Intravascular (catheter) NMR receiver probe: preliminary design analysis and application to canine iliofemoral imaging

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.

Catheter-tracking FOV MR fluoroscopy

Recent improvements in intravascular magnetic resonance imaging techniques mandate an accurate method of monitoring the introduction of MR catheter probes into the vessel of interest. For this purpose, a novel imaging protocol and a display method have been designed. First, a roadmap 3D image data set with standard pulse sequences is obtained using an external imaging coil. Subsequently, using very narrow rectangular-FOV fast-spoiled gradient recalled (SPGR), a movie of the percutaneous placement procedure of an MR catheter probe is acquired at a rate of 7.3 frames/second. In this protocol, the probe is used to transmit RF pulses and receive MR signal. A computer program was written for image unwrapping and for displaying the unwrapped movie frames on the roadmap image. In an alternative protocol, the movie frames in two projection angles were acquired in an interleaved fashion. Frames were unwrapped and combined with a 3D roadmap and displayed on a Silicon Graphics workstation equipped with stereovision goggles. Using these methods, percutaneous catheter placement in a phantom and a dog was examined. In conclusion, a new visualization technique for MR catheter placement is proposed. Combining this technique with high resolution intravascular MRI techniques may result in a very useful diagnostic tool for the evaluation of atherosclerosis and other vessel diseases.

Intravascular MR-monitored balloon angioplasty: an in vivo feasibility study

PURPOSE

To develop a new method for monitoring balloon angioplasty by using an intravascular magnetic resonance (MR) imaging technique.

MATERIALS AND METHODS

Nine New Zealand White rabbits were used: seven for technique refinement, including surgery, device insertion, stenosis creation, and MR protocol development; and two for the final MR imaging of the balloon angioplasty. The in vivo experimental method involved insertion of a catheter antenna and a balloon catheter, via femoral arteriotomies bilaterally, into the target site of the upper abdominal aorta, where a stenosis was artificially created by binding a plastic cable tie. Then, the entire process of the dilation of the stenosis with balloon inflation was monitored under MR fluoroscopy.

RESULTS

Catheter insertions were successful, and a 5-mm-long stenosis of the aorta was produced in all nine rabbits. Eight complete balloon angioplasty procedures were satisfactorily monitored and recorded, showing clearly the stenosis of the aorta at the beginning of the procedure, the dilation of the stenosis during the balloon inflation, and the complete opening of the stenosis after balloon dilation.

CONCLUSION

Preliminary results of in vivo balloon angioplasty monitored with intravascular MR imaging are presented. MR fluoroscopy, based on the intravascular MR imaging technique, may represent a potential alternative to x-ray fluoroscopy for guiding interventional treatment of cardiovascular diseases.

MR-guided percutaneous angioplasty: assessment of tracking safety, catheter handling and functionality

PURPOSE

Magnetic resonance (MR)-guided percutaneous vascular interventions have evolved to a practical possibility with the advent of open-configuration MR systems and real-time tracking techniques. The purpose of this study was to assess an MR-tracking percutaneous transluminal angioplasty (PTA) catheter with regard to its safety profile and functionality.

METHODS

Real-time, biplanar tracking of the PTA catheter was made possible by incorporating a small radiofrequency (RF) coil in the catheter tip and connecting it to a coaxial cable embedded in the catheter wall. To evaluate potentially hazardous thermal effects due to the incorporation of the coil, temperature measurements were performed within and around the coil under various scanning and tracking conditions at 1.5 Tesla (T). Catheter force transmission and balloon-burst pressure of the MR-tracking PTA catheter were compared with those of a standard PTA catheter. The dilatative capability of the angioplasty balloon was assessed in vitro as well as in vivo, in an isolated femoral artery segment in a swine.

RESULTS

The degree of heating at the RF coil was directly proportional to the power of the RF pulses. Heating was negligible with MR tracking, conventional spin-echo and low-flip gradient-echo sequences. Sequences with higher duty cycles, such as fast spin echo, produced harmful heating effects. Force transmission of the MR-tracking PTA catheter was slightly inferior to that of the standard PTA catheter, while balloon-burst pressures were similar to those of conventional catheters. The MR-tracking PTA catheter functioned well both in vitro and in vivo.

CONCLUSION

The in vivo use of an MR-tracking PTA catheter is safe under most scanning conditions.

Intravascular ultrasound and magnetic resonance imaging in the assessment of atherosclerotic lesions in rabbit aorta. Correlation to histopathologic findings

RATIONALE AND OBJECTIVES

The authors compare the usefulness of intravascular ultrasound (IVUS) and magnetic resonance imaging (MRI) for quantitation of atherosclerosis in hyperlipidemic rabbits, correlated with histopathology.

METHODS

Magnetic resonance imaging with T1- and T2-weighted spin echo sequences and three-dimensional time-of-flight MR angiography of the abdominal aorta was performed on seven rabbits using a 1.5 T MR imager and a standard head coil. X-ray angiography and IVUS examination (3.5 F/30 MHz imaging catheter) was performed via carotid artery access.

RESULTS

Time-of-flight MR angiography source images provided the best resolution and plaque-lumen contrast in visual comparison between the different MRI sequences. Intra- and interobserver reproducibilities of the lesion thickness and area measurements were similar in IVUS and MRI (Pearson correlations 0.52-0.97; P < 0.01). The measurements from IVUS and MRI correlated closely with each other as well as with those made from histopathologic specimens (Pearson correlations 0.50-0.79; P < 0.001). The measurements from IVUS were somewhat more accurate than those made from MRI.

CONCLUSIONS

Both MRI and IVUS with clinically available imaging equipments are feasible and accurate for the quantitation of experimental atherosclerosis of rabbit aorta.

An expandable intravenous RF coil for arterial wall imaging

An intravenous (iv) radiofrequency (RF) coil is proposed as a means of obtaining high resolution images of artery wall. The anatomic positioning of peripheral artery/vein pairs was investigated and a phantom mimicking the iliac artery/vein pair was constructed. Imaging results, comparing iv coils with external coils, demonstrated a potential 15- to 20-fold increase in signal-to-noise ratio (SNR) with iv coils. The SNR benefit was measured over a cylindrical volume, adjacent to the coil, and typical of artery position. Prototype expandable iv coils were constructed of Cu-Be loops and introduced via an 8-Fr catheter. The effects of local and remote iv coil tuning were investigated and local tuning was found to provide significant SNR benefits. The in vivo performance of iv RF coils was demonstrated in a porcine animal model. The iv coils were found to be an excellent alternative to intraarterial coils.

Intravascular magnetic resonance imaging of aortic atherosclerotic plaque composition

Magnetic resonance imaging (MRI) may be an excellent tool to define atherosclerotic plaque composition, but surface MRI (SMRI) suffers from a low signal-to-noise ratio and low resolution of arterial images. Intravascular MRI (IVMRI) represents a potential solution for acquiring high-quality in vivo images of atherosclerotic plaques. Isolated segments of 11 thoracic human aortas obtained at autopsy were imaged by IVMRI using an intravascular receiver catheter coil designed and built at our institution. Images obtained by IVMRI were compared with corresponding images obtained by SMRI and with histopathological aortic cross sections. The intensity of intimal thickness and plaque components was graded by IVMRI and histopathology using a score of 1 for mild, 2 for moderate, and 3 for severe intensity. IVMRI had an agreement of 75% with histopathology in fibrous cap grading (37.5% expected, kappa = 0.60, P < 0.001) and of 74% in necrotic core grading (39% expected, kappa = 0.57, P < 0.001). Intraplaque calcification was correctly graded by IVMRI in six of the eight plaques in which histopathology recognized calcium. The analysis of intimal thickness showed 80% agreement between IVMRI and histopathology (52% expected, kappa = 0.59, P < 0.001). IVMRI image features were similar to those of SMRI. In addition, IVMRI accurately determined atherosclerotic plaque size in comparison with histopathology and SMRI (slope = 1.25 cm2, r = 0.99, P < 0.001 for luminal area by IVMRI vs histopathology; slope = 0.97 cm2, r = 0.996, P < 0.001 for luminal area by IVMRI vs SMRI). IVMRI has the potential to provide important prognostic information in patients with atherosclerosis because of its ability to accurately assess both plaque composition and size.

MRI and NMR spectroscopy of the lipids of atherosclerotic plaque in rabbits and humans

The early stages of atherosclerosis are characterized by the deposition of cholesteryl esters and triglycerides into the arterial wall. In the excised human atherosclerotic plaque these lipids are in a liquid-like state at body temperature and observable via MRI and NMR spectroscopy. To assess the ability of MRI to quantitatively image the lipids of atherosclerotic plaque in vivo, we have investigated eight New Zealand White rabbits fed atherogenic diets (2 weight (wt)% cholesterol, 1 wt% cholesterol + 6 wt% peanut oil, and 1 wt% cholesterol + 6 wt% com oil). Postmortem examination indicated that all rabbits developed atherosclerosis in the aorta. Except for one animal, magnetic resonance angiography showed no noticeable obstruction in the aorta. MRI was carried out in an attempt to image atherosclerotic plaque lipids directly, but no signal was detected in vivo. However, a plaque lipid signal was observed from excised tissue using a small diameter RF coil. 1H NMR spectroscopy of the atherosclerotic plaque from excised aortas indicated that the major fraction of plaque lipids in rabbits is not in a liquid state at physiological temperature and are only marginally MRI-visible compared to human plaque lipid. The differences in the MRI characteristics of rabbit and human plaque are due to differences in the fatty acid profile of the cholesteryl esters, chiefly a decrease of linoleic acid in rabbit lesions.

Guidewire antennas for MR fluoroscopy

Before MRI can be used for guiding vascular interventions, the problem of dependably visualizing guidewires must be solved. Ideally, the position and curvature of the tip of the guidewire should be visible to facilitate steering through the vascular system. In this paper, various antennas are described that can be incorporated into a guidewire tip. These antennas allow the guidewire to be visualized with high contrast. Computer simulation and experimental evidence are presented showing the value of adding a passive MR signal source, with a short T1, internal to the coil. The internal source increases the available signal, narrows the apparent width of the guidewire, and allows the use of fast imaging sequences.

Intravascular magnetic resonance imaging using a loopless catheter antenna

Recently, intravascular catheter probes have been developed to increase signal-to-noise ratio (SNR) for MR imaging of blood vessels. Miniaturization of these catheter probes without degrading their performances is very critical in imaging small vessels such as coronary arteries. Catheter coils have a loop incorporated in their structure and have limitations in physical dimensions and electromagnetic properties. The use of a loopless intravascular catheter antenna is proposed to overcome these problems. The catheter antenna is essentially a dipole, which makes a very thin diameter possible, and its electronic circuitry can be placed outside the blood vessels without performance degradation. The theoretical foundation for the design and operation of the catheter antenna is presented. Several catheter antennae, as small as 1.5 French, were constructed and tested on phantoms and rabbits with great success. The catheter antenna has a simple structure and is easy to design, implement, and operate.

High resolution MR imaging and localization of laser-induced thermal injury in the vascular wall

With the increasing use of lasers in surgical procedures, there is need for a noninvasive imaging modality to monitor laser-tissue interactions. MRI can be used readily for imaging human anatomy and holds the potential to map laser-induced thermal injury. This study investigates high resolution T1-weighted MR imaging of human aorta samples (in vitro) that have been damaged thermally using an argon ion laser and the corresponding histology. High resolution T1-weighted MR images (voxel size, .156 x .156 x .700 mm) clearly detected residual thermal injury as areas of bright signal intensity. Effective localization of thermal injury was achieved by subtraction of preinjury and postinjury slices with pseudocoloring of positive and negative differences. The results may serve as a basis from which to guide future in vivo studies.

High resolution intravascular MRI and MRS by using a catheter receiver coil

Potentially important diagnostic information about atherosclerosis can be obtained by using magnetic resonance imaging and spectroscopy techniques. Because critical vessels such as the aorta, coronary arteries, and renal arteries are not near the surface of the body, surface coils are not adequate to increase the data quality to desired levels. A few catheter MR receiver coil designs have been proposed for imaging the walls of large blood vessels such as the aorta. These coils have limited longitudinal coverage and they are too thick to be placed into small vessels. A flexible, long and narrow receiver coil that can be placed on the tip of a catheter and will enable multi-slice high resolution imaging of small vessels has been developed. The authors describe the theory of the coil design technique, derive formulae for the signal-to-noise ratio characteristics of the coil, and show examples of high resolution cross-sectional images from isolated human aortas acquired by using this catheter coil. In addition, high resolution in vivo rabbit aorta images were obtained as well as a set of spatially resolved chemical shift spectra from a dog circumflex coronary artery.

Intracavitary birdcage resonator: applications to the human prostate

Twenty-five patients with prostatic cancer were prospectively examined with a prototype endorectal surface coil featuring a birdcage resonator circuit design. The purpose was to determine the safety of an intracavitary probe for magnetic resonance (MR) imaging of the pelvis that incorporates the "inside-out" characteristics of a volume coil design and allows high-resolution MR imaging of the prostate and potentially serves as an alternative to single-loop intracavitary surface coils. Clinically useful images supplementing images obtained with the body or external surface coils were obtained with the prototype probe. It was tolerated by all patients enrolled in the study, and none experienced side effects. The cylindrically symmetric sensitivity profile of the probe allowed identification of prostate tumors and pelvic lymph node and bone metastases. Volume-type coils may improve endopelvic MR imaging when used alone or in combination with external coil systems.

High-resolution MR imaging of human arteries

The authors studied the magnetic resonance (MR) imaging appearance of excised human arteries and correlated these findings with those from histologic sections obtained in corresponding planes. Imaging was performed with a 1.5-T clinical imager modified with an additional gradient insert capable of a 30 mT/m gradient. High-resolution images of arteries obtained at autopsy indicated that the medial and adventitial layers could be distinguished and that T2-weighted sequences offered superior contrast. Intimal thickening could be detected at a relatively early stage as a long T2 rim on the luminal surface of the artery. Atherosclerotic plaque was found to have MR properties similar to those of intimal thickening, although necrotic regions within plaque had low signal intensity. Fat suppression sequences did not significantly alter the appearance of atherosclerotic plaque. Calcified plaque produced effects ranging from slight signal loss to signal void. MR imaging findings correlated reliably with the tissue types indicated by histologic analysis.

NMR imaging with shorted coaxial line probes

At frequencies below 1 GHz, resonant sections of coaxial lines have long been used in CW-Electron Paramagnetic Resonance (EPR) with the sample placed at the position of maximum B1 at a short circuited end. Here, we show that because of the excellent separation of the B and E fields, the shield of the line can be removed in the region of the truncated end without greatly perturbing the RF properties of the line. The open region of the shield provides an aperture for local imaging in MRI. The B1 fields can be shaped by contouring the inner conductor and outer shield, and the image aperture is controlled by the shape of the shield cutout. The shield opening can range from a narrow longitudinal slit up to a full 360 degrees section that has only a few conducting strips of the shield remaining. Imaging with probes having shield diameters from 2 mm to 10 cm have been demonstrated. For imaging the useful depth is limited to approximately three to four times the probe's outer radius. Alternately, a relatively sharp cutoff at only a mm depth can be obtained by controlling the region of the shield removed, the RF power applied, and the probe diameter. The probes described here can be resonant or nonresonant. Because of the inherent broad bandwidth of the nonresonant truncated line probes, they have the potential for use in FT-EPR and FT-EPR imaging as well as other applications that require minimizing dead times.

Intravascular MR imaging in a porcine animal model

An opposed solenoid intravascular receiver coil is used in a series of five pigs to generate images of the aorta. Severe ghosting artifacts are shown to occur if the motion of the intravascular coil is not restricted with respect to the vessel wall. Restricting coil motion and saturating the blood signal superior to the imaging plane are shown to produce high quality MR images. Layers within the artery wall could be discerned and the results of the in vivo analysis compared favorably with high resolution MR images of the corresponding tissue after excision.

Prototype miniature endoluminal MR imaging catheter

PURPOSE

The feasibility of a miniature endoluminal magnetic resonance (MR) detection coil was investigated for imaging mural and perimural anatomy of small, tubular structures.

MATERIALS AND METHODS

To this end, remotely tunable, single-loop, multiturn, receive-only radio-frequency coils, housed in 6-9-F arterial sheaths, were built. A 1.9-T imager was used. Phantom excitation was accomplished with a 62-mm-diameter bird-cage quadrature coil, and ex vivo specimen excitation was accomplished with a single-turn, untuned wire loop. Phantom images obtained with use of a 9-F catheter coil showed a signal-to-noise improvement on the border of 20 dB compared with images obtained with the quadrature coil. An 8-F catheter coil was used to obtain high-resolution (100 microns in-plane pixel size, 500 microns section thickness) spin-echo images (repetition time = 2,400 msec, echo time = 53 msec) of the wall of a fresh ex vivo human popliteal artery.

RESULTS

Prospectively, these images were suggestive of the presence of diffuse intimal hyperplasia, medial calcification, and focal atherosclerotic plaque. These findings were confirmed histologically. Three-dimensional restacking of the axial images simplified examination of the normal layers and pathologic changes within the wall. The improved signal-to-noise characteristics of these miniature coils permit fast high-resolution imaging, allowing visualization of microscopic anatomic details.

CONCLUSIONS

With further development, this technology may be useful for studying atherosclerosis and for providing imaging guidance during endoluminal MR interventions.

A birdcage resonator for intracavitary MR imaging

An intracavitary probe for magnetic resonance imaging of the pelvis has been developed that takes advantage of the "inside-out" spatial characteristics of a birdcage resonator. The probe consists of an eight-leg, birdcage resonator in a low-pass configuration operating in receive-only mode. The resonator circuit is mounted on a solid rod, is encased in Teflon, and has been used to obtain detailed images of pelvic anatomy in a male canine. The approximate cylindrical symmetry of the external sensitivity profile of this type of circuit, employed in an intracavitary application, demonstrates the potential superiority of this type of probe design over single-loop intracavitary coils. Axial, coronal, and sagittal MR images, obtained with 8 and 16 cm fields of view, are presented to illustrate the advantages of this type of intracavitary probe compared with conventional body-coil images. The prototype described in this report has been designed for clinical use in human subjects and is currently undergoing testing to determine its efficacy in the evaluation of rectal, prostate, and gynecologic pathology.

MR imaging of blood vessels with an intravascular coil

A method for producing high-resolution magnetic resonance (MR) images of blood vessel walls is described. The authors review a theoretical analysis of receiver-coil design and present a coil well suited for intravascular MR imaging. The design is based on two coaxial solenoids separated by a gap region and with current driven in opposite directions. Placement of this receiver coil within the vascular space is shown to provide a substantial increase in sensitivity over that external surface coils. Experimental verification of these predictions was obtained in a vessel phantom in which a 13-cm surface coil was compared with a 3.5-mm-diameter opposed-solenoid intravascular coil. This intravascular coil had a cylindric region of high sensitivity that offered a 10-fold improvement in signal-to-noise ratio over that of an external coil near the vessel wall. The performance of this coil was also tested in the jugular vein of a swine.