 and r = .70, p = .035, respectively).

Conclusion

In this cat model, the perfusion defect revealed by myocardial perfusion MR imaging underestimated the true size of acute reperfused myocardial infarction. The defect may represent a more severely damaged area of infarction and probably has prognostic significance.     

Retrospective registration of X-ray angiograms with MR images by using vessels as intrinsic landmarks

Conventional x-ray angiography (XRA) images are projections of the vasculature with high spatial and temporal resolution, while magnetic resonance (MR) angiography (MRA) and MR imaging data show the three-dimensional locations of vessels relative to brain parenchyma. The authors have developed a retrospective method of registering these studies, which makes it practical to produce multimodality displays of this complementary information. Registration was performed by matching vessels seen on both XRA and MRA images. First, the authors determined the coordinates of the center lines of a few “landmark” vessels on the XRA image and the three-dimensional locations of the corresponding intraluminal voxels in the MRA volume. Registration was performed by rotating and translating the MRA–MR imaging volume until the perspective projection of the MRA landmark vessels matched the corresponding vessel center lines on the XRA image. Experiments with phantoms and patients indicated that the two studies were registered with an average error of less than 2 mm. A linked-cursor display was developed to show correspondence between points on the registered XRA and MRA-MR images.     

常规SE序列和动态增强MRI诊断肝局灶性病变的比较

目的比较常规SE序列与动态Gd-U．----x增强扫描对肝局灶病变的诊断价值。方法对34例肝局灶病变做了常规SE平扫和动态Gd-U．---A增强及延迟万WI增强扫描；就各序列对肝局灶病变的检出率、病变的信噪比（C／N）值和图像质展进行定员和定性分析。结果36例共142个病灶，动态Gd-lyl？l？A检出率（138／142，958％）明st高于IFZWI和延迟TW［增强（128／14，叨％；119／142，838％）（P＜005）：动态Gd-IJ：1713A增强的C／N值高于TZWI和TIWI延迟增强（P＜0．05）；动态增强的伪影较L们少（P＜0．01），而病灶清晰度各序列之间无明显差别（P＞0．历）。结论动态u-ly：1717A增强扫描在病灶检出率、图像质显反C／N值方面均优于IWI，对肝局灶病变的诊断是一种有价值的方法，     

Cardiac MRI of the normal and hypertrophied mouse heart

With the development of recent transgenic techniques, studies involving mice offer opportunities to increase understanding of cardiac disease. This provides motivation for the current study to perform noninvasive evaluation of the normal and hypertrophied mouse heart with MRI. By acquiring EGG and respiratory signals, the MR image acquisition was gated to both the cardiac and respiratory cycles. Combining a spin-warp imaging sequence with an RF surface coil resulted in short-axis images that allowed quantification of in vivo cardiac mass. Excellent agreement between MRI-determined (y) and postmortem heart weight (x) was obtained: y = 0.991x + 1.43 (r = 0.996). Isoproterenol, at 282 μMmol/kg body weight (BW) and 573 μMmol/kg BW, induced a dose-dependent increase in the ratio of heart weight to BW of 16.8 ± 1.09% and 24.1 ± 1.71 %, respectively, which was accurately measured by MRI. These results demonstrate the ability of MRI to non-invasively monitor cardiac anatomy in the mouse.