Chris J.G. Bakker

Heating Around Intravascular Guidewires by Resonating RF Waves

We examined the unwanted radiofrequency (RF) heating of an endovascular guidewire frequently used in interventional magnetic resonance imaging (MRI). A Terumo guidewire was partly immersed in an oblong saline bath to simulate an endovascular intervention. The temperature rise of the guidewire tip during an FFE sequence [average specific absorption rate (SAR) = 3.9 W/kg] was measured with a Luxtron fluoroscopic fiber. Starting from 26°C, the guidewire tip reached temperatures up to 74°C after 30 seconds of scanning. Touching the guidewire may cause sudden heating at the point of contact, which in one instance caused a skin burn. The excessive heating of a linear conductor like the guidewire can only be explained by resonating RF waves. The capricious dependencies of this resonance phenomenon on environmental factors have severe consequences for predictability and safety guidelines. J. Magn. Reson. Imaging 2000;12:79–85.

Passive tracking exploiting local signal conservation: The white marker phenomenon

This article presents a novel approach to passive tracking of paramagnetic markers during endovascular interventions, exploiting positive contrast of the markers to their background, so‐called “white marker tracking.” The positive contrast results from dephasing of the background signal with a slice gradient, while near the marker the signal is conserved because a dipole field induced by the marker compensates the dephasing gradient. Theoretical investigation shows that a local gradient induced by the local dipole field will nearly always cancel the dephasing gradient somewhere, regardless of marker composition, gradient strength, orientation, and acquisition parameters. The actual appearance of the white marker is determined by the marker strength, echo‐time, slice thickness, and gradient strength, as shown both theoretically and experimentally. The novel concept is demonstrated by tracking experiments in a flow phantom and in pig models and is shown to allow reliable and robust depiction of paramagnetic markers with positive contrast and significant suppression of the background signal. Magn Reson Med 50:784–790, 2003.

Measuring the arterial input function with gradient echo sequences.

The measurement of the arterial input function by use of gradient echo sequences was investigated by in vitro and in vivo experiments. First, calibration curves representing the influence of the concentration of Gd‐DTPA on both the phase and the amplitude of the MR signal were measured in human blood by means of a slow‐infusion experiment. The results showed a linear increase in the phase velocity and a quadratic increase in ΔR  *2 as a function of the Gd‐DTPA concentration. Next, the resultant calibration curves were incorporated in a partial volume correction algorithm for the arterial input function determination. The algorithm was tested in a phantom experiment and was found to substantially improve the accuracy of the concentration measurement. Finally, the reproducibility of the arterial input function measurement was estimated in 16 patients by considering the input function of the left and the right sides as replicate measurements. This in vivo study showed that the reproducibility of the arterial input function determination using gradient echo sequences is improved by employing a partial volume correction algorithm based on the calibration curve for the contrast agent used. Magn Reson Med 49:1067–1076, 2003.

Evaluation of permanent I-125 prostate implants using radiography and magnetic resonance imaging

PURPOSE The aim of this study is the evaluation of permanent I-125 prostate implants using radiography and magnetic resonance imaging (MRI). METHODS AND MATERIALS Twenty-one patients underwent radiography on the simulator and MRI within 3 days after implantation of the I-125 seeds. Isocentric radiographs were used for reconstruction of the seed distribution, after which registration with the seed-induced signal voids on MRI provided the seed positions in relation to the prostate. The prostate was contoured on the transversal magnetic resonance images, and dose-volume histograms were computed to evaluate the implants. The validity of the ellipsoidal prostate volume approximation, as applied in preimplant dose calculation, was assessed by comparison of ellipsoidal volumes given by prostate width, height, and length and prostate volumes obtained by a slice-by-slice contouring method, both on postimplant MRI. Prostate volume changes due to postimplant prostate swelling were assessed from radiographs taken at 3 days and 1 month after the implantation. RESULTS The seeds were readily identified on T1-weighted spin-echo images and matched with the seed distribution reconstructed from the isocentric radiographs. The matching error, averaged over 21 patients, amounted to 1.8 +/- 0.4 mm (mean +/- standard deviation). The fractions of the prostate volumes receiving the prescribed matched peripheral dose (MPD) ranged from 32 to 71% (mean +/- standard deviation: 60 +/- 10%). Prostate volumes, obtained by the contouring method on postimplant MRI, were a factor 1.5 +/- 0.3 larger than the ellipsoidal volumes given by the prostate dimensions on postimplant MRI. Prostate volumes 3 days after the implantation were a factor 1.3 +/- 0.2 larger than the prostate volumes 1 month after the implantation. Registration of the reconstructed seed distribution and the MR images showed inaccuracies in seed placement, for example, two or more seeds clustering together or seeds outside the prostate. CONCLUSIONS Registration of the reconstructed seed distribution and the MR images enabled evaluation of target coverage, which amounted to 60 +/- 10%. The discrepancy between prescribed dose and realized dose was caused by underestimation of the preimplant prostate volume due to the ellipsoidal approximation, postimplant prostate swelling at the time of evaluation, and inaccuracies in seed placement.

Correcting partial volume artifacts of the arterial input function in quantitative cerebral perfusion MRI

To quantify cerebral perfusion with dynamic susceptibility contrast MRI (DSC‐MRI), one needs to measure the arterial input function (AIF). Conventionally, one derives the contrast concentration from the DSC sequence by monitoring changes in either the amplitude or the phase signal on the assumption that the signal arises completely from blood. In practice, partial volume artifacts are inevitable because a compromise has to be reached between the temporal and spatial resolution of the DSC acquisition. As the concentration of the contrast agent increases, the vector of the complex blood signal follows a spiral‐like trajectory. In the case of a partial‐volume voxel, the spiral is located around the static contribution of the surrounding tissue. If the static contribution of the background tissue is disregarded, estimations of the contrast concentration will be incorrect. By optimizing the correspondence between phase information and amplitude information one can estimate the origin of the spiral, and thereupon correct for partial volume artifacts. This correction is shown to be accurate at low spatial resolutions for phantom data and to improve the AIF determination in a clinical example. Magn Reson Med 45:477–485, 2001.

Simultaneous quantitative cerebral perfusion and Gd-DTPA extravasation measurement with dual-echo dynamic susceptibility contrast MRI.

Quantification of cerebral perfusion using dynamic susceptibility contrast MRI generally relies on the assumption of an intact blood–brain barrier. The present study proposes a method to correct the tissue response function that does not require this assumption, thus, allowing perfusion studies in, for example, high‐grade brain tumors. The correction for contrast extravasation in the tissue during the bolus passage is based on a two‐compartment kinetic model. The method separates the intravascular hemodynamic response and the extravascular component and returns the corrected tissue response function for perfusion quantification as well as the extravasation rate constant of the vasculature. Results of simulation experiments with different degrees of contrast extravasation are presented. The clinical potential is illustrated by determination of the perfusion and extravasation of a glioblastoma multiforme. The correction scheme proves to be fast and reliable even in cases of low signal‐to‐noise ratio. It is applicable whether extravasation occurs or not. When extravasation is present, application of the proposed method is mandatory for accurate cerebral blood volume measurements. Magn Reson Med 43:820–827, 2000.

Susceptibility artifacts in 2DFT spin-echo and gradient-echo imaging: The cylinder model revisited

Susceptibility-induced geometry and intensity distortions are a familiar observation in MR imaging. In the past few years several attempts have been made to aid in the understanding of susceptibility artifacts by means of simulation studies. Although these studies, which were mostly carried out with simple test objects, have produced some qualitative insight into chi-artifacts, the results lacked precision in describing finer details. In this paper we show the discrepancy between theory and experiment in previous work to be the result of an inadequate theoretical approach. In most studies so far, delta B0 effects are taken into account in the frequency domain, that is, after Fourier transformation of the data. In our view the simulation should follow the actual sequence of events in an imaging experiment and deal with the effect of error fields in the time domain (k-space) already. The correctness of this view is demonstrated here by comparing the results of time and frequency domain simulation against experimental observation for a coaxial cylinder phantom, a widely used model in this type of work. Having established the superiority of the time domain simulation, we demonstrate its use in predicting chi-artifacts under various experimental conditions, for example, in spin-echo and gradient-echo imaging with a reduced number of phase-encoding steps.

Numerical analysis of the magnetic field for arbitrary magnetic susceptibility distributions in 2D

We describe a numerical technique for calculating the 2D magnetic field in arbitrary magnetic susceptibility distribution. The technique we used is the explicit finite difference method with an addition of the Du Fort-Frankle algorithm. The proposed algorithm is unconditionally stable and has excellent convergence properties. For simple geometries, numerical results were compared against analytical solutions and appeared to be in excellent agreement.

Improved lumen visualization in metallic vascular implants by reducing RF artifacts.

In this study, a method is proposed for MRI of the lumen of metallic vascular implants, like stents or vena cava filters. The method is based on the reduction of artifacts caused by flow, susceptibility, and RF eddy currents. Whereas both flow artifacts and susceptibility artifacts are well understood and documented, RF artifacts are not. Therefore, the present study comprises an in‐depth theoretical explanation of the factors governing the severity of these RF artifacts. It is explained that the RF caging inside cage‐like implants is caused by disturbances of the send and receive sensitivities due to coupling between the loops in the implant and the MR scanners send and receive coils. A scaled excitation angle model describing the behavior of the signal intensity inside the implants as a function of the applied nominal excitation angle is introduced. This theoretical model was validated in phantom experiments. Reduced signal from within implants due to the caging problem could be restored by increasing the applied RF power in the excitation pulse, without exceeding the generally accepted SAR safety limits. The method was tested in vitro and in vivo in a pig model and allowed adequate depiction of the interior of a nitinol stent and that of a vena cava filter in contrast‐enhanced MR angiograms. Magn Reson Med 47:171–180, 2002.

Maximum likelihood estimation of cerebral blood flow in dynamic susceptibility contrast MRI.

For quantification of cerebral blood flow (CBF) using dynamic susceptibility contrast magnetic resonance imaging (DSC‐MRI), knowledge of the tissue response function is necessary. To obtain this, the tissue contrast passage measurement must be corrected for the arterial input. This study proposes an iterative maximum likelihood expectation maximization (ML‐EM) algorithm for this correction, which takes into account the noise in T2‐ or T*2‐weighted image sequences. The ML‐EM algorithm does not assume a priori knowledge of the shape of the response function; it automatically corrects for arrival time offsets and inherently yields positive response values. The results on synthetic image sequences are presented, for which the recovered flow values and the response functions are in good agreement with their expectation values. The method is illustrated by calculating the gray and white matter flow in a clinical example. Magn Reson Med 41:343–350, 1999.