Search

Percutaneous interstitial microwave thermoablation of locally recurrent prostate carcinoma was continually guided with magnetic resonance (MR) imaging. Phase images and data were obtained with a rapid gradient-echo technique and were used to derive tissue temperature change on the basis of proton-resonance shift. Thermally devitalized regions correlated well with the phase image findings. MR imaging-derived temperatures were linearly related to the fluoroptic tissue temperatures. MR imaging can be used to guide thermoablation.

PURPOSE: To compare conventional short inversion time inversion-recovery (STIR) with fast spin-echo (FSE) STIR techniques to evaluate suspected nontraumatic musculoskeletal abnormalities. MATERIALS AND METHODS: Thirty STIR and FSE-STIR examinations in 26 pediatric patients with suspected nontraumatic musculoskeletal abnormalities were prospectively evaluated. Qualitative (subjective) and quantitative (five-point rank score) analyses of the images were performed. RESULTS: FSE-STIR was faster than STIR (mean, 2 minutes 25 seconds and 6 minutes 35 seconds, respectively). Fat suppression was slightly better with STIR. Image degradation due to motion was judged similar. Lesion contrast to muscle was slightly better with STIR than FSE-STIR, and lesion contrast to fat was equivalent. Qualitatively, lesion conspicuity was similar: All lesions were seen with both techniques. CONCLUSION: FSE-STIR can replace STIR when an inversion-recovery fat-suppression sequence is desired. Considerable imaging time is saved.

The fast spin echo sequence combines data from many echo signals in a Carr-Purcell-Meiboom-Gill echo train to form a single image. Much of the signal in the second and later echoes results from the coherent addition of stimulated echo signal components back to the spin echo signal. Because stimulated echoes experience no dephasing effects during the time that they are stored as Mz magnetization, they experience a different gradient first moment than does the spin echo. This leads to flow-related phase differences between different echo components and results in flow voids and ghosting, even when the first moment is nulled for the spin echo signal. A method of gradient moment nulling that correctly compensates both spin echo and stimulated echo components has been developed. The simplest solution involves nulling the first gradient moment at least at the RF pulses and preferably at both the RF pulses and the echoes. Phantom and volunteer studies demonstrate good suppression of flow-related artifacts.

Chemical shift artifacts and other off-resonance spatial shifts in 2DFT MRI arise from the linear time dependence in the k-space data in the readout direction. Introduction of a view-dependent time shift of the readout window adds a time dependence to the phase-encoding direction and results in a virtual frequency-encoding direction that is a linear combination of the phase-encode and readout axes. By this method, the readout and phase-encode directions can be made identical in their sensitivity to off-resonance effects and can be arbitrarily swapped with no change in chemical shift or inhomogeneity effects, improving previously reported methods that swap these axes for signal averaging or reduction of motion artifacts.

Evaluation of intracranial and intraspinal CSF flow was accomplished by the use of cardiac gated gradient echo magnetic resonance (MR) technique. Normal patterns of pulsatile flow within the ventricles, cisterns and cervical subarachnoid space were established by this technique and these observations were compared to prior description of CSF flow. With systole there is downward (caudal) flow of CSF in the aqueduct of Sylvius, the foramen of Magendie, the basal cisterns and the dorsal and ventral subarachnoid spaces while during diastole, upward (cranial) flow of CSF in these same structures is seen. The relationships between the cardiac cycle and the CSF pulsations are demonstrated on both magnitude reconstruction and phase reconstruction MR images. Calculations of actual fluid velocity within CSF containing spaces can be obtained from the phase reconstruction images and holds promise for a more accurate analysis of CSF flow. In conditions which result in alterations of flow, cine MR dramatically shows either obstruction or excessively turbulent flow within the CSF pathways. The site of obstructed flow whether in the third ventricle, aqueduct, fourth ventricle, or subarachnoid space can be appreciated by changes in or absence of the normal hypointense signal. Cystic cord lesions such as congenital syringohydromyelia and posttraumatic spinal cord cysts may show pulsatile flow of CSF, a fact which can relate to progressive enlargement of these cysts. The distinction between myelomalacia and cyst formation in the cord is facilitated by the technique. Although the use of cine MR for the analysis of CSF flow is in its infancy, our experience indicates that this technique is useful in a wide range of pathological conditions including, but not limited to, conditions resulting in hydrocephalus or cystic cord lesions.

The MR scans of seven patients with intracranial carotid occlusion (five proved, two presumed) were reviewed to evaluate the MR signal characteristics seen in this disorder. Five patients had clinical signs of cerebral infarction. Of the remaining two patients, one was asymptomatic and the other had a long-standing occlusion and headaches. We correlated the MR findings with cerebral angiography in five patients and with CT scans in six patients. All occluded vessels demonstrated MR signal predominantly isointense to brain on proton-density- T1- and T2-weighted images. Since there is an absence of flow, the MR signal is based on the intrinsic properties of the arterial thrombus and possibly on the chronicity of the occlusion. The pathogenesis and histopathology of intravascular thrombus differ significantly from extravascular hematoma, which accounts for the differences in their MR signal characteristics. The demonstration of occluded intracranial vessels may solidify the diagnosis of stroke in cases in which clinical and/or CT findings are equivocal. In patients presenting with infarction, an occluded carotid artery by MR may obviate the need for angiography; however, the demonstration of a patent carotid in conjunction with infarction suggests the possibility of an embolus, which may require angiography. We believe that MR is a valuable adjunct to CT in evaluating patients with cerebrovascular infarction.

Sixty patients were examined with and without extra gradient pulses, which compensate for motion-induced phase errors, in order to determine the effect those gradients had in suppressing the motion artifacts frequently present in the brainstem, temporal lobes, and basal ganglionic regions on routine T2-weighted brain MR imaging. Two comparative studies were performed: (1) in 50 patients the motion-artifact suppression technique (MAST) was compared with a single-echo MR examination, and (2) in 10 patients the MAST technique was compared with the second echo of a symmetric dual-echo sequence. In the first study 39 patients were examined at 1.5 T and 11 patients were examined at 0.5 T with the same pulse sequences. We found that MAST resulted in a significant improvement of image quality in 24 of 39 patients on the high-field-strength system and in two of 11 patients on the mid-field-strength system. In the second study, we found that in four of the 10 patients, MAST resulted in a suppression of artifacts greater than that achieved by even-echo rephasing alone. With MAST, artifacts were eliminated that not only obscured normal structures but that could have left doubt about the presence of a true signal abnormality. There was, however, marked suppression of the CSF flow-void phenomenon and increased signal from flowing blood, particularly in the cortical veins and dural sinuses. Because of this, the use of additional pulse sequences in which these motion-compensating gradients were not used was necessary under certain clinical circumstances. We conclude that, with these motion-compensating gradients, artifacts are reduced or eliminated, and a marked improvement in image quality can be obtained without the need for cardiac gating.