Peter Erhart

Single-loop coil concepts for intravascular magnetic resonance imaging

Compared with other coil designs that have been investigated for intravascular use, the single‐loop coil can be designed with a very small diameter for insertion into small vessels and with a longitudinal extent over several centimeters for multislice imaging. If it designed to be expandable inside the target vessel, then it combines these features with increased signal‐to‐noise ratio (SNR) and penetration depth. Expandable single‐loop coils that are capable of meeting these requirements were developed and integrated into two different commercial catheter‐based delivery systems: a self‐expandable, single‐loop made from NiTinol and a single‐loop coil mounted on an inflatable balloon. The influence of a small‐diameter coaxial cable for remote tuning and matching on the coil performance was investigated. Calculations showed the dependence of the signal on the separation between the conductors. The comparison of both catheter approaches in in vitro flow experiments and in an in vivo pig experiment revealed the influence of pulsatile flow on image quality during intravascular imaging with these designs. Magn Reson Med 41:751–758, 1999.

Temperature quantification using the proton frequency shift technique: In vitro and in vivo validation in an open 0.5 tesla interventional MR scanner during RF ablation.

Open magnetic resonance (MR) scanners allow MR‐guided targeting of tumors, as well as temperature monitoring of radio frequency (RF) ablation. The proton frequency shift (PFS) technique, an accurate and fast imaging method for temperature quantification, was used to synthesize thermal maps after RF ablation in an open 0.5 T MR system under ex vivo and in vivo conditions. Calibration experiments with 1.5% agarose gel yielded a chemical shift factor of 0.011 ± 0.001 ppm/°C (r2 = 0.96). Three gradient echo (GRE) pulse sequences were tested for thermal mapping by comparison with fiberoptic thermometer (Luxtron Model 760) readings. Temperature uncertainty decreased from high to low bandwidths (BW): ±5.9°C at BW = 15.6 kHz, ±1.4°C at BW = 3.9 kHz, and ±0.8°C at BW = 2.5 kHz. In vitro experiments (N = 9) in the paraspinal muscle yielded a chemical shift factor of 0.008 ± 0.001 ppm/°C. Temperature uncertainty was determined as ±2.7°C (BW = 3.9 kHz, TE = 19.3 msec). The same experiments carried out in the paraspinal muscle (N = 9) of a fully anesthetized pig resulted in a temperature uncertainty of ±4.3°C (BW = 3.9 kHz, TE = 19.3 msec), which is higher than it is in vitro conditions (P < 0.15). Quantitative temperature monitoring of RF ablation is feasible in a 0.5 T open‐configured MR scanner under ex vivo and in vivo conditions using the PFS technique. J. Magn. Reson. Imaging 2001;13:437–444.

Magnetic resonance-guided laparoscopic interstitial laser therapy of the liver

BACKGROUND There is a necessity for an imaging method during laparoscopy to get a three-dimensional access to the target. In this study we evaluated laparoscopic interstitial laser therapy of the liver under magnetic resonance imaging guidance. METHODS Five domestic pigs underwent laparoscopy in an open-configuration magnetic resonance system. Under simultaneous real-time magnetic resonance imaging interstitial laser therapy was applied to the liver. Magnetic resonance images, macroscopic aspects of the lesions, and light microscopic findings were compared. RESULTS The interventions could be safely performed. There was no image artifact caused by instruments or by the carbon dioxide. Dynamic gadolinium-enhanced imaging proved to significantly predict the macroscopic volume of the laser lesions. CONCLUSIONS Magnetic resonance-guided laparoscopic interstitial laser therapy of the liver combines the advantages of minimal invasive surgery and magnetic resonance imaging. Further development should focus on laparoscopic instruments and temperature sensitive sequences.

MR-guided cholecystostomy : Assessment of biplanar, real-time needle tracking in three pigs

PurposeTo demonstrate the feasibility of magnetic resonance (MR)-guided cholecystostomy using active, real-time, biplanar MR tracking in animal experiments.MethodsExperiments were performed on three fully anesthetized pigs in an interventional MR system (GE open). The gallbladder was displayed in two orthogonal planes using a heavily T2-weighted fast spin-echo sequence. These “cholangio roadmaps” were displayed on LCD monitors positioned in front of the interventionalist. A special coaxial MR-tracking needle, equipped with a small receive-only coil at its tip, was inserted percutaneously into the gallbladder under continuous, biplanar MR guidance. The MR-tracking sequence allowed sampling of the coil (needle tip) position every 120 msec. The position of the coil was projected onto the two orthogonal “cholangio roadmap” images.ResultsSuccessful insertion of the needle was confirmed by aspiration of bile from the gallbladder. The process of aspiration and subsequent instillation of Gd-DTPA into the gallbladder was documented with fast gradient-recalled echo imaging.ConclusionBiplanar, active, real-time MR tracking in combination with “cholangio roadmaps” allows for cholecystostomies in an interventional MRI environment.

Imaging temperature changes in an interventional 0.5 T magnet: In-vitro results

To evaluate the ability of monitoring laser induced temperature changes in an open, interventional 0.5T magnet, adopting fast T1‐weighted sequences.

Biopsy Needle Susceptibility Artifacts in Magnetic Resonance Imaging

Introduction Magnetic resonance imaging is attracting attention äs a potential interventional monitoring modality. MRI-guided biopsies are one of the simpler interventional procedures. One possibility is to directly visualize the needle with MRI. However, its appearance is largely determined by the susceptibility artifact. This paper presents the results of Computer simulations of the image distortion resulting from the magnetic susceptibility difference between the needle and the surrounding tissue. The simulations show not only an artifact size which is dependent on needle composition, orientation, and pulse sequence, but also a corresponding shift of the artifact center away from the actual needle center. This shift places limits on the accuracy of needle tip placement.