Steven P. Souza

MR-Guided Focused Ultrasound Surgery

Magnetic resonance guided focused ultrasound surgery provides a minimally invasive controlled method for selectively destroying deep-lying tissue. A thermal analysis of focused ultrasound provides an estimate of the time-dependent temperature distribution and thermal dose required for ultrasound surgery. The temperature distribution is estimated by accumulating heat sources, considering the effects of thermal conductivity, heat content, and perfusion. In this study, both gel phantoms and excised in vitro bovine muscle specimens were imaged in a 1.5 T MR system while heated with a 5 cm diameter, 10 cm focal length, 1.1 MHz transducer. During sonication, the thermal effects were observed with T1-weighted pulse sequences. Below a critical temperature, the heat zone appeared as a dark spot that moved with the focal spot. Above a critical thermal dose, the in vitro tissue was irreversibly altered and the focal lesion was observed on both the MR image and the specimen slice.

Tracking system to monitor the position and orientation of a device using magnetic resonance detection of a sample contained within the device

A tracking system employs magnetic resonance signals to monitor the position and orientation of a device, such as a catheter, within a subject. The device has an MR active sample and a receiver coil which is sensitive to magnetic resonance signals generated by the MR active sample. These signals are detected in the presence of magnetic field gradients and thus have frequencies which are substantially proportional to the location of the coil along the direction of the applied gradient. Signals are detected responsive to sequentially applied mutually orthogonal magnetic gradients to determine the devices position in several dimensions. The position of the device as determined by the tracking system is superimposed upon independently acquired medical diagnostic images. One or more devices can be simultaneously tracked.

Tracking system and pulse sequences to monitor the position of a device using magnetic resonance

A tracking system employs magnetic resonance signals to monitor the position of a device such as a catheter within a subject. The device has a receiver coil which is sensitive to magnetic resonance signals generated in the subject. These signals are detected in the presence of magnetic field gradients and thus have frequencies which are substantially proportional to the location of the coil along the direction of the applied gradient. Signals are detected responsive to sequentially applied mutually orthogonal magnetic gradients to determine the position of the device in several dimensions. The position of the device as determined by the tracking system is superimposed upon independently acquired medical diagnostic images.

Tracking system to monitor the position of a device using multiplexed magnetic resonance detection

A tracking system employs magnetic resonance signals to monitor the position of a device such as a catheter within a subject. The device has a receiver coil which is sensitive to magnetic resonance signals generated in the subject. These signals are detected in the presence of magnetic field gradients and thus have frequencies which are substantially proportional to the location of the coil along the direction of the applied gradient. Signals are detected responsive to applied magnetic gradients to determine the position of the device in several dimensions. Sensitivity of the measured position to resonance offset conditions such as transmitter frequency misadjustment, chemical shift and the like is minimized by repeating the process a plurality of times with selected amplitudes and polarities for the applied magnetic field gradient. Linear combinations of the data acquired responsive to the different applied magnetic field gradients are computed to determine the position of the device in three orthogonal dimensions. The position of the device as determined by the tracking system is superimposed upon independently acquired medical diagnostic images.

3D surface rendered MR images of the brain and its vasculature.

Both time-of-flight and phase contrast magnetic resonance angiography images are combined with stationary tissue images to provide data depicting two contrast relationships yielding intrinsic discrimination of brain matter and flowing blood. A computer analysis is based on nearest neighbor segmentation and the connection between anatomical structures to partition the images into different tissue categories: from which, high resolution brain parenchymal and vascular surfaces are constructed and rendered in juxtaposition, aiding in surgical planning.

Quantitative flow measurement in phase contrast MR angiography.

The quantitative nature of phase contrast magnetic resonance angiography is explored, and a technique of blood flow measurement that is independent of system and patient parameters is presented. Phantom and patient studies demonstrate that quantitative flow measurements by phase contrast angiography can be routinely obtained with good accuracy despite nonuniform sensitivity profiles, blood flow pulsatility, and patient-to-patient changes in system gain. The calibration method requires the acquisition of only two flow images and thus can be performed as part of an angiographic session. Flow calibration of any flow profile can be accomplished with this technique. The solutions for plug flow and parabolic flow are presented.

SIMA: simultaneous multislice acquisition of MR images by Hadamard-encoded excitation.

We present a method of multislice magnetic resonance imaging that utilizes simultaneous binary-encoded excitation. Signals are acquired from all slices at once, and the images are separated in the reconstruction process. This simultaneous multislice acquisition method has been implemented for multislice spin-echo imaging, and the results are compared with those for a standard interleaved multislice method. Advantages include improved signal to-noise ratios and flexible slice placement. Phantom and volunteer studies are presented and evaluated in comparison with competing methods.

Two- and three-dimensional phase contrast MR angiography of the abdomen.

Protocols for two- and three-dimensional phase contrast magnetic resonance angiography of the major vessels in the abdomen were developed and demonstrated in healthy volunteers. Phase contrast imaging provides excellent suppression of stationary tissue signal and can be used to quantify the speed and direction of flow in a given vessel. The two-dimensional protocols provide rapid breath-held images, which proved useful for anatomical localization. Trie three-dimensional studies required longer acquisition times but provided superior image quality. Artifacts from bowel and respiratory motion were not significant with either protocol. Vasculature detected includes the renal, splenic, hepatic, and mesenteric systems as well as the aorta and inferior vena cava.

A broadband phased-array system for direct phosphorus and sodium metabolic MRI on a clinical scanner

Despite their proven gains in signal‐to‐noise ratio and field‐of‐view for routine clinical MRI, phased‐array detection systems are currently unavailable for nuclei other than protons (1H). A broadband phased‐array system was designed and built to convert the 1H transmitter signal to the non‐1H frequency for excitation and to convert non‐1H phased‐array MRI signals to the 1H frequency for presentation to the narrowband 1H receivers of a clinical whole‐body 1.5 T MRI system. With this system, the scanner operates at the 1H frequency, whereas phased‐array MRI occurs at the frequency of the other nucleus. Pulse sequences were developed for direct phased‐array sodium (23Na) and phosphorus (31P) MRI of high‐energy phosphates using chemical selective imaging, thereby avoiding the complex processing and reconstruction required for phased‐array magnetic resonance spectroscopy data. Flexible 4‐channel 31P and 23Na phased‐arrays were built and the entire system tested in phantom and human studies. The array produced a signal‐to‐noise ratio improvement of 20% relative to the best‐positioned single coil, but gains of 300–400% were realized in many voxels located outside the effective field‐of‐view of the single coil. Cardiac phosphorus and sodium MRI were obtained in 6–13 min with 16 and 0.5 mL resolution, respectively. Lower resolution human cardiac 23Na MRI were obtained in as little as 4 sec. The system provides a practical approach to realizing the advantages of phased‐arrays for nuclei other than 1H, and imaging metabolites directly. Magn Reson Med 43:269–277, 2000.

Evaluation of the carotid artery bifurcation : comparison of magnetic resonance angiography and digital subtraction arch aortography

SummaryThirty-four carotid artery bifurcations were examined using both magnetic resonance angiography (MRA) and digital subtraction arch aortography to determine their accuracy when compared to selective carotid angiography. The sensitivity of MRA was 73% and its specificity was 91% when compared with selective carotid angiography. The sensitivity of arch aortography was 27% and its specificity was 100%.