Faculty Bibliography

PURPOSE: To quantify femoral-popliteal vessel deformation during thigh contraction. MATERIALS AND METHODS: Eleven subjects underwent a magnetic resonance (MR) examination of the femoral-popliteal vasculature on a 1.5 T system. A custom 3D balanced steady-state free precession (SSFP) sequence was implemented to image a 15-20-cm segment of the vasculature during relaxation and voluntary isometric thigh contraction. The arterial and venous lumina were outlined using a semiautomated method. For the artery, this outline was fit to an ellipse whose aspect ratio was used to describe arterial deformation, while venous deformation was characterized by its cross-sectional area. RESULTS: Focal compression of the femoral-popliteal artery during contraction was observed 94-143 mm superior to the condyle that corresponds to the distal adductor canal (AC) immediately superior to the adductor hiatus. This was illustrated by a significant reduction (P < or = 0.05) in aspect ratio from 0.88 +/- 0.06 during relaxation to 0.77 +/- 0.09 during contraction. A negligible change in arterial aspect ratio was observed inferior to the AC and in the proximal AC. Similarly, venous area was dramatically reduced in the distal AC region during contraction. CONCLUSION: Rapid 3D SSFP MR angiography of the femoral-popliteal vasculature during thigh contraction demonstrated focal compression of the artery in the distal AC region. This may help explain the high stent failure rate and the high likelihood of atherosclerotic disease in the AC. J. Magn. Reson.

For pharmacokinetic modeling of tissue physiology, there is great interest in measuring the arterial input function (AIF) from dynamic contrast-enhanced (DCE) magnetic resonance imaging (MRI) using paramagnetic contrast agents. Due to relaxation effects, the measured signal is a nonlinear function of the injected contrast agent concentration and depends on sequence parameters, system calibration, and time-of-flight effects, making it difficult to accurately measure the AIF during the first pass. Paramagnetic contrast agents also affect susceptibility and modify the magnetic field in proportion to their concentration. This information is contained in the MR signal phase which is discarded in a typical image reconstruction. However, quantifying AIF through contrast agent susceptibility induced phase changes is made difficult by the fact that the induced magnetic field is nonlocal and depends upon the contrast agent spatial distribution and thus on organ and vessel shapes. In this article, the contrast agent susceptibility was quantified through inversion of magnetic field shifts using a piece-wise constant model. Its feasibility is demonstrated by a determination of the AIF from the susceptibility-induced field changes of an intravenous bolus. After in vitro validation, a time-resolved two-dimensional (2D) gradient echo scan, triggered to diastole, was performed in vivo on the aortic arch during a bolus injection of 0.1 mmol/kg Gd-DTPA. An approximate geometrical model of the aortic arch constructed from the magnitude images was used to calculate the spatial variation of the field associated with the bolus. In 14 subjects, Gd concentration curves were measured dynamically (one measurement per heart beat) and indirectly validated by independent 2D cine phase contrast flow rate measurements. Flow rate measurements using indicator conservation with this novel quantitative susceptibility imaging technique were found to be in good agreement with those obtained from the cine phase contrast measurements in all subjects. Contrary to techniques that rely on intensity, the accuracy of this signal phase based method is insensitive to factors influencing signal intensity such as flip angle, coil sensitivity, relaxation changes, and time-of-flight effects extending the range of pulse sequences and contrast doses for which quantitative DCE-MRI can be applied.

Magnetic properties characterized by susceptibility and chemical shift linearly modify the local magnetic field experienced by spins. A piece-wise constant solution using magnetic resonance imaging is found to the challenging inversion problem from field to magnetic properties. The magnetic field shifts were estimated from MR phase images. The MR magnitude images were segmented into many regions of uniform magnetic properties. Standard linear regression using the calculated magnetic field from each region allowed accurate susceptibility quantification. The technique was experimentally validated on a variety of samples including water, vegetable oil, air, Gadolinium, and superparamagnetic iron oxides. Susceptibility was measured with a precision better than 0.1 ppm, in a range of 10 ppm. In vivo feasibility was shown on the forearm for which soft-tissue, cortical bone, and bone marrow susceptibility, and chemical shift values in good agreement with literature data were obtained.

PURPOSE: To investigate the application of a coil array consisting of multiple birdcages for bolus chase magnetic resonance angiography (MRA) of the lower extremities. MATERIALS AND METHODS: The prototype consisted of four birdcage coils; two adjacent birdcages for thigh imaging, and two for calf imaging. Decoupling between adjacent coils was achieved using shared capacitors. Bench measurements and MR images were used to evaluate the decoupling scheme. Image signal-to-noise ratios (SNR) were compared between the birdcage array and four commercially available coils. Contrast-enhanced imaging experiments were performed on 10 volunteers and parallel imaging was simulated. This study was approved by the local institutional review board and written informed consent was obtained from each volunteer. RESULTS: Capacitive decoupling resulted in a reduction in signal leakage. The calf birdcages provided an 84% SNR improvement over a four element array, while the thigh birdcages provided a 53% improvement. Angiographic images illustrated the utility of the coil for peripheral MRA. Parallel imaging was demonstrated with a two-fold reduction factor. CONCLUSION: Birdcage coils were demonstrated to be valuable for lower extremity imaging due to their homogenous sensitivity, good SNR, and cylindrical geometry. Coupling was controlled using shared capacitors that allowed a single birdcage to encompass each leg individually, providing a novel approach to signal reception for peripheral imaging.

Noise correlation between multiple receiver coils is discussed using principles of statistical physics. Using the general fluctuation-dissipation theorem we derive the prototypic correlation formula originally determined by Redpath (Magn Res Med 1992;24:85-89), which states that correlation of current spectral noise depends on the real part of the inverse impedance matrix at a given frequency. A distinct correlation formula is also derived using the canonical partition function, which states that correlation of total current noise over the entire frequency spectrum depends on the inverse inductance matrix. The Kramers-Kronig relation is used to equate the inverse inductance matrix to the spectral integral of the inverse impedance matrix, implying that the total noise is equal to the summation of the spectral noise over the entire frequency spectrum. Previous conflicting arguments on noise correlation may be reconciled by differentiating between spectral and total noise correlation. These theoretical derivations are verified experimentally using two-coil arrays

The optimization problem for coil arrays is largely unsolved, even for the case of a two-coil system. This paper reports a systematic computer simulation to investigate the maximal achievable signal-to-noise ratio (SNR) with a two-coil receiver system where, using cancellation circuitry, mutual inductance is made zero. Both symmetrical and asymmetrical solutions with respect to two-coil geometry are considered. SNR is measured at a single point at a certain depth and also along a longitudinal or transverse line at the same depth. The conducting medium containing these regions of interest is assumed to be an infinite half space, an infinite cylinder or a finite sphere. The previous coil array design using a "magical" overlap only approximates the optimal solution for the infinite half space. For the infinite cylinder and the finite sphere, optimal solutions can be quite different from the "magical" overlap.

A receive-only phased-array coil was designed to image the lower extremities. The array consists of four volume coils placed on two cylindrical formers. The coil array has the ability to image both legs simultaneously over a 40 cm longitudinal field of view (FOV). Experiments using phantoms show an increase in signal-to-noise ratio (SNR) in regions of interest through the center of the coil by an average factor of 2.8 over the body coil and 1.5 over the GE 4-channel torso array. In vivo data acquired from 10 subjects show that the X array provided similar SNR improvement in spin-echo images and more vascular details in angiographic images compared to the torso array.