Jeffrey L. Duerk

Magnetic Resonance Fingerprinting

Magnetic resonance is an exceptionally powerful and versatile measurement technique. The basic structure of a magnetic resonance experiment has remained largely unchanged for almost 50 years, being mainly restricted to the qualitative probing of only a limited set of the properties that can in principle be accessed by this technique. Here we introduce an approach to data acquisition, post-processing and visualization—which we term ‘magnetic resonance fingerprinting’ (MRF)—that permits the simultaneous non-invasive quantification of multiple important properties of a material or tissue. MRF thus provides an alternative way to quantitatively detect and analyse complex changes that can represent physical alterations of a substance or early indicators of disease. MRF can also be used to identify the presence of a specific target material or tissue, which will increase the sensitivity, specificity and speed of a magnetic resonance study, and potentially lead to new diagnostic testing methodologies. When paired with an appropriate pattern-recognition algorithm, MRF inherently suppresses measurement errors and can thus improve measurement accuracy.

Motion artifact suppression technique (MAST) for MR imaging

A technique has been developed that significantly improves the image resolution and reduces motion artifacts in conventional two-dimensional Fourier transform and three-dimensional Fourier transform magnetic resonance imaging sequences. Modifications on the gradient waveforms completely refocus the transverse magnetization at the echo time, regardless of the motion occurring between the time of the 90 degrees radiofrequency excitation and the echo time (within-view). This accomplishes suppression of motion artifacts and regains the signal from flowing blood and CSF. Images of the head, abdomen, chest, and spine are reproduced which show the increase in signal and anatomical detail that would otherwise be degraded and lost in artifact noise. This technique has reduced the practical difficulty of obtaining clinically diagnostic T2-weighted abdominal images. It also has allowed diagnostic quality T1- and T2-weighted images to be obtained with one acquisition per view, thus reducing the total scan time.

Cortical localization of human sustained attention: Detection with functional MR using a visual vigilance paradigm

PURPOSE Our goal was to determine whether functional MRI on a standard 1.5 T system can localize activation during a visual vigilance sustained attention task and whether this corresponds to results described in a PET investigation of a similar task. METHOD Sixteen volunteers were studied on a 1.5 T system using a gradient echo technique. A single axial section was oriented within a stereotaxic coordinate space, 40 mm superior to the anterior-posterior commissure line. Images with eyes closed were followed by images during subject concentration on a small dim spot. Motion correction and pixel-by-pixel statistical analysis were performed. Talairach grids were applied for summary statistical analysis and comparison to PET data, with analysis using a series of planned contrasts within a repeated measures analysis of variance. RESULTS Predominantly right-sided frontal and parietal activation was observed, with statistical significance across subjects in the right frontal lobe (F > or = 5.9, p < or = 0.041). Comparison with previously reported PET data yielded a very similar pattern of activation (F = 13.2; df = 1,8; p = 0.007). CONCLUSION Activation of the right middle frontal gyrus and right parietal lobe during visual vigilance is detectable across functional imaging modalities.

Semiautomatic 3-D image registration as applied to interventional MRI liver cancer treatment

The authors evaluated semiautomatic, voxel-based registration methods for a new application, the assessment and optimization of interventional magnetic resonance imaging (I-MRI) guided thermal ablation of liver cancer. The abdominal images acquired on a low-field-strength, open I-MRI system contain noise, motion artifacts, and tissue deformation. Dissimilar images can be obtained as a result of different MRI acquisition techniques and/or changes induced by treatments. These features challenge a registration algorithm. The authors evaluated one manual and four automated methods on clinical images acquired before treatment, immediately following treatment, and during several follow-up studies. Images were T2-weighted, T1-weighted Gd-DTPA enhanced, T1-weighted, and short-inversion-time inversion recovery (STIR). Registration accuracy was estimated from distances between anatomical landmarks. Mutual information gave better results than entropy, correlation, and variance of gray-scale ratio. Preprocessing steps such as masking and an initialization method that used two-dimensional (2-D) registration to obtain initial transformation estimates were crucial. With proper preprocessing, automatic registration was successful with all image pairs having reasonable image quality. A registration accuracy of /spl ap/3 mm was achieved with both manual and mutual information methods. Despite motion and deformation in the liver, mutual information registration is sufficiently accurate and robust for useful applications in I-MRT thermal ablation therapy.

Active device tracking and high-resolution intravascular MRI using a novel catheter-based, opposed-solenoid phased array coil.

A novel two‐element, catheter‐based phased array coil was designed and built for both active MR device tracking and high‐resolution vessel wall imaging. The device consists of two independent solenoid coils that are wound in opposite directions, connected to separate receive channels, and mounted collinearly on an angiographic catheter. The elements were used independently or together for tracking or imaging applications, respectively. The arrays dual functionality was tested on a clinical 1.5 T MRI scanner in vitro, in vivo, and in situ. During real‐time catheter tracking, each element gave rise to a high‐amplitude peak in the respective projection data, which enabled reliable and robust device tracking as well as automated slice positioning. In vivo microimaging with 240 μm in‐plane resolution was achieved in 9 s using the device and TrueFISP imaging. Therefore, a single device was successfully implemented that met the combined requirements of intravascular device tracking and imaging. Magn Reson Med 51:668–675, 2004.

Automatic MR volume registration and its evaluation for the pelvis and prostate.

A three-dimensional (3D) mutual information registration method was created and used to register MRI volumes of the pelvis and prostate. It had special features to improve robustness. First, it used a multi-resolution approach and performed registration from low to high resolution. Second, it used two similarity measures, correlation coefficient at lower resolutions and mutual information at full resolution, because of their particular advantages. Third, we created a method to avoid local minima by restarting the registration with randomly perturbed parameters. The criterion for restarting was a correlation coefficient below an empirically determined threshold. Experiments determined the accuracy of registration under conditions found in potential applications in prostate cancer diagnosis, staging, treatment and interventional MRI (iMRI) guided therapies. Images were acquired in the diagnostic (supine) and treatment position (supine with legs raised). Images were also acquired as a function of bladder filling and the time interval between imaging sessions. Overall studies on three patients and three healthy volunteers, when both volumes in a pair were obtained in the diagnostic position under comparable conditions, bony landmarks and prostate 3D centroids were aligned within 1.6 +/- 0.2 mm and 1.4 +/- 0.2 mm, respectively, values only slightly larger than a voxel. Analysis suggests that actual errors are smaller because of the uncertainty in landmark localization and prostate segmentation. Between the diagnostic and treatment positions, bony landmarks continued to register well, but prostate centroids moved towards the posterior 2.8-3.4 mm. Manual cropping to remove voxels in the legs was necessary to register these images. In conclusion, automatic, rigid body registration is probably sufficiently accurate for many applications in prostate cancer. For potential iMRI-guided treatments, the small prostate displacement between the diagnostic and treatment positions can probably be avoided by acquiring volumes in similar positions and by reducing bladder and rectal volumes.

Slice-to-volume registration and its potential application to interventional MRI-guided radio-frequency thermal ablation of prostate cancer

In this study, we registered live-time interventional magnetic resonance imaging (iMRI) slices with a previously obtained high-resolution MRI volume that in turn can be registered with a variety of functional images, e.g., PET, SPECT, for tumor targeting. We created and evaluated a slice-to-volume (SV) registration algorithm with special features for its potential use in iMRI-guided radio-frequency (RF) thermal ablation of prostate cancer. The algorithm features included a multiresolution approach, two similarity measures, and automatic restarting to avoid local minima. Imaging experiments were performed on volunteers using a conventional 1.5-T MR scanner and a clinical 0.2-T C-arm iMRI system under realistic conditions. Both high-resolution MR volumes and actual iMRI image slices were acquired from the same volunteers. Actual and simulated iMRI images were used to test the dependence of SV registration on image noise, receive coil inhomogeneity, and RF needle artifacts. To quantitatively assess registration, we calculated the mean voxel displacement over a volume of interest between SV registration and volume-to-volume registration, which was previously shown to be quite accurate. More than 800 registration experiments were performed. For transverse image slices covering the prostate, the SV registration algorithm was 100% successful with an error of <2 mm, and the average and standard deviation was only 0.4 mm /spl plusmn/ 0.2 mm. Visualizations such as combined sector display and contour overlay showed excellent registration of the prostate and other organs throughout the pelvis. Error was greater when an image slice was obtained at other orientations and positions, mostly because of inconsistent image content such as that from variable rectal and bladder filling. These preliminary experiments indicate that MR SV registration is sufficiently accurate to aid image-guided therapy.

An optical system for wireless detuning of parallel resonant circuits.

A new optical method of detuning parallel resonant circuits is described. This method involves the integration of a photoresistor in parallel with the inductor and capacitor of a parallel resonant circuit, in this case a magnetic resonance imaging (MRI) receiver coil. A fiberoptic cable extending the length of the interventional device is used in conjunction with an external light source to deliver light to the photoresistor. Exposing the photoresistor to light changes its bulk resistance and greatly lowers the Q of the parallel resonant circuit, effectively detuning it. By combining this optical detuning scheme with inductive coupling of the interventional device-mounted microcoils to a standard MRI coil, a completely wireless device for active device tracking has been created. This new device improves on current technology by simplifying device complexity and reducing patient risk by eliminating the need for electrical connections between the device-mounted microcoils to the MR receiver channel.A new optical method of detuning parallel resonant circuits is described. This method involves the integration of a photoresistor in parallel with the inductor and capacitor of a parallel resonant circuit, in this case a magnetic resonance imaging (MRI) receiver coil. A fiberoptic cable extending the length of the interventional device is used in conjunction with an external light source to deliver light to the photoresistor. Exposing the photoresistor to light changes its bulk resistance and greatly lowers the Q of the parallel resonant circuit, effectively detuning it. By combining this optical detuning scheme with inductive coupling of the interventional device‐mounted microcoils to a standard MRI coil, a completely wireless device for active device tracking has been created. This new device improves on current technology by simplifying device complexity and reducing patient risk by eliminating the need for electrical connections between the device‐mounted microcoils to the MR receiver channel. J. Magn. Reson. Imaging 2000;12:632–638.

How does alteration of hepatic blood flow affect liver perfusion and radiofrequency-induced thermal lesion size in rabbit liver?

The purpose of this study was to test the hypothesis that decreasing liver perfusion in rabbits results in an increase in thermal lesion size and that these effects can be accurately monitored using magnetic resonance imaging (MRI). We additionally tested the hypothesis that the increase in thermal lesion size would depend on the particular vessel or vessels occluded (hepatic artery, portal vein, or both). Using an Institutional Animal Care and Use Committee approved protocol, 20 New Zealand white rabbits were randomly assigned to four treatment groups (five in each group): control and ligation of portal vein (PV), hepatic artery (HA), or both PV and HA (HAPV). Surgical ligation of the appropriate vessel was performed under general anesthesia. Immediately after ligation, the rabbits were placed in a 0.2‐T open MR system, and an 18‐G copper radiofrequency (RF) electrode with a 2‐cm exposed tip was inserted into the liver. RF was applied for 10 minutes with the tip temperature maintained at 90° ± 2°C. Before and after ablation, perfusion data were obtained for 90 seconds using 30 3‐second sequential single oblique‐slice fast imaging with steady‐state progression (FISP) acquisitions after injection of gadolinium‐diethylene triamine pentaacetic acid (Gd‐DTPA) via the inferior vena cava. Postablation scanning included axial and oblique turbo spin‐echo (TSE) T2‐weighted (T2w), STIR, and Gd‐enhanced T1w sequences. Lesion size was determined perpendicular to the RF electrode using software calipers on the imager. The rabbits were sacrificed after completion of the post‐therapy scans, and their livers were harvested for histologic analysis. The liver showed a mean increase in signal amplitude (SA) of 76% 24 seconds after Gd contrast injection in the control group. After contrast injection, the SA increased to a mean of only 66% in the group with ligated hepatic arteries, with no difference in the time to peak compared with the control group. No significant SA increase over baseline could be found in the groups with ligated PV or ligated PV and HA. T2‐weighted images demonstrated the highest lesion‐to‐liver contrast‐to‐noise ratios (CNRs; mean −5.5) on postprocedure images, followed by STIR images (mean −2.2) in the control group. The lesions were poorly delineated on the Gd‐enhanced images. Average maximum lesion sizes (mean ± 95% confidence interval) were 22 ± 4.3 mm after ligation of PV, 22 ± 2.6 mm after ligation of both PV and HA, 14 ± 2.0 mm after ligation of HA, and 13 ± 1.9 mm in the control group. We accept the hypothesis that the diameter of the region of coagulation necrosis achieved by standardized RF ablation in the liver increases with reduced organ perfusion and that this effect can be accurately monitored using MRI. The major factor influencing the size of the coagulation area is the portal venous flow. Occlusion of the hepatic artery alone does not significantly increase lesion size. T2w sequences are best suited for postprocedure imaging due to the high lesion‐to‐liver CNR in rabbits with normal hepatic perfusion. J. Magn. Reson. Imaging 2001;13:57–63.

MRI-guided radiofrequency thermal ablation of implanted VX2 liver tumors in a rabbit model : Demonstration of feasibility at 0.2 T

Successful radiofrequency (RF) thermal ablation was performed on VX2 tumors implanted in 23 rabbit livers under magnetic resonance (MR) guidance using a C‐arm‐shaped low‐field 0.2 T system. RF application and immediate postprocedure MRI of all animals was performed [T2‐weighted, turbo short tau inversion recovery (STIR), T1‐weighted before and after gadopentetate dimeglumine administration). Follow‐up MRI with a superparamagnetic iron oxide (SPIO) contrast medium was performed in nine rabbits at 2 weeks and in four rabbits at 1 month post RF ablation. All livers were harvested for pathologic examination. T2‐weighted and turbo‐STIR images demonstrated the highest tumor‐to‐RF‐thermal lesion contrast‐to‐noise ratios (CNRs; means 4.5 and 3.8, respectively) on postprocedure images; this was redemonstrated at 2‐ and 4‐week follow‐up imaging. T2‐weighted imaging never overestimated pathologic lesion size by more than 2 mm, and the radiologic‐pathologic correlation coefficient was not less than 0.90. In conclusion, MRI‐guided RF thermal ablation in implanted liver tumor is feasible using a C‐arm‐shaped low‐field 0.2 T system. The thermal lesion size can be most accurately monitored with T2‐weighted and turbo‐STIR images. Magn Reson Med 42:141–149, 1999.