Biomedical subjects
Publications and source records attributed to T L Chenevert.
We describe the appearance of an adrenal pseudocyst on MRI and CT. The MR characteristics of the lesion were noteworthy in that the lesion had two components with different imaging characteristics. The larger component was of low signal intensity on both T1- and T2-weighted images and might have been confused with an adrenal adenoma.
A hybrid fat-suppression sequence in magnetic resonance (MR) imaging was used to evaluate inflammatory muscle disorders in seven children: five patients with dermatomyositis, one patient with vasculitis, and one patient with viral myositis. Fat-suppressed multisection axial images obtained with the same repetition and echo times as those used to obtain standard spin-echo (SE) images enabled direct comparison of images, with little variation of T1 and T2 weighting. In six patients, the contrast on images obtained with T2 fat suppression was 15%-20% greater than contrast on conventional T2-weighted SE images. In all seven patients, the subjective judgment was that T2-weighted fat-suppression sequences improved visualization of muscle abnormalities. It is concluded that T2 fat suppression is useful in evaluation of inflammatory muscle disorders in children because it increases contrast and eliminates fat as a cause of muscle abnormality.
Magnetic resonance imaging techniques designed for sensitivity to microscopic motions of water diffusion and blood flow in the capillary network are also exceptionally sensitive to bulk motion properties of the tissue, which may lead to contrast artifact and large quantitative errors. The magnitude of bulk motion error that exists in human brain perfusion/diffusion imaging and the inability of cardiac gating to adequately control this motion are demonstrated by direct measurement of phase stability of voxels localized in the brain. Two methods are introduced to reduce bulk motion phase error. The first, a postprocessing phase correction algorithm, reduces coarse phase error but is inadequate by itself for quantitative perfusion/diffusion MRI. The second method employs orthogonal slice selection gradients to define a column of tissue in the object, from which echoes may be combined in a phase-insensitive manner to measure more reliably the targeted signal attenuation. Applying this acquisition technique and a simplistic model of perfusion and diffusion signal attenuations yields an estimated perfusion fraction of 3.4 +/- 1.1% and diffusion coefficient of 1.1 +/- 0.2 x 10(-5) cm2/s in the white matter of one normal volunteer. Successful separation of perfusion and diffusion effects by this technique is supported in a dynamic study of calf muscle. Periods of normal blood flow, low flow, and reactive hyperemia are clearly distinguished in the quantitative perfusion results, whereas measured diffusion remained nearly constant.
A method is presented for designing motion-compensated gradients in a progressive manner. The method is easily applicable to many types of waveforms, and can compensate for any order of motion. It can be implemented graphically or numerically. Underlying theory and examples of its application are provided.
The effect of irreproducible bulk tissue motions upon quantification of tissue perfusion and diffusion was studied via computer simulation of random phase error in conventional phase-encoded perfusion/diffusion MRI. Simulations using acquisition parameters typical for human brain studies demonstrate that bulk motion irreproducibility of approximately 60 microns/s can produce phase instability on the order of 20 degrees which overwhelms estimates of perfusion fraction and produces significant errors in diffusion values. Bulk tissue motion control of the human brain via cardiac gating and substantial head restraint was studied by direct measurement of voxel phase stability. Phase instability of 10 degrees to 20 degrees was observed from right-to-left and anterior-to-posterior motions and significantly greater phase variability from superior-to-inferior motion. The spatial pattern of phase variability indicates the source is likely a mixture of cardiac pulsation and respiration.
To determine the clinical feasibility and applicability of phosphorus-31 magnetic resonance (MR) spectroscopy and to assess its potential for characterization of human hepatic tissue, one-dimensional chemical shift imaging (CSI) was performed in 37 patients with various malignant hepatic neoplasms (30 metastases from a variety of primary tumors and seven hepatocellular carcinomas) and seven healthy volunteers. Tumors were grouped according to the percentage of the analyzed section that was occupied by tumor: less than 50% (group A) or more than 50% (group B). In group B, all phosphomonoester/beta-adenosine triphosphate ratios were significantly higher than normal (P less than .001). Hepatocellular carcinomas and metastases from various primary neoplasms could not be differentiated on the basis of spectral characteristics and metabolite ratios. Limitations of one-dimensional surface coil CSI prevented separation of spectra of small tumors and tumors deep within the liver parenchyma from spectra of normal liver parenchyma.
Recent methods of magnetic resonance imaging involve the placement of a grid of planes of saturation over the imaging plane; distortion of the grid corresponds to tissue displacement in two dimensions. An extension to this method that allows measurement of motion in the third dimension involves a second acquisition that tilts the grid, allowing analysis of motion normal to the imaging plane. A rotating phantom was used to verify the accuracy of the motion measurements, and the technique was applied to the heart wall and skeletal muscle. Phantom results show that the measure of z motion can be as accurate as that of x and y motion. Three-dimensional displacements of heart-wall and skeletal muscle are shown. With an accurate measure of three-dimensional motion, more complete analysis of heart-wall motion and contraction is possible.
Many MR imaging systems are limited in their ability to successfully display inversion-recovery images. The reason is that part of the contrast is encoded as phase differences between pixels, whereas in the more commonly used spin-echo pulse sequence all the information is contained in the pixel magnitude. Inversion-recovery images are often displayed in magnitude form, resulting in loss of potentially useful phase information contained in the data. Before this phase information can be used, phase errors which result from scanner imperfections must be removed. While most of the necessary correction can be accomplished using data obtained by scanning a uniform phantom, this approach has several disadvantages. An alternative method by which phase errors can be readily removed without phantom data is described. This method has been applied to images of the head, knee, and liver with good results. It is concluded that this technique is useful for producing phase corrected inversion-recovery MR images.
Sixteen anesthetized New Zealand white rabbits were subjected to thoracotomy, and a reversible snare occluder was attached around a large branch of the left circumflex coronary artery. A 1.3 cm. diameter nuclear magnetic resonance (NMR) surface coil was placed adjacent to the myocardium perfused by this vessel. The animals were divided into two groups of eight animals each, treatment and control. The rabbits were studied using a 2.0 T magnetic resonance (MR) spectrometer, and baseline spectra were acquired. The treatment animals then received intravenous propranolol (1.5 mg/kg) and the control animals received an equal volume of saline. Spectra were then acquired during a 20-minute occlusion period and during subsequent reperfusion. Animals in both groups showed expected decreases in phosphocreatine and adenosine triphosphate and an increase in inorganic phosphate during occlusion; these changes reverted toward baseline values with reperfusion. There were no significant differences between the two groups. The myocardium became acidotic during occlusion in both groups, but significantly more so in the control animals: during the first 10 minutes of occlusion pH was 7.30 +/- 0.41 in the treatment group versus 6.55 +/- 0.24 for controls (p = 0.0005). During the second 10 minutes of occlusion pH was 7.05 +/- 0.65 in the treatment group versus 6.24 +/- 0.25 in controls (p = 0.0053). We conclude that attenuation of intracellular acidosis by propranolol during myocardial ischemia was evident by MR spectroscopy in this animal model.
Documentation of muscle involvement in a child thought to have dermatomyositis may require the performance of invasive procedures such as electromyography and/or muscle biopsy. We describe four patients with dermatomyositis in whom magnetic resonance imaging (MRI) demonstrated the muscle involvement. The involved muscles had increased signal intensity on the T2-weighted images (SE 2500/80) and normal appearance on the T1-weighted images (SE 600/20). The involvement of the muscles was not uniform. There was good correlation between the distribution of muscle involvement by MRI and functional testing. Follow-up MRI scans in patients with favorable outcome demonstrated that the affected muscles had returned to normal signal intensity. Although the MRI findings are not specific, in the proper clinical context they may be helpful in establishing the diagnosis of dermatomyositis. MRI may also be used in establishing an appropriate muscle biopsy site. In addition, MRI may be used for monitoring the progress of the disease.
The effect of a graded exercise protocol on phosphorus-31 magnetic resonance (MR) spectroscopy of calf skeletal muscle in nine healthy (control) subjects and 16 patients with symptomatic peripheral arterial occlusive disease (PAOD) was assessed. Ankle-brachial pressure indexes were obtained in all 16 patients, and 10 patients underwent peripheral arteriography. Temporal profiles of pH and the inorganic phosphorus (Pi) index were calculated from the spectra. A Pi-index recovery rate constant was calculated for each subject. Arteriograms were graded by calculating the runoff resistance in the limb of interest. The pH profiles during exercise did not differ significantly between the PAOD patients and control subjects. The Pi-index recovery rate constant in the PAOD patients was significantly (P less than .01) smaller than in the control subjects. There was no significant correlation between recovery rate and the ankle-brachial pressure indexes, but there was a strong negative correlation between recovery rates and angiographic resistance grades, with smaller recovery rate constants in patients with increased arterial resistance. It is concluded that P-31 MR spectroscopy shows promise as a direct measure of tissue perfusion.
Quantitative measurements of perfusion and molecular diffusion were made in human white matter in two orientations of the motion-sensitization gradient to document anisotropy of these parameters. Measurements were localized to a 10 X 10-mm tissue column oriented in an anterior-to-posterior direction in the left cerebral hemisphere just above the body of the left ventricle. This region was selected because of the relatively high directionality of white matter fibers. In this study of five healthy volunteers, strong diffusion anisotropy was observed in all cases. Twofold or greater anisotropy was commonly observed, with the higher diffusion value associated with motion sensitivity along the fiber directions. By combining data from both gradient orientations in all cases, diffusion values of solid tissue ranged from 0.38 X 10(-3) mm2/sec to 1.12 X 10(-3) mm2/sec, and measured perfusion fractions were in the range of 2%-5% (excluding areas highly contaminated by cerebrospinal fluid). Little or no perfusion-fraction anisotropy was observed; however, perfusion measurements were limited by noise. Data were collected without cardiac gating by using a technique that offers good immunity to bulk tissue motion artifacts.
The objective of this work is refinement of an MR angiography technique via postprocessing removal of phase errors which inhibit static signal subtraction. Projective views of the object are obtained using interleaved flow-compensated and noncompensated gradient waveforms. Complex subtraction of data sets is required since a projection dephase pulse is used for static signal suppression. This renders the difference image susceptible to systematic phase errors which are modeled as a smoothly varying multiplicative phase function. The error function is estimated by comparison of heavily spatial filtered renditions of the object acquired without projection dephasing in order to minimize influence of flow. The phase correction is then applied to high-resolution data sets collected with projection dephasing to enhance flow sensitivity. The technique is demonstrated by improvement of MR angiograms of rats acquired on a 2-T, 31-cm-bore system.
Image localized 31P magnetic resonance (MR) spectroscopy of the liver was performed in twelve normal volunteers and seven patients with hepatic tumours. The tumours which were clearly imaged by proton MR could also be distinguished from normal tissue because of spectral differences. The malignant tumours had significantly elevated phosphomonoester/inorganic phosphate and phosphomonoester/beta-adenosine triphosphate ratios, probably due to elevated tumour concentrations of phosphocholine and phosphoethanolamine, which are intermediates in the synthesis of membrane phospholipids. The pH values of the malignant tumours were elevated compared to normal hepatic parenchyma. Liver spectra in two patients with the commonest benign hepatic neoplasm, cavernous haemangioma, differed from both normal tissue and the malignant tumours in having a very low signal/noise ratio but apparently normal relative levels of phosphomonoester.
Narrowing of the pulmonary arteries was created surgically in twelve dogs. In six of the dogs the narrowing was central (main pulmonary artery), and in the remaining six the narrowing was located peripherally at the hilar level of the right pulmonary artery beyond the pericardial reflection. MRI and angiography were performed in all dogs. MRI clearly delineated the site of the pulmonary band and the caliber of the pulmonary artery at the site of the band in all dogs (N = 6). MRI was not able to visualize any of the stenosis of the right pulmonary arteries at the hila, beyond the pericardial reflection. In addition, optimal imaging planes to depict each segment of the central pulmonary arteries were determined. The capability to image in oblique planes is essential in evaluating the morphology of the central pulmonary arteries.
Diltiazem may provide a protective effect to ischemic and reperfused myocardium through preservation of high-energy phosphate metabolism. To test this hypothesis, rabbits had a 1.3 cm solenoidal coil placed over the myocardium to be rendered ischemic. Data were acquired with a 22 cm bore nuclear magnetic resonance spectrometer at 2.0 T. Animals were treated with diltiazem (200 micrograms/kg intravenous bolus of drug followed by a 15 micrograms/kg/min continuous intravenous infusion, n = 10) or by an equal volume of saline (n = 6). The left circumflex artery was occluded and reperfused using a reversible snare while electrocardiogram-gated spectra were accumulated. Levels of phosphocreatine were decreased during occlusion in both groups; however, this decrease was attenuated in the diltiazem treated animals compared to control (in relative percent area: 7.8 +/- 1.0 to 2.5 +/- 0.5, p less than 0.01). Levels of phosphocreatine promptly returned to baseline following reperfusion and there was no difference between the two groups. The inorganic phosphate metabolites of high-energy phosphate consumption increased with occlusion, though more so in the control group compared with the diltiazem-treated rabbits (in relative percent area: 72.5 +/- 0.9 to 55.4 +/- 1.3, p less than 0.01). With reperfusion, levels of inorganic phosphates returned toward baseline in both groups; however, the diltiazem group had a more complete recovery relative to control (in relative percent area: 38.8 +/- 2.1 to 47.6 +/- 2.7, p less than 0.05). Levels of adenosine triphosphate decreased in both groups relative to baseline; however, the amount of decrease was similar in the two groups. With reperfusion there was a definite though incomplete recovery of levels of adenosine triphosphate in the diltiazem-treated group (in relative percent area: 10.7 +/- 1.0 at occlusion, 12.3 +/- 0.4 during reperfusion, p less than 0.05), but in the control group levels of adenosine triphosphate remained depressed (in relative percent area: 9.8 +/- 0.6 at occlusion, 9.8 +/- 0.8 during reperfusion, p = NS). During ischemia there was a trend toward attenuation of intracellular acidosis in the diltiazem group; however, this trend did not reach statistical significance. These data indicate that diltiazem provides a protective effect on myocardial high-energy phosphate metabolism during regional ischemia and reperfusion in the intact animal.
Few studies have examined metabolic consequences of coronary occlusion and reperfusion using phosphorus31 nuclear magnetic resonance (31P-NMR) in an intact animal model. Accordingly, we developed a model to study serial changes in myocardial metabolism in the intact open-chest rabbit. Ten animals underwent 20 +/- 2 minutes of regional coronary occlusion and 60 +/- 10 minutes of reperfusion followed by reocclusion. Cardiac-gated 31P-NMR spectra were obtained with a regional surface coil over the ischemic area during baseline, occlusion, reperfusion, and reocclusion conditions. Phosphocreatine fell with both the initial and second ischemic insults to 65% +/- 5% of baseline for the first occlusion (p less than 0.01) and tended to decrease to 89% +/- 8% of baseline for the second occlusion (p = 0.07), with normal levels reattained in the intervening period of reperfusion (99% +/- 5% of baseline, p = NS). Concordant inverse changes were seen with inorganic phosphates. At occlusion levels of inorganic phosphates were 135% +/- 10% of baseline (p less than 0.05) and 139% +/- 10% of baseline at reocclusion (p less than 0.05). Levels of adenosine triphosphate decreased during occlusion to 78% +/- 9% of baseline and were significantly lower than baseline during the second occlusion (75% +/- 5% of baseline, p less than 0.01). The ratio of phosphocreatine to inorganic phosphates, when compared with values at baseline, decreased at occlusion (49.6% +/- 4.7% of baseline, p less than 0.01) and at reocclusion (64.7% +/- 4.9% of baseline, p less than 0.01), with a normal ratio reattained in the intervening period of reperfusion (93.3% +/- 3.1% of baseline, p = NS). We conclude that reperfusion restores levels of phosphocreatine and adenosine triphosphate while returning levels of inorganic phosphates to baseline. Deleterious changes in high-energy phosphate metabolism are not potentiated by reocclusion in this model. 31P-NMR spectroscopy holds promise as a technique to noninvasively monitor intracellular biochemical processes serially during various interventions in the intact animal model.
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