Mark E. Ladd

Reduction of resonant RF heating in intravascular catheters using coaxial chokes

The incorporation of RF coils into the tips of intravascular devices has been shown to enable the localization of catheters and guidewires under MR guidance. Furthermore, such coils can be used for endoluminal imaging. The long cable required to connect the coil with the scanner input inadvertently acts as a dipole antenna which picks up RF energy from the body coil during transmit. Currents are induced on the cable which can lead to localized heating of surrounding tissue. Cables of various lengths were measured to determine if a resonance in the heating as a function of cable length could be found. Coaxial chokes with a length of λ/4 were added to coaxial cables to reduce the amplitude of the currents induced on the cable shield. A 0.7‐mm diameter triaxial cable, small enough to fit into a standard intravascular device, was developed and measured both with and without a coaxial choke. It is demonstrated that resonant heating does occur and that it can be significantly reduced by avoiding a resonant length of cable and by including coaxial chokes on the cable. Magn Reson Med 43:615–619, 2000.

Vascular stents as RF antennas for intravascular MR guidance and imaging.

Stent deployment is used to improve the immediate and long‐term results of vascular interventions in various vascular sites. X‐ray angiography as an imaging modality is often limited in providing an accurate assessment with regard to vessel size, plaque calcification, or stent deployment. In this study, the potential of using the stent endoprothesis as a radiofrequency (RF) receive‐only probe for MR guidance and lesion imaging was investigated. Three different principles were developed to visualize stents actively, the first employing the stent as a loop antenna, the second employing the stent in an electrical dipole configuration, and the third employing the stent in a hybrid configuration as a coaxial line antenna. The three configurations resulted in different signal characteristics. Based on two of these antenna configurations, stent deployment devices were built and evaluated in in vitro as well as in vivo sheep experiments. Active stent visualization allows real‐time MR guidance through the vessel tree and monitoring of stent deployment. In addition, the stent antenna may become useful for high resolution imaging of the vessel wall. Magn Reson Med 42:738–745, 1999.

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.

Autoperfused balloon catheter for intravascular MR imaging

An intravascular magnetic resonance (MR) imaging catheter for high‐resolution imaging of vessel walls was developed. The catheter design is based on an autoperfusion balloon catheter that allows passive perfusion of blood during balloon inflation. The blood enters a central lumen through multiple sideholes of the catheter shaft proximal to the balloon. A remotely tuned, matched, and actively decoupled, expandable single‐loop radiofrequency coil was mounted onto the balloon to receive intravascular MR signals. The autoperfusion rate through the catheter was determined experimentally relative to perfusion pressure. The catheter concept was evaluated in vitro on human femoral artery specimens and in vivo in the internal carotid artery of two pigs. The proposed catheter design allowed for maintained blood perfusion during the acquisition of high‐resolution intravascular images. During perfusion, image quality remained unaffected by flow, motion, and pulsatility artifacts. The availability of an autoperfused intravascular catheter design can be considered an important step toward high‐resolution atherosclerotic plaque imaging in critical vessels such as the carotid and coronary arteries. J. Magn. Reson. Imaging 1999; 9:428–434.

Prenatal diagnosis of fetal malformations by ultrafast magnetic resonance imaging

Prenatal ultrasonography is the primary imaging modality in pregnancy as it allows direct real‐time fetal examination. Antenatal magnetic resonance imaging (MRI) has so far been of limited clinical value owing to poor image quality. This was due to the long acquisition times that were needed to achieve a high enough spatial resolution for assessment of the small fetal anatomic structures resulting in severe motion artefacts. This problem has now been overcome by recent technical improvements.

Magnetresonanztomographie und -spektroskopie

Seit der Einfuhrung der Magnetresonanztomographie (MRT), fruher auch Kernspintomographie oder bildgebende „Nuclear Magnetic Resonance“ (NMR) genannt, Anfang der 1980er-Jahre, hat sich diese Untersuchungstechnik zu einer weitverbreiteten medizinischen Bildgebungsmethode entwickelt, die unterschiedlichste anatomische Regionen abbildet und verschiedenste morphologische und funktionelle Fragestellungen beantwortet. Dieses Kapitel soll einen Einblick in diese vielseitige diagnostische Technologie und ihre Moglichkeiten geben. Zunachst wird ein kurzer Uberblick uber die geschichtliche Entwicklung der Methode gegeben, im Folgenden werden nahere Details zur Entstehung des MR-Signals und dessen Verarbeitung erlautert. Es wird erklart, welche Hardware-Voraussetzungen erforderlich sind, um Bilder anzufertigen. Dann wird erlautert, wie Bildkontraste entstehen und welche kernphysikalischen Eigenschaften die MRT-Schnittbilder eigentlich abbilden.

MR Angiography and High Field Strength: 3.0 T and Higher

The motivation for performing MR angiography (MRA) at higher magnetic field strength can be appreciated by answering a few simple questions: Would you like to increase your spatial resolution in time-of-flight or contrast-enhanced (CE) MRA, increase your temporal resolution in dynamic MRA applications, decrease your contrast agent dose in CE-MRA, or even use new imaging contrasts for MRA not available at lower field strength? Higher static magnetic field strengths open opportunities in all of these areas.

Guidance of Intravascular Therapeutic Procedures: Current Status and Future Prospects

For most of its short history, the clinical utilization of magnetic resonance (MR) has been largely confined to the area of diagnostic imaging. However, excellent soft tissue contrast and multiplanar imaging capabilities have motivated several groups to explore the possibility of using MR to guide simple interventional procedures. These were primarily biopsies (Mueller et al. 1986; Lufkin et al. 1987; Van Sonnenberg et al. 1988; Orel et al. 1994), but interest has also been shown regarding vascular interventions (Rubin et al. 1990; Kochli et al. 1994; Bakker et al. 1996, 1997). Insufficient patient access afforded by the long magnet tunnel limited initial success.