Vincent Colucci

Noninvasive arterial occlusion using MRI-guided focused ultrasound

The purpose of this work was to test the hypothesis that reproducible and sustainable arterial occlusion can be induced by focused ultrasound energy deposition noninvasively within deep tissue. An MRI-compatible focused ultrasound transducer was used to sonicate a branch of the renal artery (diameter about 0.6 mm) in vivo (nine rabbits). An intravenous MRI contrast agent bolus was injected about 30 min and up to 7 days after the sonication. After follow-up, in vitro magnification x-ray angiograms were obtained and the kidneys were fixed in formaldehyde for histologic study. The ultrasound pulses resulted in complete cessation of blood flow, as shown by the gradient echo images. In seven of the nine rabbits, a wedge-shaped unenhanced area was seen at the part of the kidney that was perfused by the vessel after the contrast agent injection. This area extended laterally (outside of the sonicated volume) to the cortical surface of the kidney. The x-ray angiograms showed that the artery was completely occluded. Postmortem histologic evaluation showed an infarcted tissue volume corresponding to the wedge shape seen in the images. This study showed that appropriately focused ultrasound can be used to close arteries noninvasively. This finding has significant clinical potential.

Potential adverse effects of high-intensity focused ultrasound exposure on blood vessels in vivo

The aim of the study was to evaluate the potential adverse effects of high intensity ultrasound exposure on blood vessels during noninvasive focused ultrasound surgery. A hydraulic MR-compatible positioning device was used to manipulate a focused ultrasound transducer (frequency 1.49 MHz, f-number = 0.8) in an MRI scanner. The system was used to sonicate a branch of the femoral artery and vein of 19 rabbits (26 thighs) in vivo at intensity levels above the threshold for transient cavitation; i.e., between 4400 and 8800 W cm-2 with multiple 1 s pulses stepped across the vessels (step size = 0.7 mm). The vessels were located and followed by MR angiography. In 13 rabbits, x-ray angiograms were also performed after the animals were euthanized. The results demonstrated that the 1 s high-intensity exposures caused the arteries to constrict at all exposure levels tested. At the intensity of 5800 W cm-2 and above, the MRI angiogram immediately after the sonications showed no flow. The x-ray angiograms (1-2 h later) showed that the blood vessels were open, but constricted to about 50% or less of their diameter. Both the MR and x-ray angiograms showed that the vessel diameters relaxed toward their initial diameter during the first week after sonication. In five cases, hemorrhage or vessel rupture was caused by the sonication. This study demonstrates that short, high-intensity focused ultrasound exposure can cause vessel spasm and hemorrhage when transient cavitation is present. This condition should be avoided during noninvasive focused ultrasound surgery.

Feasibility of using ultrasound phased arrays for MRI monitored noninvasive surgery

The purpose of this paper was to evaluate the in vivo feasibility of using phased arrays for MRI guided ultrasound surgery. Two different array concepts were investigated: a spherically curved concentric ring array to move the focus along the central axis and a spherically curved 16 square element array to make the focus larger. Rabbit thigh muscles were exposed in vivo in a 1.5 T MRI scanner to evaluate the array performance. The results showed that both of the arrays performed as expected, and the focus could be moved and enlarged. In addition, adequate power could be delivered from the arrays to necrose in vivo muscle tissue in 10 s. This study was the first implementation of phased arrays for MRI guided ultrasound surgery. The results demonstrate that phased arrays have significant potential for noninvasive tissue coagulation.

FOCUSED ULTRASOUND EFFECTS ON NERVE ACTION POTENTIAL IN VITRO

Minimally invasive applications of thermal and mechanical energy to selective areas of the human anatomy have led to significant advances in treatment of and recovery from typical surgical interventions. Image-guided focused ultrasound allows energy to be deposited deep into the tissue, completely noninvasively. There has long been interest in using this focal energy delivery to block nerve conduction for pain control and local anesthesia. In this study, we have performed an in vitro study to further extend our knowledge of this potential clinical application. The sciatic nerves from the bullfrog (Rana catesbeiana) were subjected to focused ultrasound (at frequencies of 0.661 MHz and 1.986 MHz) and to heated Ringers solution. The nerve action potential was shown to decrease in the experiments and correlated with temperature elevation measured in the nerve. The action potential recovered either completely, partially or not at all, depending on the parameters of the ultrasound exposure. The reduction of the baseline nerve temperature by circulating cooling fluid through the sonication chamber did not prevent the collapse of the nerve action potential; but higher power was required to induce the same endpoint as without cooling. These results indicate that a thermal mechanism of focused ultrasound can be used to block nerve conduction, either temporarily or permanently.

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 exvivo 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.

MR-based temperature monitoring for hot saline injection therapy

We applied magnetic resonance (MR) phase mapping methods to monitor the thermal frequency shift of water in order to study temperature changes from percutaneous hot saline injection therapy (PSIT) using in vitro swine livers and in vivo rabbit livers. The thermal coefficients calculated from the shifts of the water frequency with thermocouple based temperature measurements were −0.0085 ± 0.0019 ppm/°C for the in vitro studies and −0.0089 ppm/°C for the in vivo studies. The error range was estimated to be ± 3°C and ± 4.5 °C, respectively. Color‐coded temperature maps were compared with macroscopic lesion sizes of the specimen. Regions defined using a 20°C elevation in the initial images following hot saline injection (around 55°C in absolute temperature) closely correlated with visible coagulation in size. We conclude that MR temperature monitoring of PSIT is quite feasible and may be helpful in expanding the clinical use of this thermal therapeutic tool for liver tumors. J. Magn. Reson. Imaging 2000;12:330–338.

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.

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.

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.

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.