Bruce L. Daniel

Referenceless PRF shift thermometry

The proton resonance frequency (PRF) shift provides a means of measuring temperature changes during minimally invasive thermotherapy. However, conventional PRF thermometry relies on the subtraction of baseline images, which makes it sensitive to tissue motion and frequency drift during the course of treatment. In this study, a new method is presented that eliminates these problems by estimating the background phase from each acquired image phase. In this referenceless method, a polynomial is fit to the background phase outside the heated region in a weighted least‐squares fit. Extrapolation of the polynomial to the heated region serves as the background phase estimate, which is then subtracted from the actual phase. The referenceless method is demonstrated on a phantom during laser heating, 0° temperature rise images of in vivo human liver, interstitial laser ablation of porcine liver, and transurethral ultrasound ablation of canine prostate. A good correlation between temperature maps reconstructed with the referenceless and subtraction methods was found. Magn Reson Med 51:1223–1231, 2004.

Triggered, navigated, multi-baseline method for proton resonance frequency temperature mapping with respiratory motion.

A technique is presented for the acquisition of temperature maps in the presence of variable respiratory motion using the proton resonance frequency (PRF) shift. The technique uses respiratory triggering, diaphragm position determination with a navigator echo, and the collection of multiple baseline images to generate temperature maps. Laser ablations were performed in an ex vivo liver phantom undergoing variable simulated respiratory motion and in vivo in four porcine livers, demonstrating a reduction of artifacts in the computed temperature maps compared with conventional single baseline techniques, both uncorrected and corrected for motion. Magn Reson Med 50:1003–1010, 2003.

Relaxation times of breast tissue at 1.5T and 3T measured using IDEAL.

To accurately measure T1 and T2 of breast fibroglandular tissue and fat at 1.5T and 3T, and note the partial volume effects of the admixture of fibroglandular tissue and fat on the relaxation rates using an approach termed iterative decomposition of water and fat with echo asymmetry and least squares estimation (IDEAL) imaging.

Real-Time Estimation of 3-D Needle Shape and Deflection for MRI-Guided Interventions

We describe a MRI-compatible biopsy needle instrumented with optical fiber Bragg gratings for measuring bending deflections of the needle as it is inserted into tissues. During procedures, such as diagnostic biopsies and localized treatments, it is useful to track any tool deviation from the planned trajectory to minimize positioning errors and procedural complications. The goal is to display tool deflections in real time, with greater bandwidth and accuracy than when viewing the tool in MR images. A standard 18 ga × 15 cm inner needle is prepared using a fixture, and 350-μm-deep grooves are created along its length. Optical fibers are embedded in the grooves. Two sets of sensors, located at different points along the needle, provide an estimate of the bent profile, as well as temperature compensation. Tests of the needle in a water bath showed that it produced no adverse imaging artifacts when used with the MR scanner.

Image-guided focused ultrasound ablation of breast cancer: current status, challenges, and future directions

Image-guided focussed ultrasound (FUS) ablation is a non-invasive procedure that has been used for treatment of benign or malignant breast tumours. Image-guidance during ablation is achieved either by using real-time ultrasound (US) or magnetic resonance imaging (MRI). The past decade phase I studies have proven MRI-guided and US-guided FUS ablation of breast cancer to be technically feasible and safe. We provide an overview of studies assessing the efficacy of FUS for breast tumour ablation as measured by percentages of complete tumour necrosis. Successful ablation ranged from 20% to 100%, depending on FUS system type, imaging technique, ablation protocol, and patient selection. Specific issues related to FUS ablation of breast cancer, such as increased treatment time for larger tumours, size of ablation margins, methods used for margin assessment and residual tumour detection after FUS ablation, and impact of FUS ablation on sentinel node procedure are presented. Finally, potential future applications of FUS for breast cancer treatment such as FUS-induced anti-tumour immune response, FUS-mediated gene transfer, and enhanced drug delivery are discussed. Currently, breast-conserving surgery remains the gold standard for breast cancer treatment.

Mapping of the prostate in endorectal coil-based MRI/MRSI and CT: a deformable registration and validation study.

The endorectal coil is being increasingly used in magnetic resonance imaging (MRI) and MR spectroscopic imaging (MRSI) to obtain anatomic and metabolic images of the prostate with high signal-to-noise ratio (SNR). In practice, however, the use of endorectal probe inevitably distorts the prostate and other soft tissue organs, making the analysis and the use of the acquired image data in treatment planning difficult. The purpose of this work is to develop a deformable image registration algorithm to map the MRI/MRSI information obtained using an endorectal probe onto CT images and to verify the accuracy of the registration by phantom and patient studies. A mapping procedure involved using a thin plate spline (TPS) transformation was implemented to establish voxel-to-voxel correspondence between a reference image and a floating image with deformation. An elastic phantom with a number of implanted fiducial markers was designed for the validation of the quality of the registration. Radiographic images of the phantom were obtained before and after a series of intentionally introduced distortions. After mapping the distorted phantom to the original one, the displacements of the implanted markers were measured with respect to their ideal positions and the mean error was calculated. In patient studies, CT images of three prostate patients were acquired, followed by 3 Tesla (3 T) MR images with a rigid endorectal coil. Registration quality was estimated by the centroid position displacement and image coincidence index (CI). Phantom and patient studies show that TPS-based registration has achieved significantly higher accuracy than the previously reported method based on a rigid-body transformation and scaling. The technique should be useful to map the MR spectroscopic dataset acquired with ER probe onto the treatment planning CT dataset to guide radiotherapy planning.

A truly hybrid interventional MR/X-ray system: feasibility demonstration.

A system enabling both x‐ray fluoroscopy and MRI in a single exam, without requiring patient repositioning, would be a powerful tool for image‐guided interventions. We studied the technical issues related to acquisition of x‐ray images inside an open MRI system (GE Signa SP). The system includes a flat‐panel x‐ray detector (GE Medical Systems) placed under the patient bed, a fixed‐anode x‐ray tube overhead with the anode‐cathode axis aligned with the main magnetic field and a high‐frequency x‐ray generator (Lunar Corp.). New challenges investigated related to: 1) deflection and defocusing of the electron beam of the x‐ray tube; 2) proper functioning of the flat panel; 3) effects on B0 field homogeneity; and 4) additional RF noise in the MR images. We have acquired high‐quality x‐ray and MR images without repositioning the object using our hybrid system, which demonstrates the feasibility of this new configuration. Further work is required to ensure that the highest possible image quality is achieved with both MR and x‐ray modalities. J. Magn. Reson. Imaging 2001;13:294–300.

Temperature mapping of frozen tissue using eddy current compensated half excitation RF pulses

Cryosurgery has been shown to be an effective therapy for prostate cancer. Temperature monitoring throughout the cryosurgical iceball could dramatically improve efficacy, since end temperatures of at least –40°C are required. The results of this study indicate that MR thermometry based on tissue R  *2 has the potential to provide this information. Frozen tissue appears as a complete signal void on conventional MRI. Ultrashort echo times (TEs), achievable with half pulse excitation and a short spiral readout, allow frozen tissue to be imaged and MR characteristics to be measured. However, half pulse excitation is highly sensitive to eddy current distortions of the slice‐select gradient. In this work, the effects of eddy currents on the half pulse technique are characterized and methods to overcome these effects are developed. The methods include: 1) eddy current compensated slice‐select gradients, and 2) a correction for the phase shift between the first and second half excitations at the center of the slice. The effectiveness of these methods is demonstrated in R  *2 maps calculated within the frozen region during cryoablation. Magn Reson Med 46:985–992, 2001.

Management of biopsy needle artifacts: Techniques for RF-refocused MRI†

Several methods were investigated to improve the depiction of biopsy needles in radiofrequency (RF)‐refocused magnetic resonance imaging. Distortion correction is performed by the use of view angle tilting (VAT): a gradient is employed on the slice‐select axis during readout. Needle conspicuity is increased by offsetting the gradient echo from the spin echo and by inverting the 90° RF pulse slice‐select gradient. VAT effectively re‐registers in‐plane shifts. Since this method changes the projection angle through the slice, some structures appear blurred, while other structures appear sharper. VAT does not correct errors in slice selection. Offsetting the spin echo from the gradient echo increases needle conspicuity but can result in a shift in the apparent location of the needle. Inverting the 90° slice‐select gradient effectively increases the needle conspicuity with no shift in the needle location. These methods provide an easy and interactive means to manipulate needle artifacts but should be used cautiously.J. Magn. Reson. Imaging 1999;9:586–595.

MR-guided Transjugular Intrahepatic Portosystemic Shunt Creation with Use of a Hybrid Radiography/MR System

PURPOSE To evaluate the performance of a combined hybrid radiography/magnetic resonance (MR) unit to guide portal vein (PV) puncture during human transjugular intrahepatic portosystemic shunt (TIPS) creation. MATERIALS AND METHODS Fourteen patients undergoing TIPS creation were studied during standard clinical applications. Patients were anesthetized and then positioned in an open MR unit containing a flat-panel radiographic fluoroscopic unit. With use of a combination of fluoroscopy and MR imaging, the PV was accessed and the TIPS procedure was performed. A noncovered nitinol stent or a covered stent-graft was placed in the TIPS tract. Number of punctures required, total procedure time, fluoroscopy time, procedural success rate, complications, and ultrasonographic and clinical follow-up were recorded. RESULTS Clinical success was obtained in 13 of 14 patients. In one patient, extrahepatic puncture of the PV occurred, resulting in hemorrhage and requiring placement of a covered stent to control the bleeding. The mean number of punctures required to access the PV was 2.6 +/- 1.7, and the total procedure time was 2.5 hours +/- 0.6. Mean fluoroscopy time was 22.3 minutes +/- 5.5. Results of clinical and ultrasonographic follow-up compare favorably to previously published reports. CONCLUSION TIPS creation with a combination hybrid radiography/MR unit is feasible and may reduce the number of needle passes required and radiation exposure, with similar overall outcomes compared with studies reported in the literature.