Catheter Coils

Magnetic resonance imaging (MRI) is a very powerful tool for diagnostic imaging; however, it is very seldom used for guiding interventional procedures. On the other hand, in many procedures the quality of the guiding image modality is less than perfect, and therefore, MRI can be considered as an alternative. Among these interventional procedures, the percutaneous cardiovascular procedures have a special place. Today, most of these procedures are conducted under the guidance of X-ray fluoroscopy. Using this guidance method, the catheters include guidewires that are visible, but this projection-based imaging modality provides very little information on the tissue or organ of interest. MRI appears to be a natural alternative to X-ray based imaging for this purpose because of its high tissue contrast. In addition, MRI has no ionizing radiation. High X-ray exposure to the patient, especially to children, can be harmful. This is also a significant problem for the physician who is conducting the procedure on a daily basis. MRI solves this problem as well. Therefore, MRI has a great potential to replace X-ray in guiding some of the percutaneous cardiovascular procedures. While the advantages of MRI over X-ray fluoroscopy are apparent, it is also very obvious why currently MRI cannot replace X-rays. The patient access is poor in MRI scanners, although development of short and wide-bore magnets alleviated this problem. MRI-compatible equipment such as patient monitoring systems has limited availability. Most catheters and guidewires are not compatible with MRI. While these or similar problems are real, they are not fundamental limitations. The most important limitation of MRI compared with X-ray fluoroscopy is its poor visualization of the interventional devices. In MRI, most interventional devices appear dark, and therefore it is possible to visualize them only if a thin slice imaging method is used. The interventional devices get lost in the body when thick slice imaging is used. This very fundamental problem of the poor visualization of catheters gave rise to the research field of catheter tracking under MRI. This effort resulted in the development of the catheter coils, i.e., the catheters that can be tracked using the MRI signals that are received by small coils embedded inside the catheters. The development of catheter coils did not have a single motivation. The other motivation is to obtain high-resolution images of the blood vessels. As is well known in the field of MRI, the shape, size, and position of the receiver radiofrequency (RF) coils have critical importance for the signal-to-noise ratio (SNR) of the acquired images. With rather straightforward manipulation of the pulse sequences, the increase in the SNR can be used for higher image resolution. As the RF coils get closer to the point of interest, the SNR is expected to be improved and therefore higher image resolution can be obtained. With this motivation, researchers have investigated the placement of small receiver coils, so-called catheter coils, inside the blood vessels. The interest in development of catheter coils developed immediately after commercial MRI scanners became available. In 1984, Dr Howard Kantor, while working at the NIH, tested the first catheter coil concept for increasing the SNR in 31P NMR spectra.1 As is well known, the 31P signal is very weak compared to the signal of proton and therefore obtaining a clinically useful result is often difficult. On the other hand, one of the key components in the SNR in a magnetic resonance experiment is the position and the design of the receiver coil. SNR can be increased significantly by reducing the size of the coil and placing it close to the region of interest. In Dr Kantor’s case, the region of interest was the heart. He and his colleagues placed a small loop (7.5 × 24 mm, two turns) inside the heart. They tuned the coil using a single capacitor and obtained 31P spectra of the myocardium, as shown in Figure 1. This pioneering approach triggered interest in catheter coils and resulted in many more publications on the subject. Three research groups led by Dr Gregory C. Hurst of Case Western Reserve University,2 Dr Alastair J. Martin of the University of Toronto,3 and Dr Krishna Kandarpa of Brigham and Women’s Hospital4 demonstrated the use of catheter coils for high-resolution imaging of blood vessels almost simultaneously. While the designs of Dr Hurst and Dr Martin were based on opposed solenoid coils (Figure 2), the design of Dr Kandarpa was a small rectangular loop (elongated loop) placed in an 8 Fr (1 Fr is 1/3 mm) catheter (Figure 3). Using this design, Dr Kandarpa showed high-resolution images of excised human arteries. These designs are discussed later. In the mean time, the first catheter tracking was initiated by Dr Jerome L. Ackerman of Massachusetts General Hospital (MGH) with his pioneering work published as a 1986 Society of Magnetic Resonance in Medicine Annual Meeting abstract.5 In this work, Dr Ackerman explains the use of a small RF coil for tracking the position of interventional devices (Figure 4). Later, this work was extended by Dr Charles L. Dumoulin in a 1993 article in an elegant way.6 In this method, Dr Dumoulin applies non-slice-selective RF pulses followed by three orthogonal readout gradients to obtain the projection of the catheter in three orthogonal axes. From this information, it becomes rather trivial to find the location of the tip of the catheter. This method was later used by many researchers. In the next section, the designs of catheter coils are discussed.