Kim Butts

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.

Reduction of blurring in view angle tilting MRI

Magnetic resonance imaging (MRI) in the presence of metallic objects suffers from slice‐selection errors and in‐plane distortions. View angle tilting (VAT) corrects for in‐plane distortions by adding a gradient on the slice‐select axis during readout, but can suffer from image blurring. This work demonstrates that the major source of blurring is a slice profile modulation of the data, and proposes several solutions to prevent such blurring. Multiple high‐bandwidth readouts are demonstrated to reduce the blurring while improving the signal‐to‐noise ratio (SNR) over a single high‐bandwidth readout. Magn Reson Med 53:418–424, 2005.

Study of focused ultrasound tissue damage using MRI and histology.

This paper reports on an experimental study of in vivo tissue damage in the rabbit brain with focused ultrasound (FUS) using magnetic resonance imaging (MRI) and histopathological analysis. Ten ultrasonic lesions (tissue damage) were created in five rabbits using a focused ultrasound beam of 1.5 MHz, electrical power input to the transducer of 70–85 W, and an exposure duration of 15–20 seconds. T1‐ and T2‐weighted fast spin‐echo (FSE) and Fluid attenuated inversion recovery (FLAIR) sequences were used to detect the ultrasonic lesions after treatment. Imaging was performed for 4–8 hours after treatment, after which the animals were immediately sacrificed. Ultrasonic lesion diameter was measured on MRI and histological sections after correction for tissue shrinkage during the histological processing. The T1‐weighted images showed lesions poorly, whereas both T2‐weighted and FLAIR images showed lesions clearly. The lesion diameters on both T2 and FLAIR imaging correlated well with measurements from histology. The time delay before lesions appeared on T2‐weighted imaging was 15 minutes to 1 hour, depending on the exposure location in the brain. J. Magn. Reson. Imaging 1999;10:146–153.

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.

Segmental lumbar mobility in individuals with low back pain: in vivo assessment during manual and self-imposed motion using dynamic MRI.

BackgroundAltered spinal mobility is thought to be related to current or past episodes of low back pain; however evidence of that relationship in younger subjects has not been established. The purpose of this study was to compare lumbar segmental mobility in asymptomatic and symptomatic subjects during posterior to anterior (PA) manual spinal mobilization and a self-initiated prone press-up (PU) maneuver. We hypothesized that persons with central low back pain would have an altered lumbar segmental mobility pattern compared to those without pain.MethodForty-five individuals (age 32.1 ± 8.5) with non-specific low back pain and 20 persons (age 31.1 ± 7.0) without low back pain participated. Each subject underwent dynamic imaging of the lumbar spine during a PA mobilization procedure and while performing a PU. Segmental motion was quantified as the change in the intervertebral angle between the resting and end-range vertebral positions.ResultsThe symptomatic group had a larger percentage of subjects with evidence of single level segmental hypermobility than the asymptomatic group during the PA (40.0% vs. 5%) and PU (26.7% vs. 15%) procedures. Single lumbar motion-segment analysis revealed hyper-mobility in symptomatic subjects at L5 – S1 (Chi-square = 10.0, p ≤ 0.01) and L4 – L5 (Chi-square = 4.18, p ≤ 0.05) during the PA test.ConclusionPersons with non-specific low back pain have a tendency to demonstrate single level lumbar segmental hypermobility when compared to age specific asymptomatic subjects.

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.

Magnetic resonance imaging of frozen tissues: Temperature-dependent MR signal characteristics and relevance for MR monitoring of cryosurgery

Previously, the magnetic resonance (MR) imaging appearance of frozen tissues created during cryosurgery has been described as a signal void. In this work, very short echo times (1.2 msec) allowed MR signals from frozen tissues to be measured at temperatures down to −35°C. Ex vivo bovine liver, muscle, adipose tissue, and water were imaged at steady‐state temperatures from −78° to +6°C. Signal intensity, T2*, and T1 were measured using gradient‐echo imaging. Signal intensity and T2* decrease monotonically with temperature. In the future, these MR parameters may be useful for mapping temperatures during cryosurgery. Magn Reson Med 41:627–630, 1999.