Optimized single-slab three-dimensional spin-echo MR imaging of the brain

The development and optimization of spin-echo-based, single-slab, three-dimensional techniques for magnetic resonance imaging of the whole brain are described. T1-weighted and T2-weighted image sets with a volume resolution of 1 mm(3) and fluid-attenuated inversion-recovery image sets with a volume resolution of 3 mm(3) were obtained in acquisition times of less than 10 minutes per image set.     

MR plaque imaging of the carotid artery

Atherosclerotic carotid plaque represents a major cause of cerebral ischemia. The detection of vulnerable plaque is important for preventing future cardiovascular events. The key factors in advanced plaque that are most likely to lead to patient complications are the condition of the fibrous cap, the size of the necrotic core and hemorrhage, and the extent of inflammatory activity within the plaque. Magnetic resonance (MR) imaging has excellent soft tissue contrast and can allow for a more accurate and objective estimation of carotid wall morphology and plaque composition. Recent advances in MR imaging techniques have permitted serial monitoring of atherosclerotic disease evolution and the identification of intraplaque risk factors for accelerated progression. The purpose of this review article is to review the current state of techniques of carotid wall MR imaging and the characterization of plaque components and surface morphology with MR imaging, and to describe the clinical practice of carotid wall MR imaging for the determination of treatment plan.     

Assessment of patency capsule retention using MR diffusion-weighted imaging

Objectives

Evaluate the ability of MR diffusion-weighted imaging (DWI) to predict patency capsule retention in Crohn’s disease (CD).

Methods

Clinical and imaging data were prospectively reviewed for 80 CD patients following patency capsule administration and MR-DWI under institutional review board (IRB) approval with informed consent. Two radiologists separately assessed the presence/absence of restricted diffusion in the distal ileum. Apparent diffusion coefficients (ADC) from three regions of interest on the ileal wall were averaged. The association between restricted diffusion and retention, and sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) were calculated. Ability of ADC to predict retention was assessed with receiver operating characteristic (ROC) curve analysis.

Results

Restricted diffusion in the distal ileum was associated with capsule retention (p?=?0.001, p?<?0.0001). Sensitivity, specificity, PPV and NPV of restricted diffusion for capsule retention were 100.0%, 46.2%, 30.0%, 100% and 100.0%, 56.9%, 34.9%, 100%, respectively, for two radiologists. Accuracy of ADC to predict retention was high (area under the curve?=?0.851, p?<?0.0001). An ADC of 1.47 mm²/s showed 90.0% sensitivity and 50.0% specificity for retention.

Conclusions

Sensitivity and NPV of restricted diffusion for patency capsule retention were 100%, suggesting that DWI may predict gastrointestinal tract capability to pass video camera endoscopy.

Key Points

? Capsule endoscopy enables assessment of the gastrointestinal mucosa in Crohn’s disease ? Prior patency capsule administration is recommended to evaluate gastrointestinal tract patency ? MR diffusion-weighted imaging may detect pathological constriction of the ileum ? Restricted diffusion in the distal ileum was associated with capsule retention ? MR-DWI may predict gastrointestinal tract capability to pass capsule endoscopy

     

Comparison of spin-echo MR pulse sequences for imaging of the brain.

PURPOSETo determine the value of the gradient- and spin-echo (GRASE) technique as compared with the fast spin-echo and conventional spin-echo techniques in MR imaging of the brain.METHODSSixty-six patients with ischemic and neoplastic brain lesions were examined with T2-weighted spin-echo, fast spin-echo, and GRASE sequences. Three independent observers evaluated the contrast characteristics of anatomic and pathologic structures and of artifacts. Quantitative image analysis included region-of-interest measurements of anatomic structures and lesions.RESULTSThe contrast of anatomic structures was superior in images obtained with conventional and fast spin-echo techniques as compared with those obtained with the GRASE technique. Extended lesions, such as tumors and territorial infarcts, were identified equally with all techniques. For delineation of small ischemic lesions, GRASE was slightly inferior to fast and conventional spin-echo sequences. Flow artifacts were considerably reduced with fast spin-echo and GRASE sequences. Chemical-shift artifacts were significantly reduced, but ringing artifacts were more pronounced with GRASE.CONCLUSIONFast spin-echo remains the standard technique in MR imaging of the brain. However, GRASE might be useful in special cases, such as with uncooperative patients whose conventional or fast spin-echo images show severe motion artifacts.     

Coupled-spin fast spin-echo MR imaging

Distinguishing between lipid and water-containing tissues is clinically important. Current techniques rely on the chemical shift difference between fat and water resonances or differences in relaxation times of the tissues, or a combination of both. A method is presented for separating the signals of lipid protons from those of water protons by using fast spin-echo magnetic resonance imaging based on the principle that lipid protons behave differently from water protons in mul-tiecho sequences. Two images are acquired with different echo train lengths and echo spacing but with identical TEs, and then subtracted to exploit differences in the behavior of lipid and water protons in mul-tiecho sequences. The method is insensitive to B₀ inhomoge-neities or susceptibility effects and provides separate lipid and water images with a high signal-to-noise ratio. The advantages of the method are demonstrated with phantom studies and clinical examples.     

Rapid MR imaging of the pediatric brain using the fast spin-echo technique.

PURPOSE: To evaluate diagnostic reliability and to establish optimal scanning techniques of a recently developed Fast Spin-echo MR pulse sequence that allows rapid proton density-weighted and T2-weighted imaging. METHODS: We compared lesion conspicuity and signal intensity measurements on Fast Spin-echo and conventional spin-echo sequences in 81 patients ranging from 1 week to 25 years in age on a 1.5-T MR unit. A total of 28 Fast Spin-echo dual-echo images (14 slice locations) were obtained in 2:08 minutes with a 256 x 128 matrix or in 3:12 minutes with a 256 x 192 matrix at a TR of 2000 msec and two excitations. RESULTS: Lesion conspicuity and characterization on Fast Spin-echo images compared favorably with conventional spin-echo images in our series when pseudo-TEs of 15 and 90 msec were employed for proton density-weighted and T2-weighted images, respectively. Fast Spin-echo images yielded diagnostic information in four nonsedated patients whose conventional spin-echo images were either degraded by motion or unobtainable. Fat signal remained bright on T2-weighted Fast Spin-echo images. Magnetic-susceptibility effects were slightly reduced with Fast Spin-echo but did not pose any diagnostic problem in our series. CONCLUSION: Diagnostically reliable rapid dual-echo brain images can be obtained with Fast Spin-echo sequences.     

Fast spin-echo MR imaging of the eye

Magnetic resonance imaging of the eye usually includes T2-weighted images both for screening purposes and for characterization of melanoma. Conventional T2-weighted spin-echo (SE) imaging suffers both from long acquisition times and incomplete recovery of the vitreous' signal. A fast SE sequence was therefore compared prospectively with conventional sequences in 29 consecutive patients with lesions of the eye. Fast SE images delineated melanoma and other lesions of the eye from vitreous better than conventional T2-weighted images. Image quality and lesion conspicuity were improved on the fast sequence. Whereas melanoma appeared hypointense to vitreous on both types of images, subretinal effusion was hypointense on fast images and hyperintense on conventional T2-weighted images. Ghosting of the globe, which, however, did not decrease diagnostic value, was more pronounced on fast images. Conventional T2-weighted images may be replaced by fast SE images in MR studies of the eye with a gain in lesion conspicuity and significant time saving.Correspondence to: N. HostenThis work was supported by grant 70-01847-Ho 1, Deutsche Krebshilfe.     

A study of coronary artery bypass graft patency using MR imaging

This is a prospective study comparing selective angiography and magnetic resonance (MR) imaging in the evaluation of the patency of coronary artery bypass grafts (CABGs). Twenty-eight patients with 52 grafts were studied (10 internal mammary artery grafts, 19 saphenous vein grafts including 2 sequential grafts, and 15 right coronary artery saphenous vein grafts). The mean interval between MR imaging and the surgical procedure was 13.2 months. Results obtained with angiography were as follows: 39 patent grafts; 4 patent but stenotic (greater than 50%); 9 occluded CABGs. Magnetic resonance imaging was performed with axial imaging, spin echo images, and electrocardiographic gating. Grafts that were normal on angiographic examination appeared without a signal on both the first and the second echoes in 38 of 39 cases. Grafts that were patent but presented significant stenosis as evidenced by angiography presented the same appearance. In one case, there was an aspect corresponding to a decreased blood flow (presence of a signal on the first echo that became stronger on the second echo). The CABG occlusion was determined in seven of nine cases. Occluded grafts twice presented a signal variation corresponding to an old thrombus formation. In two cases, only the origin of the graft was visible without any visualization of its distal portion. In three other cases, neither the proximal nor the distal portions of the graft were identified. Among six CABGs that could not be identified, three were patent. This study demonstrated that MR imaging makes it possible to correctly identify patent CABGs, but this modality has significant limitations because patent but stenotic CABGs may present in the same way as patent CABGs without stenosis. On the other hand, the correct identification of occluded grafts may be accurately performed using MR imaging.     

Pulsed magnetization transfer spin-echo MR imaging

Cross relaxation between macromolecular protons and water protons is known to be important in biologic tissue. In magnetic resonance (MR) imaging sequences, selective saturation of the characteristically short T2 macromolecular proton pool can produce contrast called magnetization transfer contrast, based on the cross-relaxation process. Selective saturation can be achieved with continuous wave irradiation several kilohertz off resonance or short, intense 0° pulses on resonance. The authors analyze 0° binomial pulses for T2 selective saturation, present design guidelines, and demonstrate the use of these pulses in spin-echo imaging sequences in healthy volunteers and patients. Using the phenomenologic Bloch equations modified for two-site exchange, the authors derive the analytic expressions for water proton relaxation under periodic pulsed saturation of the macromolecular protons. This relaxation is shown to be monoexpo-nential, with a rate constant dependent on the saturation pulse repetition rate and the individual and cross-relaxation rates.     

Coronary artery bypass graft patency: noninvasive evaluation with MR imaging

A noninvasive means of determining coronary artery bypass graft (CABG) patency in symptomatic patients would be an important clinical asset. The accuracy of magnetic resonance (MR) imaging was evaluated for this purpose. Multiphasic electrocardiographically gated MR imaging examinations were performed in 25 patients with a total of 72 grafts. Transverse images of the heart at ten anatomic levels were obtained at five or six phases of the cardiac cycle. The MR images were read blindly to determine CABG patency versus occlusion, and these results were compared with those of coronary angiography performed within 2 months before the MR imaging. MR imaging correctly disclosed 43 patent grafts and 13 occluded grafts (predictive accuracy, 78%). Patency and occlusion were incorrectly diagnosed from MR imaging findings in five and four CABGs, respectively. CABG status could not be determined in seven (10%) grafts because the grafts were visualized at only one anatomic level. Thus, the accuracy of a definitive MR imaging evaluation was 91% (43 of 47 grafts) for patency determination and 72% (13 of 18 grafts) for occlusion determination. MR imaging appears to be a useful method for the noninvasive evaluation of CABGs.     

Assessment of brain perfusion with MR imaging

A technique for assessing brain perfusion with magnetic resonance (MR) imaging is described. This technique uses two spin echo sequences that are identical except that the second is sensitised to blood flow by use of a pair of unipolar gradients on either side of the 180 degree pulse. Differences in phase between the two sequences are displayed with a sensitivity to flow rates of +/- 0.5, +/- 1, and +/- 2 mm/s per full scale (+/- pi radians) deflection. The technique was validated for measurement of flow at these rates with a water phantom. Ten patients with cerebrovascular disease, multiple sclerosis, cerebral tumor, periventricular leukomalacia, and meningitis were studied. Differences between grey and white matter were normally seen in adults. Infants displayed differences between central and peripheral regions of the brain. Abnormalities were seen in all clinical cases. The technique will require further validation but it appears to provide a totally noninvasive method for assessing brain perfusion.     

MR imaging in carotid artery atherosclerosis plaque characterization.

AIM: To evaluate the potential role of carotid artery atherosclerosis plaque magnetic resonance (MR) microimaging as magnetic resonance imaging (MRI) marker, ex vivo MR images were acquired at optimized parameters on 9.4T Bruker animal imager for occluded tissue resected by carotid endarterectomy (CEA) and corresponding histopathological analysis was made. METHODS AND MATERIALS: For imaging, CEA tissues of size 2-6 cm long and 0.5-1.5 cm wide, were transferred to 15 ml co-polymer laboratory culture tubes containing either 10% formalin in phosphate buffered saline (PBS) or in 50% glycerol in PBS. Imaging protocol was set at TE=30 ms, TR=1.5 s, matrix size=265 x 512, NEX=128, slice thickness=1 mm and in-plane resolution=0.1 mm for total sample size 2.5 cm. Soon after imaging done, carotid artery tissues were cut into 5-mm segments and processed for histological section for successive 5-micrometer slices. To compare morphology of 5 mum thin CEA section with that of 1 mm MR slices, registration was obtained between histologic sections and MR slices. Contrast and magnetic resonance relaxation characteristics were analyzed. RESULTS: Total carotid artery area computed by MR imaging was correlated with areas determined from histologic sections (r(2)=0.989, p=0.0001). For the lumen area, the correlation between MR images and histologic area was (r(2)=0.942, p=0.0001). Relaxation times and T(2) parametric images of different plaque components were determinant for contrast resolution. Scan parameters were optimized for fibrous cap and atheroma. Scan parameters were characteristic for comparison at 1.5T and 9.4T MR imagers. CONCLUSION: The observed correlation validated MR microimaging to assess morphological features of carotid artery plaques and contrast resolution highlighted the potential of in vivo MR imaging as non-invasive MRI marker to monitor carotid artery plaque morphometry and plaque composition.     

MR fluid-attenuated inversion recovery imaging as routine brain T2-weighted imaging

We tried to investigate if magnetic resonance (MR) fluid-attenuated inversion recovery (FLAIR) imaging can be used as a routine brain screening examination instead of spin-echo T2-weighted imaging. Three hundred and ninety-four patients with clinically suspected brain diseases were randomly selected and examined with both brain MR FLAIR and T2-weighted imaging on the axial plane. These two imaging techniques were evaluated by two neuroradiologists as to which imaging was better for routine brain T2-weighted imaging. In 123 of 394 cases (31%), FLAIR imaging was superior to spin-echo T2-weighted imaging. Especially in cases with inflammatory diseases, traumatic diseases and demyelinating diseases, FLAIR imaging was particularly useful. Small lesions bordering cerebrospinal fluid (CSF) are often detected only by FLAIR imaging. In 259 cases (66%), including 147 normal cases (37%), they were equally evaluated. Only in 12 cases (3%) was conventional T2-weighted imaging superior to FLAIR imaging. Cerebrovascular lesions like cerebral aneurysm and Moyamoya disease could not be detected on FLAIR images because these structures were obscured by a low signal from the CSF. Also, because old infarctions tend to appear as low signal intensity on FLAIR images, the condition was sometimes hard to detect. Finally, FLAIR imaging could be used as routine brain T2-weighted imaging instead of conventional spin-echo T2-weighted imaging if these vascular lesions were watched.     

Clinical experience with routine Gd-DTPA administration for MR imaging of the brain

A retrospective analysis of intracranial magnetic resonance (MR scans of 514 patients who underwent nonselective Gd-DTPA enhanced MR imaging was performed to determine the efficacy of this protocol for Gd-DTPA administration. This report reviews the frequency and clinical significance of abnormally enhancing areas that were entirely undetectable on precontrast images or would have been missed without the retrospective knowledge of enhancement. Fifty-seven patients (11% of the 514 patients studied) showed enhancing lesions, with 16 (3.1% of the total) of these patients demonstrating one or more lesions identifiable only on postcontrast images. Of those 16 patients, 8 had other focal abnormalities on precontrast studies, whereas the remaining 8 (1.6% of the total) had negative precontrast studies. The new diagnoses affected clinical management directly in five patients and in another nine contributed potentially significant information. Considerations regarding a selective versus nonselective protocol for Gd-DTPA administration for intracranial MR imaging and the use of clinical information to augment these protocols are discussed.     

Diffusion-tensor MR imaging of the human brain with gradient- and spin-echo readout: technical note

Diffusion-tensor MR imaging of the brain is an objective method that can measure diffusion of water in tissue noninvasively. Five adult volunteers participated in this study that was performed to evaluate the potential of gradient- and spin-echo readout for diffusion-tensor imaging by comparing it with single-shot spin-echo echo-planar imaging. Gradient- and spin-echo readout provides comparable measures of water diffusion to single-shot spin-echo echo-planar readout with significantly less geometrical distortion at the expense of a longer imaging time.