Human peripheral arteries: feasibility of transvenous intravascular MR imaging of the arterial wall

An optically powered CMOS tracking system for 3 T magnetic resonance environment

In this work, a fully optical Complementary Metal Oxide Semiconductor (CMOS) based catheter tracking system designed for 3 T Magnetic Resonance Imaging (MRI) environment is presented. The system aims to solve the Radio Frequency (RF) induced heating problem present in conventional wired catheter tracking systems used in MRI. It is based on an integrated circuit, consisting of a receiver and an optical power supply unit. The optical power supply unit includes a single on-chip photodiode and a DC-DC converter that boosts the low photodiode voltage output to voltages greater than 1.5 V. Through an optically driven switch, the accumulated charge on an a storage capacitor is transferred to the rest of the system. This operation is novel in the way that it is fully optical and the switch control is done through modulation of the applied light. An on-chip local oscillator signal for the receiver is avoided by application of an RF signal that is generated by the MRI machine at the receiving period. The signals received by a micro-coil antenna are processed by the on-chip direct conversion receiver. The processed signal is then transferred, also optically, to the outside world for tracking purposes. The frequency encoding method is used for MRI tracking. Operation with various levels of external optical power does not generate noticeble temperature increase in the system. The overall system is successfully tested in a 3 T MRI machine to demonstrate its full operation.

Development of an intrabiliary MR imaging-monitored local agent delivery technique: a feasibility study in pigs

PURPOSE

To investigate the feasibility of using magnetic resonance (MR) imaging to monitor intrabiliary delivery of motexafin gadolinium (MGd) into pig common bile duct (CBD) walls.

MATERIALS AND METHODS

Animal studies were approved by the Institutional Animal Care and Use Committee. Initially, human cholangiocarcinoma cells were treated with various concentrations of MGd, a compound serving as a T1-weighted MR imaging contrast agent, chemotherapy drug, and cell marker. These cells were then examined by means of confocal microscopy to confirm the intracellular uptake of MGd. In addition, an MGd/trypan blue mixture was locally infused into CBD walls of six cadaveric pigs using a microporous balloon catheter. CBDs of six pigs were infused with saline to serve as controls. Ex vivo T1-weighted MR imaging of these CBDs was performed. For in vivo technical validation, the microporous balloon catheter was placed in the CBD by means of a transcholecytic access to deliver MGd/trypan blue into CBD walls of six living pigs. T1-weighted images were obtained with both a surface coil and an intrabiliary MR imaging guidewire, and contrast-to-noise ratios of CBD walls before and after MGd/trypan blue infusions were compared in the two groups by means of paired t test, with subsequent histologic analysis to confirm the penetration and distribution of the MGd/trypan blue agent into CBD walls.

RESULTS

In vitro experiments confirmed uptake of MGd by human cholangiocarcinoma cells. The ex vivo experiments demonstrated the penetration of MGd/trypan blue into the CBD walls. The in vivo experiment confirmed the uptake of MGd/trypan blue, showing an increased contrast-to-noise ratio for the CBD after administration of the mixture, compared with images obtained prior to MGd/trypan blue administration (11.6 ± 4.2 [standard deviation] vs 5.7 ± 2.8; P = .04). Histologic results depicted the blue dye stains and red fluorescence of MGd in CBD walls, confirming the imaging findings.

CONCLUSION

It is feasible to use MR imaging to monitor the penetration of locally delivered MGd into pig CBD walls.

High-resolution intravascular magnetic resonance quantification of atherosclerotic plaque at 3T

BACKGROUND

The thickness of fibrous caps (FCT) of atherosclerotic lesions is a critical factor affecting plaque vulnerability to rupture. This study tests whether 3 Tesla high-resolution intravascular cardiovascular magnetic resonance (CMR) employing tiny loopless detectors can identify lesions and accurately measure FCT in human arterial specimens, and whether such an approach is feasible in vivo using animal models.

METHODS

Receive-only 2.2 mm and 0.8 mm diameter intravascular loopless CMR detectors were fabricated for a clinical 3 Tesla MR scanner, and the absolute signal-to-noise ratio determined. The detectors were applied in a two-step protocol comprised of CMR angiography to identify atherosclerotic lesions, followed by high-resolution CMR to characterize FCT, lesion size, and/or vessel wall thickness. The protocol was applied in fresh human iliac and carotid artery specimens in a human-equivalent saline bath. Mean FCT measured by 80 μm intravascular CMR was compared with histology of the same sections. In vivo studies compared aortic wall thickness and plaque size in healthy and hyperlipidemic rabbit models, with post-mortem histology.

RESULTS

Histology confirmed plaques in human specimens, with calcifications appearing as signal voids. Mean FCT agreed with histological measurements within 13% on average (correlation coefficient, R = 0.98; Bland-Altman analysis, -1.3 ± 68.9 μm). In vivo aortic wall and plaque size measured by 80 μm intravascular CMR agreed with histology.

CONCLUSION

Intravascular 3T CMR with loopless detectors can both locate atherosclerotic lesions, and accurately measure FCT at high-resolution in a strategy that appears feasible in vivo. The approach shows promise for quantifying vulnerable plaque for evaluating experimental therapies.

Interventional cardiovascular magnetic resonance: still tantalizing

The often touted advantages of MR guidance remain largely unrealized for cardiovascular interventional procedures in patients. Many procedures have been simulated in animal models. We argue these opportunities for clinical interventional MR will be met in the near future. This paper reviews technical and clinical considerations and offers advice on how to implement a clinical-grade interventional cardiovascular MR (iCMR) laboratory. We caution that this reflects our personal view of the "state of the art."

MR-guided intravascular interventions: techniques and applications

Magnetic resonance imaging (MRI) offers several advantages over other imaging modalities that make it an attractive imaging tool for diagnostic and therapeutic procedures. This tremendous potential of MRI has provided the rationale for increased attention toward MR-guided endovascular interventions. MR guidance has been used recently to navigate endovascular catheters and deliver stents, vena cava filters, embolization materials, and septum closure devices. However, its potential goes beyond just copying existing procedures toward the development of new minimally invasive techniques that cannot be performed with conventional guiding techniques. Because of technical limitations and safety issues associated with some of the currently available devices, a limited number of clinical studies have been performed so far. The overall success for this developing field requires considerable interdisciplinary research within both the interventional and the MR community. Only through a combined effort can this complex technology find its way into clinical practice. This review discusses the hardware and software improvements that have helped to advance endovascular interventions under MR imaging guidance from a pure research tool to become a clinical reality. In addition, technical and safety issues specific to endovascular MR image guidance will be described and practical applications will be shown that take advantage of the benefits of MR for endovascular interventions.

Intravascular magnetic resonance imaging

The purpose of this article is to review the current state of the art with respect to intravascular magnetic resonance imaging, including intravascular coils, their implementation for plaque identification and characterization, and strategies for future approaches to coronary imaging and other cardiovascular applications.

High-resolution 3D arteriography of chronic total peripheral occlusions using a T1-W turbo spin-echo sequence with inner-volume imaging

Percutaneous revascularization of peripheral artery chronic total occlusion (CTO) is challenging under X-ray guidance without direct image feedback, due to poor visualization of the obstructed segment and underappreciation of vessel tortuosity. Operators are required to steer interventional devices relatively "blindly," and therefore procedural failure or perforation may occur. Alternatively, MRI may allow complete visualization of both patent and occluded arterial segments. We designed and implemented a 3D high-resolution, T(1)-weighted (T(1)-W) turbo spin-echo (TSE) MRI sequence with inner-volume (IV) imaging to enable detailed peripheral artery CTO imaging. Using this sequence, high-resolution volumes of interest (VOIs) around the vessel were achieved within 5-10 min. This imaging approach may be used for rapid pre- and postprocedural evaluations, and as a 3D roadmap that can be overlaid during real-time X-, MR-, or XMR-guided catheterization. Experiments were successfully performed on a carotid CTO model in swine ex vivo, and in peripheral arteries in normal volunteers and patients in vivo. Delineation of the vascular architecture, including contrast differences between the patent and occluded artery segments, and lesion morphology heterogeneity were visualized.

Cardiovascular interventional MR imaging: a new road for therapy and repair in the heart

Over the last 5 years, interventional MR imaging has been fertile ground for research. Real-time MR imaging, combined with recent advances in other MR imaging modalities such as perfusion imaging and intravascular imaging, has opened up new paths for cardiac therapy. The recent reports on cardiac stem cell therapy guided and monitored by MR imaging suggest that we are already seeing the establishment of an important role for cardiac MR imaging in cardiac restoration. The collaborative effort from a multidisciplinary team of basic biologists, engineers, and clinicians will refine stem cell incubation and labeling for MR-guided transcatheter endomyocardial injections, and this in turn may facilitate new studies in humans. Several groups have demonstrated in animal studies the feasibility of MR-guided catheter interventions for the treatment of congenital heart disease and arrythmia therapy. Hence, applications in humans remain the challenge for the next years. Although there have been first reports of cardiac catheterizations in humans by combined use of x-ray fluoroscopy and MR imaging, there are no reports in the literature suggesting that active tracking methods by MR imaging have been applied to humans. Safety issues (namely, heating of catheters and wires) hamper clinical use, particularly in infants and children. Current reports are promising that these limitations will be overcome in the near future and will eventually reduce x-ray usage during catheterization. In its current state, cardiac MR imaging offers a unique opportunity to investigate new therapeutic strategies for the treatment of congenital and acquired heart disease.

Magnetic resonance imaging-guided vascular interventions

Magnetic resonance imaging (MRI), which provides superior soft-tissue imaging and no known harmful effects, has the potential as an alternative modality to guide various medical interventions. This review will focus on MR-guided endovascular interventions and present its current state and future outlook. In the first technical part, enabling technologies such as developments in fast imaging, catheter devices, and visualization techniques are examined. This is followed by a clinical survey that includes proof-of-concept procedures in animals and initial experience in human subjects. In preclinical experiments, MRI has already proven to be valuable. For example, MRI has been used to guide and track targeted cell delivery into or around myocardial infarctions, to guide atrial septal puncture, and to guide the connection of portal and systemic venous circulations. Several investigational MR-guided procedures have already been reported in patients, such as MR-guided cardiac catheterization, invasive imaging of peripheral artery atheromata, selective intraarterial MR angiography, and preliminary angioplasty and stent placement. In addition, MR-assisted transjugular intrahepatic portosystemic shunt procedures in patients have been shown in a novel hybrid double-doughnut x-ray/MRI system. Numerous additional investigational human MR-guided endovascular procedures are now underway in several medical centers around the world. There are also significant hurdles: availability of clinical-grade devices, device-related safety issues, challenges to patient monitoring, and acoustic noise during imaging. The potential of endovascular interventional MRI is great because as a single modality, it combines 3-dimensional anatomic imaging, device localization, hemodynamics, tissue composition, and function.