Magnetic resonance imaging of the acute effects of interstitial neodymium:YAG laser irradiation on tissues.

RATIONALE AND OBJECTIVES: Laser irradiation therapy in deep tissues requires a monitoring method other than visual guidance. Magnetic resonance imaging (MRI) can be used for this purpose because it visualizes soft tissue structures and heat distribution. METHODS: The authors performed interstitial laser irradiations in rat livers with various laser outputs and measured the sizes of laser-induced lesions. MRI of these lesions was done ex vivo and compared with the histologic findings. Laser-induced lesions also were studied in rabbit brain, liver, and skeletal muscle to show the influences of tissue optical and thermal properties. Imaging of interstitial laser irradiation also was performed in vivo in rabbit brains. RESULTS: MRI depicted the laser-induced lesions produced with different laser outputs and tissue types. MRIs of rabbit brain in vivo effectively demonstrated the signal decrease during heating and acute tissue changes. CONCLUSION: MRI has potential for monitoring interstitial laser surgery or hyperthermia.

Differentiation of experimental white matter lesions using multiparametric magnetic resonance measurements.

The potential of multiparametric proton magnetic resonance (MR) measurements for characterizing white matter lesions was investigated. The authors compared acute experimental allergic encephalomyelitis (EAE), which is distinguished by inflammatory lesions, with an immunologically potentiated hyperacute form of the disease in which demyelinating lesions (DEM) also are present. Tissue samples containing cervical spinal cord and brain stem were excised and in vitro measurements of T1, T2, and two components of T2 were performed. Discriminant analysis was applied using MR parameters singly and in various combinations. When the disease was clearly manifested, discrimination between treated and normal animals was satisfactory with single parameters. The use of biexponential T2 components improved the distinction of normal from treated but asymptomatic animals, and differentiated between EAE and DEM. These results suggest that improved characterization of white matter lesions is possible with multiparametric MR in vivo, especially if sampling is performed with imaging and the T2 decay curves are obtained with a sufficient number of echoes to perform biexponential analysis.

Response to and control of destructive energy by magnetic resonance.

Magnetic resonance imaging techniques can be used to control and monitor the deposition of destructive energy. The authors evaluated the feasibility of phosphorus-31 magnetic resonance spectroscopy for the control, monitoring, and prediction of the three-dimensional extent of tissue destruction during interstitial laser surgery. Characteristic metabolic changes were demonstrated within the lesion and in the adjacent normal tissue during the deposition of thermal energy.

Cocaine- and methamphetamine-induced acute cerebral vasospasm: an angiographic study in rabbits.

Stroke is a frequently reported, though uncommon, complication of drug abuse, primarily cocaine. The pathogenesis is uncertain, although such cerebrovascular events may result from sympathetically mediated vasoconstriction of cerebral vessels. Amphetamine, another sympathomimetic amine that is commonly abused, may also cause strokes by producing cerebral vasospasm and/or vasculitis. Amphetamine and its derivatives are frequent adulterants of illegally obtained cocaine and may also be used concurrently; the effect(s) of this combination of drugs upon the cerebral vasculature is not known. Our aim was to develop an animal model that would enable us to study the ability of these drugs to produce acute cerebral vasospasm and to observe the response to IV administration of amphetamine and cocaine, either alone or together. Magnified basilar artery arteriograms were obtained in 12 New Zealand white rabbits before and after IV administration of cocaine, methamphetamine, or both, at various dosages. Low doses produced mild vasodilatation. At higher doses, the animals who received cocaine and methamphetamine alone showed little or no basilar artery spasm, but coadministration produced definite basilar artery vasospasm, reflecting a synergistic vasoconstrictive effect. If a similar response exists in the human cerebral vasculature, then this could help explain the cause of strokes associated with drug abuse.

In vivo characterization of cytotoxic intracellular edema by multicomponent analysis of transverse magnetization decay curves.

RATIONALE AND OBJECTIVES: We investigated the multicompartmental nature of T2 decay in a specific white matter edema model. METHODS: Triethyltin (TET) intoxication was produced in six male New Zealand White rabbits. Images were obtained over the 23-day study duration using a 64-echo Carr-Purcell-Meiboom-Gill (CPMG) sequence (repetition time = 3000 msec, echo time = 20 msec). T2 decay curves were extracted from 0.7 x 0.7 x 3.0 mm3 voxels in the corpus callosum and contiguous white matter tracts, cortex, thalamic nuclei, hypothalamic nuclei, and the masseter muscles. The curves were fit with biexponential functions. RESULTS: Increased signal intensity in the corpus callosum was evident 2-3 days after the first TET injection. At this time, a substantial slowly relaxing component appeared in the decay curves of the corpus callosum and, to a lesser extent, in the thalamus and hypothalamus. Changes in the rabbits' body weight, general physical condition, and neurologic state paralleled the growth and regression of the second, slowly relaxing component. CONCLUSION: The appearance and regression of a slowly decaying second component in the T2 decay curve is consistent with the formation and shrink-age of intracellular vesicles in the intramyelin sheaths of central white matter.

Experimental arterial aneurysms:modification of the production technique.

Vein pouches obtained from the external jugular vein were used to create experimental aneurysms on the carotid arteries of 13 rabbits and the abdominal aortas of 18 rabbits. Eighteen of these vein pouch aneurysms were grafted onto a linear incision in the artery; 13 of the aneurysms were grafted onto an elliptical incision. In 4 of the rabbits in which the vein pouch was grafted onto an elliptical arteriotomy in the carotid, a new approach for producing an intracranial aneurysm was attempted by relocating the aneurysm segment to the subarachnoid space at the cranial vertex. The results indicate that the aneurysms grafted onto an elliptical arteriotomy had a higher patency rate than the aneurysms grafted onto a linear arteriotomy in both the aortic and carotid models. Also, the attempt to produce an intracranial model demonstrated that short-term patency of the aneurysms could be achieved and that a two-stage approach to creating such aneurysms would be more appropriate than a one-stage approach.

Interstitial Nd:YAG laser ablation in normal rabbit liver: trial to maximize the size of laser-induced lesions.

In order to apply interstitial laser ablation to relatively small liver tumors in humans, it will be necessary to optimize the irradiation schedule. Nd:YAG laser was applied to normal rabbit liver in vivo at various power and energy outputs, including a protocol in which irradiation was repeated twice, with and without fiber tip advancement during the intermission. Ex vivo and in vivo tissue were also irradiated to determine the effect of perfusion on the lesion size. We obtained the same monotonic relationship between laser settings and lesion size in rabbit liver as we previously reported in rat liver. MR-guided fiber advancement between heating periods increased the transverse diameter of the lesion, and MR monitoring demonstrated this process. Our results suggest that repeated irradiation with brief intermissions, when combined with fiber advancement, may increase the lesion size.

MR monitoring during cryotherapy in the liver: predictability of histologic outcome.

For well-controlled application of cryotherapy to focal liver lesions, real-time monitoring is necessary to limit the final necrotic effect in the treated area. In this study, near real-time magnetic resonance (MR) monitoring images of normal rabbit liver were obtained during the freezing procedure. The MR-monitored, freezing-induced lesions were followed with MR images for up to 3 weeks. Corresponding histologic samples were also obtained over the same time period. Our results indicate that MR images obtained during the freezing procedure can adequately depict the area of final necrosis. Furthermore, histologic changes at each stage of lesion development correlated well with MR signal intensities on follow-up images. With the development of an MR-compatible cryogen probe, MR imaging may prove to be a robust method for monitoring, controlling, and following up cryotherapy in the liver.

MR appearance and spectral features of injected ethanol in the liver: implication for fast MR-guided percutaneous ethanol injection therapy.

PURPOSE: Our goal was to evaluate several fast MR strategies for monitoring ethanol distributions so that percutaneous ethanol injection might be guided with MRI. METHOD: Fast RF spoiled GRE sequences (SPGR) and T2-weighted rapid acquisition with relaxation enhancement (RARE) sequences with and without spectroscopic-quality water suppression techniques were assessed for their ability to depict the distribution of injected ethanol in ex vivo pig liver. A line scan Carr-Purcell-Meiboom-Gill spectroscopic imaging sequence was used to validate observations and measure spectral relaxation characteristics of the ethanol signal in liver. Injected deuterated ethanol was also tested as an alternative possibility to depict the distribution of ethanol. RESULTS: The water-suppressed T2-weighted RARE sequence depicted the distribution of ethanol better than other sequences. Deuterated ethanol appeared as a signal void on all sequences. CONCLUSION: Water-suppressed T2-weighted RARE sequences could be useful to rapidly monitor MR-guided PEI.

MR studies of brain oedema in the developing animal.

Assessment of perinatal brain oedema is complicated by normal changes in brain water that accompany the marked physiological, biochemical and morphological alterations occurring during this phase of development. Multiexponential analysis of transverse decay curves (TDCs), derived from 128 echo CPMG images, of white matter (WM) made oedematous by either exposure of animals to triethyltin (TET) or cryogenic cortical lesions revealed a second, slower decay component not apparent in controls. More significantly, an obvious difference was noted between the TET and cryogenic lesion fast decay components which might serve as a basis to differentiate non-invasively cytotoxic and vasogenic oedemas.

Transcatheter fiberoptic laser coagulation of blood vessels.

In ten rabbit ears the central auricular artery was percutaneously catheterized and subjected to doses of laser energy transmitted through a flexible optical fiber within the artery. Arterial occlusion was not produced in the initial three ears using energy levels less than 600 mW. Higher levels of laser energy, in the range of 800-1000 mW, caused arterial coagulation in six of the remaining seven rabbit ears. Serial observations by transillumination of the ears, postmortem angiography, and histologic examination demonstrated complete arterial ablation. The experiments showed that segmental arterial ablation is feasible using an intra-arterial optical fiber to direct laser energy to a specific arterial target. Technical refinements and potential therapeutic application are discussed.