Thomas Kwok-Fah Foo

Automated bolus chase peripheral MR angiography: Initial practical experiences and future directions of this work-in-progress†

Bolus chase 3‐dimensional MR angiography (3D MRA) is a recent development that extends the effective field of view for arterial imaging from the typical single 40–50 cm to over 100 cm. This technique is well suited for imaging long vascular territories such as the lower extremity. Bolus chase peripheral 3D MRA is achieved with overlapping 3D gradient‐echo scans during the arterial transit of a single intravenous injection of gadolinium‐chelate contrast media. This technique can depict the arteries from the infrarenal aorta to the ankles in less than 2 minutes. The initial experiences with bolus chase peripheral MRA using an automated algorithm that controls both table translation and 3D data acquisition are described. Suggestions for future refinements to the technique are also discussed. J. Magn. Reson. Imaging 1999;10:376–388. Published 1999 Wiley‐Liss, Inc.

128-channel body MRI with a flexible high-density receiver-coil array.

To determine whether the promise of high‐density many‐coil MRI receiver arrays for enabling highly accelerated parallel imaging can be realized in practice.

Electroanatomic Mapping of the Left Ventricle in a Porcine Model of Chronic Myocardial Infarction With Magnetic Resonance–Based Catheter Tracking

Background— X-ray fluoroscopy constitutes the fundamental imaging modality for catheter visualization during interventional electrophysiology procedures. The minimal tissue discriminative capability of fluoroscopy is mitigated in part by the use of electroanatomic mapping systems and enhanced by the integration of preacquired 3-dimensional imaging of the heart with computed tomographic or magnetic resonance (MR) imaging. A more ideal paradigm might be to use intraprocedural MR imaging to directly image and guide catheter mapping procedures. Methods and Results— An MR imaging–based electroanatomic mapping system was designed to assess the feasibility of navigating catheters to the left ventricle in vivo using MR tracking of microcoils incorporated into the catheters, measuring intracardiac ventricular electrograms, and integrating this information with 3-dimensional MR angiography and myocardial delayed enhancement images to allow ventricular substrate mapping. In all animals (4 normal, and 10 chronically infarcted swine), after transseptal puncture under fluoroscopic guidance, catheters were successfully navigated to the left ventricle with MR tracking (13 to 15 frames per second) by both transseptal and retrograde aortic approaches. Electrogram artifacts related to the MR imaging gradient pulses were successfully removed with analog and digital signal processing. In all animals, it was possible to map the entire left ventricle and to project electrogram voltage amplitude maps to identify the scarred myocardium. Conclusions— It is possible to use MR tracking to navigate catheters to the left ventricle, to measure electrogram activity, and to render accurate 3-dimensional voltage maps in a porcine model of chronic myocardial infarction, completely in the MR imaging environment. Myocardial delayed enhancement guidance provided dense sampling of the proximity of the infarct and accurate localization of complex infarcts.

Peripheral nerve stimulation characteristics of an asymmetric head-only gradient coil compatible with a high-channel-count receiver array

To characterize peripheral nerve stimulation (PNS) of an asymmetric head‐only gradient coil that is compatible with a commercial high–channel‐count receive‐only array.

Fat‐referenced MR thermometry in the breast and prostate using IDEAL

To demonstrate a three‐echo fat‐referenced MR thermometry technique that estimates and corrects for time‐varying phase disturbances in heterogeneous tissues.

Estimating amounts of iron oxide from gradient echo images

Rat legs directly injected with superparamagnetic iron oxide (SPIO) were studied by dual‐echo, gradient‐echo imaging. The amount of iron injected was estimated using a point dipole model for the SPIO injection site. Saturation magnetization of 6:1 PEG/amino modified silane‐coated iron oxide particles with 5‐ to 6‐nm core and 20–25 hydrodynamic diameter was ∼110 emu/g of iron. Estimates of the amount of iron injected made from signal void volumes surrounding SPIO centers yielded erroneous results varying with sample orientation in the scanner and echo time (TE). For example, a 10 μL, 3‐μg iron injection produced signal void volumes of 80 and 210 μL at TE of 9.8 and 25 ms, respectively, giving apparent iron contents of 6 ± 1 and 10 ± 2 μg respectively. A more effective approach uses the phase difference between two gradient recalled echo images. To estimate iron content, this approach fits the expected (3 cos2θ − 1)/|r|3 spatial phase distribution to the observed phase differences. Extraneous phase effects made fitting phase at a single TE ineffective. With the dual echo method, 18 independent estimates were 2.48 ± 0.26 μg std, independently of sample orientation. Estimates in empty control regions were −90 and −140 ng. A 1‐μg injection indicated 0.5, 1.2, and 1.2 μg. Magn Reson Med, 2009.

Bolus-chase peripheral 3D MRA using a dual-rate contrast media injection.

In this pilot study, using a standard 40 mL gadolinium (Gd) chelate contrast dose, dual‐rate (first 20 mL at 0.5 mL/sec; remaining 20 mL at 1.5 mL/sec) and fixed‐rate (entire 40 mL dose at either 0.7 mL/sec or 2.0 mL/sec) injection schemes for multistation, bolus‐chase magnetic resonance angiography (MRA) were compared in normal volunteers. Signal‐to‐noise ratio, contrast‐to‐noise ratio, and physician preference were determined for nine arterial segments. At the terminal station (calf), the dual‐rate contrast injection improved arterial signal and contrast compared with both fixed‐rate injection schemes and improved subjective vessel appearance compared with the 2.0 mL/sec, but not the 0.7 mL/sec, fixed‐rate scheme. J. Magn. Reson. Imaging 2000;12:769–775.

High slew-rate head-only gradient for improving distortion in echo planar imaging: Preliminary experience.

To investigate the effects on echo planar imaging (EPI) distortion of using high gradient slew rates (SR) of up to 700 T/m/s for in vivo human brain imaging, with a dedicated, head‐only gradient coil.

High resolution imaging using short TE and TR pulse sequences with asymmetric NMR echo acquisition

An NMR system acquires a series of spin-echo signals during a scan and these are used to reconstruct an image. The spin-echo signals are partially acquired by locating the data acquisition window asymmetrically with respect to the echo peak. A homodyne reconstruction is employed to fill in the missing data during the image reconstruction.

High temporal resolution breathheld 3D FIESTA CINE imaging: Validation of ventricular function in patients with chronic myocardial infarction

To develop a gated single‐breathhold, high temporal resolution three‐dimensional (3D) CINE imaging technique and to evaluate its accuracy in volumetric and functional quantification in patients with chronic myocardial infarction.