High-resolution intravascular magnetic resonance imaging: monitoring of plaque formation in heritable hyperlipidemic rabbits

Intravascular optical coherence tomography [Invited]

Shortly after the first demonstration of optical coherence tomography for imaging the microstructure of the human eye, work began on developing systems and catheters suitable for intravascular imaging in order to diagnose and investigate atherosclerosis and potentially to monitor therapy. This review covers the driving considerations of the clinical application and its constraints, the major engineering milestones that enabled the current, high-performance commercial imaging systems, the key studies that laid the groundwork for image interpretation, and the clinical research that traces intravascular optical coherence tomography (OCT) from early human pilot studies to current clinical trials.

Intravascular MRI for Plaque Characterization: Are We Close to Reality?

Non-invasive external magnetic resonance imaging (MRI) of large vessel atherosclerosis is a robust and promising imaging modality that can be applied for the evaluation of the atherosclerotic process in large vessels. However, it requires expertise for setup and time for data acquisition and analysis. Intravascular MRI is a promising tool, but its use remains at the pre-clinical stage within selected research groups. In this review, the current status and future role of intravascular MRI for atherosclerotic plaque characterization are summarized, along with important challenges which will be necessary to overcome prior to the wide adoption of this technique.

MRI study of atherosclerotic plaque progression using ultrasmall superparamagnetic iron oxide in Watanabe heritable hyperlipidemic rabbits

OBJECTIVE

The purpose of this study was to evaluate plaque progression by using MRI with ultrasmall superparamagnetic iron oxide (USPIO) and by histopathological studies.

METHODS

We divided 12 Watanabe heritable hyperlipidemic (WHHL) rabbits into 4 groups based on their age (3, 9, 14 and 26 months) and injected them intravenously with 0.8 mmol (Fe) kg(-1) of USPIO (size, 32 nm; concentration, 15 mg dl(-1)). On the fifth post-injection day, they were again given an intravenous injection with 40 μmol kg(-1) of the same USPIO, and MR angiography (MRA) was performed. The signal-to-noise ratio (SNR) in regions of interest in the wall of the upper abdominal aorta was calculated on coronal images. Specimens from the same level of the aorta were subjected to iron staining and RAM-11 immunostaining and used for histopathological study. For statistical analysis of the MRA and histopathological findings, we used analysis of variance [Tukey's honest significant difference (HSD) test].

RESULTS

In 9-month-old rabbits, the SNR was significantly lower than in rabbits of the other ages (p < 0.01), and the area of RAM-11 (DAKO Corporation, Glostrup, Denmark) and iron uptake in the aortic wall was significantly larger (RAM-11, p < 0.01; iron, p < 0.05). These areas were the smallest in 3-month-old rabbits.

CONCLUSION

Histopathologically, the number of macrophages was the greatest in 9-month-old rabbits. Our findings indicate that the SNR on MRI scans reflects the number of macrophages in the aortic wall of WHHL rabbits.

ADVANCES IN KNOWLEDGE

USPIO-enhanced MRI visualized the accumulation of macrophages in early atherosclerotic plaques of WHHL rabbits in the course of natural progression.

Initial feasibility testing of limited field of view magnetic resonance thermometry using a local cardiac radiofrequency coil

The visualization of lesion formation in real time is one potential benefit of carrying out radiofrequency ablation under magnetic resonance (MR) guidance in the treatment of atrial fibrillation. MR thermometry has the potential to detect such lesions. However, performing MR thermometry during cardiac radiofrequency ablation requires high temporal and spatial resolution and a high signal-to-noise ratio. In this study, a local MR coil (2-cm diameter) was developed to investigate the feasibility of performing limited field of view MR thermometry with high accuracy and speed. The local MR coil allowed high-resolution (1 × 1 × 3 mm(3)) image acquisitions in 76.3 ms with a field of view 64 × 32 mm(2) during an open-chest animal experiment. This represents a 4-fold image acquisition acceleration and an 18-fold field of view reduction compared to that achieved using external MR coils. The signal sensitivity achieved using the local coil was over 20 times greater than that achievable using external coils with the same scan parameters. The local coil configuration provided fewer artifacts and sharper and more stable images. These results demonstrate that MR thermometry can be performed in the heart wall and that lesion formation can be observed during radiofrequency ablation procedures in a canine model.

Evaluation of atherosclerotic lesions using dextran- and mannan-dextran-coated USPIO: MRI analysis and pathological findings

Magnetic resonance imaging (MRI) can detect atherosclerotic lesions containing accumulations of ultrasmall superparamagnetic iron oxides (USPIO). Positing that improved USPIO with a higher affinity for atherosclerotic plaques would yield better plaque images, we performed MRI and histologic studies to compare the uptake of dextran- and mannan-dextran-coated USPIO (D-USPIO and DM-USPIO, respectively) by the atherosclerotic walls of rabbits. We intravenously injected atherosclerotic rabbits with DM-USPIO (n = 5) or D-USPIO (n = 5). Two rabbits were the controls. The doses delivered were 0.08 (dose 1) (n = 1), 0.4 (dose 2) (n = 1), or 0.8 (dose 3) (n = 3) mmol iron/Kg. The dose 3 rabbits underwent in vivo contrast-enhanced magnetic resonance angiography (MRA) before and 5 days after USPIO administration. Afterwards, all animals were euthanized, the aortae were removed and subjected to in vitro MRI study. The signal-to-noise ratio (SNR) of the aortic wall in the same region of interest (ROI) was calculated in both in vivo and in vitro studies. Histological assessment through measurement of iron-positive regions in Prussian blue-stained specimens showed that iron-positive regions were significantly larger in rabbits injected with DM- rather than D-USPIO (P < 0.05) for all doses. In vivo MRA showed that the SNR-reducing effect of DM- was greater than that of D-USPIO (P < 0.05). With in vitro MRI scans, SNR was significantly lower in rabbits treated with dose 2 of DM-USPIO compared with D-USPIO treatment (P < 0.05), and it tended to be lower at dose 3 (P < 0.1). In conclusion, we suggest that DM-USPIO is superior to D-USPIO for the study of atherosclerotic lesions in rabbits.

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.

Detection of the vulnerable coronary atheromatous plaque. Where are we now?

Atherosclerosis is a progressive process with potentially devastating consequences and has been identified as the leading cause of morbidity and mortality, especially in the industrial countries. The underlying mechanisms include endothelial dysfunction, lipid accumulation and enhanced inflammatory involvement resulting in plaque disruption or plaque erosion and subsequent thrombosis. However, it has been made evident, that the majority of rupture prone plaques that produce acute coronary syndromes are not severely stenotic. Conversely, lipid-rich plaques with thin fibrous cap, heavily infiltrated by inflammatory cells have been shown to predispose to rupture and thrombosis, independently of the degree of stenosis. Therefore, given the importance of plaque composition, a continuously growing interest in the development and improvement of diagnostic modalities will promptly and most importantly, accurately detect and characterize the high-risk atheromatous plaque. Use of these techniques may help risk stratification and allow the selection of the most appropriate therapeutic approach.

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.

Imaging of atherosclerosis in WHHL rabbits using high-resolution ultrasound

Watanabe heritable hyperlipidemic (WHHL) rabbits provide an animal model of hypercholesterolemia and atherosclerotic progression. However, a large individual variation in plaque progression rate calls for serial investigations, as do treatment studies. In contrast to histopathology, transthoracic ultrasound imaging of the aortic arch is a noninvasive technique suitable for repeated investigations. We studied 34 WHHL rabbits by both techniques. Ultrasound correctly interpreted plaque morphology compared with histopathology of the same spot (location verified by needle puncture). Intima media thickness (IMT) measured by the two methods agreed well. Ultrasonic values were similar to the histopathologic average circumferential values when these were corrected for postmortem shrinkage. Finally, the transthoracic ultrasound technique demonstrated a significant increase in IMT over a 15-week period (p = 0.0002). We conclude that transthoracic ultrasound of aortic arch IMT in WHHL rabbits is a reliable and feasible technique for studies of plaque progression and the evaluation of interventions.

Investigation of atherosclerotic plaques with MRI at 3 T using ultrasmall superparamagnetic particles of iron oxide

This study aims to investigate the uptake of the experimental ultrasmall superparamagnetic particles of iron oxide (USPIO) contrast agent DDM43/34 (Schering AG, Berlin, Germany) by aortic atherosclerotic plaques using magnetic resonance imaging (MRI) at 3 T. Six Watanabe heritable hyperlipidemic rabbits were injected with USPIO at doses of 0.1-1.0 mmol/kg Fe. Parasagittal magnetic resonance angiography (MRA) scans were acquired using 3D gradient-echo sequences before and after USPIO administration, then again after 6 h, 1 day, 2 days and 5 days. At later time points, when the USPIO concentration was too low to enhance blood signal, additional MRA scans were acquired during the infusion of gadopentate dimeglumine (Magnevist; Schering AG). In the images, widespread susceptibility artifacts demonstrated readily detectable USPIO uptake in the liver, bone marrow and lymphatic vessels. Surprisingly, however, no such effects could be associated specifically with the aortic vessel wall, in contrast to previous studies that showed strong uptake with similar pulse sequences. Histological analysis was performed on aortic slices from two animals, demonstrating that aortic plaques were active but showed very little USPIO uptake, consistent with MRI findings. We conclude that, despite the exciting potential of plaque detection using USPIO, some caution is advised since the absence of susceptibility effects does not necessarily imply the absence of plaque, even at 3 T, which offers increased sensitivity to susceptibility. Future work will investigate the dependence of such results on stage of plaque development, magnetic field strength and choice of contrast agent.

High-Resolution 3 T MR Microscopy Imaging of Arterial Walls

PURPOSE

To achieve a high spatial resolution in MR imaging that allows for clear visualization of anatomy and even histology and documentation of plaque morphology in in vitro samples from patients with advanced atherosclerosis. A further objective of our study was to evaluate whether T2-weighted high-resolution MR imaging can provide accurate classification of atherosclerotic plaque according to a modified American Heart Association classification.

METHODS

T2-weighted images of arteries were obtained in 13 in vitro specimens using a 3 T MR unit (Medspec 300 Avance/Bruker, Ettlingen, Germany) combined with a dedicated MR microscopy system. Measurement parameters were: T2-weighted sequences with TR 3.5 sec, TE 15-120 msec; field of view (FOV) 1.4 x 1.4; NEX 8; matrix 192; and slice thickness 600 microm. MR measurements were compared with corresponding histologic sections.

RESULTS

We achieved excellent spatial and contrast resolution in all specimens. We found high agreement between MR images and histology with regard to the morphology and extent of intimal proliferations in all but 2 specimens. We could differentiate fibrous caps and calcifications from lipid plaque components based on differences in signal intensity in order to differentiate hard and soft atheromatous plaques. Hard plaques with predominantly intimal calcifications were found in 7 specimens, and soft plaques with a cholesterol/lipid content in 5 cases. In all specimens, hemorrhage or thrombus formation, and fibrotic and hyalinized tissue could be detected on both MR imaging and histopathology.

CONCLUSION

High-resolution, high-field MR imaging of arterial walls demonstrates the morphologic features, volume, and extent of intimal proliferations with high spatial and contrast resolution in in vitro specimens and can differentiate hard and soft plaques.

Toward rapid high resolution in vivo intravascular MRI: evaluation of vessel wall conspicuity in a porcine model using multiple imaging protocols

PURPOSE

To assess magnetic resonance (MR) pulse sequences for high resolution intravascular imaging.

MATERIALS AND METHODS

Intravascular imaging of the abdominal aorta and iliac arteries was performed in vivo in a porcine model at 1.5 T using catheter-mounted micro-receive coils. Ten protocols, including spin-echo (SE)-echo planar imaging (SE-EPI), segmented EPI, half-Fourier single-shot turbo spin-echo (HASTE), fast imaging with steady-state free precession (TrueFISP), turbo spin-echo (TSE), and SE acquisition schemes were employed in 13 trials. Images were analyzed by six expert raters with respect to wall-conspicuity, wall-to-lumen/tissue contrast, visible layers of the arterial wall, anticipated clinical usefulness, and overall image quality. Mean differences between sequence-types were evaluated using analysis of variance (ANOVA) between groups with planned comparisons.

RESULTS

The vessel wall was delineated in almost all protocols. Motion artifacts from physiological and device motion were reduced in fast techniques. The best contrast between the wall and surrounding tissue was provided by a HASTE protocol. Anatomic layers of the vessel wall were best depicted on dark blood T2-weighted TSE. Overall, TrueFISP was ranked highest on the remaining measures.

CONCLUSION

Dedicated catheter-coils combined with fast sequences have potential for in vivo characterization of vessel walls. TrueFISP offered the best overall image quality and acquisition speed, but suffered from the inability to delineate the multiple layers of the wall, which seems associated with dark blood- and T2-weighted contrast. We believe future intra-arterial trials should proceed from this study in normal artery imaging and initially focus on fast T2-weighted dark blood techniques in trials with pathology.

Acquired diseases of the thoracic aorta: role of MRI and MRA

Diseases of the thoracic aorta can present with a broad clinical spectrum of symptoms and signs. Their prevalence appears to be increasing in western populations, most likely corresponding to aging and heightened clinical awareness but also influenced by the progress of high-resolution, noninvasive imaging modalities. Among them, MRI provides an excellent visualization of vascular structures and is well suited for evaluation of thoracic aorta disease. Currently, in many centers, noninvasive imaging modalities are the first choice in the cardiovascular system evaluation and diagnosis, reserving conventional angiography for use only before therapeutic intervention. Understanding the principle MRA techniques is essential for acquiring consistent diagnostic images. Basic technical considerations, which include fast spin-echo, fast gradient-echo, and MRA techniques with phase contrast and contrast-enhanced methods, are discussed and applied in the evaluation of acquired thoracic aorta diseases.

Vulnerable Plaque Imaging

The concept of vulnerable plaque is well established with increasing evidence from clinical and basic research. The paradigm has shifted from focusing exclusively on the hemodynamic effects of plaque (ie, resulting lumenal stenosis alone as a predictor of stroke risk) to assessment of the structure and composition of plaque (eg, denuded endothelium with inflammatory elements as a nidus for platelet-fibrin clumping). It is increasingly evident that methods to detect and characterize vulnerable plaque must be developed and optimized. Although MR imaging, CT, and ultrasound provide data regarding single lesions, future investigations relying heavily on nuclear medicine techniques may offer functional assessment of the entire cardiovascular system.

Transmission line for improved RF safety of interventional devices

A new concept is proposed to improve the safety of transmission lines with respect to heating during RF transmission. It is based on the integration of transformers into the transmission line. The concept was applied to an active tracking device. Miniature transformers were designed, and two types of tracking devices were built based on a standard line and a transformer line. Temperature measurements were performed for both devices during high specific absorption rate (SAR) scanning, and the suppression of RF heating to a physiologically non-relevant level was demonstrated for the transformer device. The transmission properties of the transformer line were examined in simulations and RF measurements. Active tracking with the transformer device performed robustly in the phantom. Because of the favorable signal transmission properties of the tested device, it is expected that the concept can be applied to the construction of clinical devices for tracking and intravascular imaging, which are RF-safe under clinical SAR conditions. Since the transformer line has a large bandwidth, the concept may also be applied for RF-safe transmission of non-MR signals.

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

Feasibility of in vivo transvenous intravascular magnetic resonance (MR) imaging of the human arterial wall was determined. All subjects provided written informed consent, and institutional review board approved the study. Six arteries in six patients were imaged with a guidewire placed in the iliac vein (n = 5) or left renal vein (n = 1). Pre- and postcontrast T1-weighted and T2-weighted transvenous MR imaging were performed. An atherosclerotic plaque with a fibrous cap was identified on 27 (42%) of 64 images of veins without stents; intimal hyperplasia in a renal artery with a stent was identified on 12 images. Contrast-to-noise ratios (CNRs) on arterial wall postcontrast T1-weighted images were superior to those on images obtained with other sequences (P < .001), and the postcontrast images demonstrated the greatest number of plaques with a low-signal intensity core and fibrous cap. Preliminary results show that transvenous MR imaging is feasible for high-spatial-resolution imaging of the arterial wall and atherosclerotic plaque. Postcontrast T1-weighted imaging affords greatest CNR for the arterial wall.

Quand, comment et pourquoi réaliser une imagerie des artères des membres inférieurs ?

Noninvasive screening of lower limb arterial disease has long been performed using color Doppler ultrasonography, whereas surgical or endovascular treatment planning relied on conventional angiography. With continued improvements of noninvasive imaging modalities, it is now possible to image the entire lower limb vasculature without arterial catheterization. Multidetector row helical CT angiography has the advantage of visualizing the arterial lumen and arterial wall calcifications, and nephrotoxicity is reduced by decreasing amounts of contrast medium. Three-dimensional MR angiography is a safe procedure, with high contrast sensitivity, and has recently benefited from step table technology that allows a single injection of contrast medium. The literature shows that both helical CT and MR angiography have high levels of accuracy, but outcome studies of their respective role in the setting of acute and chronic lower limb ischemia are lacking. These noninvasive techniques should play a major role in two main situations: first, in critical ischemia where therapeutic planning should ideally be achieved non invasively; second, in the follow-up of treated patients presenting with recurrent ischemia.

Diffusivity- and T2 imaging at 3 Tesla for the detection of degenerative changes in human-excised tissue with high resolution: atherosclerotic arteries

RATIONALE AND OBJECTIVES

We investigated whether it is possible to investigate degenerative changes in human tissue on a sub-100-microm resolution scale not only on special high-field small-bore MR-microscopy systems but also on a 3T whole-body MR-scanner.

METHODS

Spin-spin relaxation, proton density, and diffusion microimaging were investigated in studying human atherosclerotic arteries. Strong diffusion weighting and high spatial resolution was achieved by means of a strong dedicated gradient system and a small birdcage radiofrequency resonator.

RESULTS

Quantitative parameter maps were obtained at voxel sizes down to 73 x 73 x 600 microm3. The morphologic structure and pathology connected to lipid deposits, plaques, small thrombi, and bifurcations were well visualized.

CONCLUSION

High-resolution parameter-weighted and parameter-imaging at sub-100-microm pixel resolution can be achieved for excised tissue on a 3.0 T whole body MR system. Perspectives for the characterization of atherosclerotic plaques imply not only cost advantages but also equivalence of contrast, especially as to T(2), for in vivo and high-resolution ex vivo investigations on the same MR scanner.

Pair of resonant fiducial markers for localization of endovascular catheters at all catheter orientations

PURPOSE

To test wireless resonance circuits (RC) to be used as fiducial marker of endovascular catheters during MR-guided interventions. Current markers loose their signal enhancement for certain catheter orientations. The purpose of this study was to test a marker setup which overcomes this orientation problem.

MATERIALS AND METHODS

The markers were constructed from a pair of two RCs. The RCs were individually tuned and the coil axes were oriented perpendicular to each other in order to decouple the two RCs. The markers were mounted on the tip of endovascular catheters and tested in vitro and in one porcine in vivo experiment.

RESULTS

An intense MR signal at similar signal levels was noted at all catheter orientations. In the in vivo experiment the markers allowed for fast and reliable MR guidance of the catheters.

CONCLUSION

A pair of two individually tuned and decoupled RCs is well suited for MR guidance of endovascular catheters.

Initial experiences with in vivo intravascular coronary vessel wall imaging

PURPOSE

To evaluate potential use of a loopless internal receiver coil for in vivo coronary artery vessel wall imaging in five domestic swine.

MATERIALS AND METHODS

Intravascular free-breathing black blood coronary vessel wall imaging was performed using a previously described double inversion fast spin echo technique after x-ray guided placement of an internal receiver coil in or in close proximity to the target vessel (LAD, LCX).

RESULTS

Image quality using the phased array coil was reproducible, while image quality with the internal receiver coil was heavily dependent on coil position with respect to the examined artery, and likely also dependent on blood flow and/or cardiac-related coil motion. With internal coil placement in the left circumflex coronary artery, images of the left anterior descending vessel wall appeared similar or superior compared to commercially available phased array surface coil images. With coil placement in the target vessel itself, imaging was suboptimal because of the extremely high signal intensity (hotspot) in close proximity to the vessel wall, leading to low contrast between the vessel wall and the surrounding tissues and blood.

CONCLUSION

In this study, we demonstrate the feasibility of in vivo intravascular coronary vessel wall imaging. Continued research is necessary to minimize coil motion and optimize coil sensitivity algorithms.

Intravascular MR imaging and intravascular MR-guided interventions

Intravascular MR technology, using an intravascularly placed MR receiver probe to acquire high-resolution angiographic MR images (i.e. intravascular MR imaging) and to guide cardiovascular interventional therapies (i.e. intravascular MR-guided interventions), is a new, very attractive development in the field of MR imaging. The new technology offers unique advantages for cardiovascular imaging and interventions, including superior contrast capability and multiplanar imaging capabilities without the use of contrast agents and with no risk of ionizing radiation. Thecombination of intravascular MR techniques with other advanced MR imaging techniques, such as functional MR imaging, will open new avenues for the future comprehensive management of cardiovascular atherosclerotic disease. Further improvements in intravascular MR fluoroscopy with true real-time display, analogous to X-ray fluoroscopy, will dramatically establish the role of intravascular MR technology in modern medicine.

Interventional MRA using actively visualized catheters, TrueFISP, and real-time image fusion

An integrated system for performing interventional magnetic resonance angiography (MRA) with actively visualized instruments and real-time image fusion was implemented on a 1.5 T scanner. True fast imaging with steady precession (TrueFISP) imaging provided high acquisition speed paired with high signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) for the simultaneous visualization of active instruments and arterial morphology. The system enabled simultaneous image reconstruction and image postprocessing of multiple receiver channels, with subsequent image fusion display in real time. Optional interleaved image acquisition in two planes provided additional important information for biplanar instrument guidance. Various vascular interventions, including selective catheterization and subsequent selective MRA of the abdominal aorta, renal arteries, superior mesenteric artery (SMA), hepatic artery, and aortic arch, were performed on 10 pigs under MR guidance. In terms of instrument contrast, image acquisition, reconstruction, and fusion speed, the setup represents a powerful platform for performing interventional MRA procedures.

Visualization of coronary atherosclerotic plaques in patients using optical coherence tomography: comparison with intravascular ultrasound

OBJECTIVES

The aim of this study was to evaluate the feasibility and the ability of intravascular optical coherence tomography (OCT) to visualize the components of coronary plaques in living patients.

BACKGROUND

Disruption of a vulnerable coronary plaque with subsequent thrombosis is currently recognized as the primary mechanism for acute myocardial infarction. Although such plaques are considered to have a thin fibrous cap overlying a lipid pool, imaging modalities in current clinical practice do not have sufficient resolution to identify thin (< 65 microm) fibrous caps. Optical coherence tomography is a new imaging modality capable of obtaining cross-sectional images of coronary vessels at a resolution of approximately 10 microm.

METHODS

The OCT images and corresponding histology of 42 coronary plaques were compared to establish OCT criteria for different types of plaques. Atherosclerotic lesions with mild to moderate stenosis were identified on angiograms in 10 patients undergoing cardiac catheterization. Optical coherence tomography and intravascular ultrasound (IVUS) images of these sites were obtained in all patients without complication.

RESULTS

Comparison between OCT and histology demonstrated that lipid-rich plaques and fibrous plaques have distinct OCT characteristics. A total of 17 IVUS and OCT image pairs obtained from patients were compared. Axial resolution measured 13 +/- 3 microm with OCT and 98 +/- 19 microm with IVUS. All fibrous plaques, macrocalcifications and echolucent regions identified by IVUS were visualized in corresponding OCT images. Intimal hyperplasia and echolucent regions, which may correspond to lipid pools, were identified more frequently by OCT than by IVUS.

CONCLUSIONS

Intracoronary OCT appears to be feasible and safe. Optical coherence tomography identified most architectural features detected by IVUS and may provide additional detailed structural information.

Detection of atherosclerosis using a novel positron-sensitive probe and 18-fluorodeoxyglucose (FDG)

Inflammation contributes to atherosclerotic plaque remodeling, enlargement and rupture. Non-invasive imaging of coronary artery inflammation could help target therapy to 'vulnerable' atheromata, but is limited because of small tissue mass and arterial motion. Local radiopharmaceutical imaging may overcome some of these limitations. We used a positron-sensitive fiberoptic probe, which can distinguish positron emissions from annihilation photons, to identify diseased from healthy endothelium in an atherosclerotic model. New Zealand White rabbits underwent Fogarty-catheter injury of an iliac artery and then were fed a high-fat diet for 3 weeks. Fasted animals received 90-180 MBq of 18-fluorodeoxyglucose (FDG) 2-4 h before sacrifice and harvest of injured and uninjured iliacs. Arteries were incised longitudinally and the probe was placed in contact with the arterial intima. Multiple measurements were obtained along 1 cm artery segments in 60 s intervals, and corrected for 18F decay and background. Measurements were recorded over 93 injured and normal artery segments in 11 animals. Mean probe Z-scores were 4.8-fold higher (CI 3.4-6.3) over injury atherosclerosis compared with uninjured normal iliac artery segments (P<0.001). Gamma counting confirmed that injured artery segments accumulated more FDG per gram than did normal segments (0.203% x kg injected dose per gram of tissue versus 0.042, P<0.001). Non-arterial tissue also accumulated FDG avidly, particularly reticuloendothelial tissues and blood. Delayed sacrifice, 4 h compared with 2 h after animal FDG injection, further reduced blood background counts and improved the signal-to-noise ratio. Histopathology confirmed that injured iliac artery had significantly higher intimal and medial cross-sectional area compared with uninjured artery. Injured artery also had significantly higher macrophage and smooth muscle cell density. Positron-sensitive probe counts correlated with the intima to media ratio (r =0.63, P = 0.03). Our positron-sensitive probe distinguishes atherosclerotic from healthy artery in a blood-free field. Intravascular study of plaque biology may be feasible using FDG and a positron-sensitive probe.

Carotid plaque characterization by magnetic resonance imaging: review of the literature

Magnetic resonance imaging (MRI) of carotid plaque has undergone significant improvements in the last decade. Early studies utilizing ex vivo specimens and spin-echo or fast spin-echo imaging led to the conclusion that T2 weighting is the best single contrast to characterize carotid plaque morphology. On these images, the fibrous plaque appears bright and the lipid core is dark; thrombus can have variable intensity. There can be an overlap in T2-weighted signal intensities among the various plaque components, which can be partially offset by the use of multispectral analysis of multiple contrast images. With improvements in coil design, sequence design, and main field and gradient capabilities, accurate in vivo differentiation and measurement of these various carotid plaque components should be possible in 3 to 5 years. Ex vivo and in vivo studies have yielded high-resolution measurements of the complex three-dimensional lumen geometry, which are being used to predict hemodynamic forces acting on the lumenal surface. Carotid plaque burden can be accurately measured in vivo today; ongoing longitudinal studies should lead to a better understanding of the relationship between plaque burden and the risk of thromboembolic complications, as well as the effect of diet and drug therapy in hyperlipidemic patients. With these developments in place or soon to be available, MRI of the diseased carotid artery wall may prove to be even more important than magnetic resonance angiography.

Possibilities of transesophageal MRI for assessment of aortic disease: a review

The thoracic aortic wall is a common site of atherosclerotic plaque in humans. Tools for serial, non-invasive assessment of these plaques are of value for addressing gaps in our basic understanding of the biology of plaque rupture and its relationship to atherosclerotic disease progression as well as for monitoring response to anti-atherosclerotic interventions in therapeutic clinical trials. Common approaches to assessment of the wall of the thoracic aorta in vivo are limited. Here we discuss some of the challenges and limitations encountered by conventional techniques and review a novel approach, transesophageal MRI (TEMRI). Initial experiences in applying the TEMRI approach to assessment of aortic morphology and pathology are discussed.

In vivo measurement of pulsewave velocity in small vessels using intravascular MR

A one-dimensional intravascular MR (IVMR) technique for the measurement of pulsewave velocity in a single cardiac cycle is presented. The technique was used to measure pulsewave velocity in vivo in the intact rabbit model, where its sensitivity to different hemodynamic states was demonstrated using a pharmacological intervention with phenylephrine and nitroprusside. IVMR measurements of pulsewave velocity were found to increase with mean arterial pressure, as expected. Further, IVMR-based pulsewave velocity estimates were in agreement with those measured by pressure catheters and direct distensibility measurement. Because of their rapidity and highly localized nature, these measurements of vessel elasticity may complement the high-resolution vascular imaging information gained in an IVMR examination. This could allow assessment of atherosclerotic plaques and facilitate immediate treatment decisions. Magn Reson Med 45:53-60, 2001.

High-resolution MRI characterization of human thrombus using a novel fibrin-targeted paramagnetic nanoparticle contrast agent

In this study, the sensitivity of a novel fibrin-targeted contrast agent for fibrin detection was defined in vitro on human thrombus. The contrast agent was a lipid-encapsulated perfluorocarbon nanoparticle with numerous Gd-DTPA complexes incorporated into the outer surface. After binding to fibrin clots, scanning electron microscopy of treated clots revealed dense accumulation of nanoparticles on the clot surfaces. Fibrin clots with sizes ranging from 0.5-7.0 mm were imaged at 4.7 T with or without treatment with the targeted contrast agent. Regardless of sizes, untreated clots were not detectable by T(1)-weighted MRI, while targeted contrast agent dramatically improved the detectability of all clots. Decreases in T(1) and T(2) relaxation times (20-40%) were measured relative to the surrounding media and the control clots. These results suggest the potential for sensitive and specific detection of microthrombi that form on the intimal surfaces of unstable atherosclerotic plaque.

Real-time projection MR angiography: feasibility study

Intraarterial injections of small doses of gadopentetate dimeglumine were combined with a fast spoiled-gradient-echo magnetic resonance (MR) sequence to obtain real-time projection angiographic images of the rabbit aorta and canine coronary arteries. Arterial filling and washout, as well as venous and perfusion phases, were clearly displayed, demonstrating that arterial fluoroscopy in which an MR technique is used is feasible.

Interventional MRA and intravascular imaging

Several attributes make magnetic resonance imaging (MRI) attractive for guidance of intravascular therapeutic procedures, including high soft tissue contrast, imaging in arbitrary oblique planes, lack of ionizing radiation, and the ability to provide functional information, such as flow velocity or flow volume per unit time, in conjunction with morphologic information. For MR guidance of vascular interventions to be safe, the interventionalist must be able to visualize catheters and guidewires relative to the vascular system and surrounding tissues. A number of approaches for rendering instruments visible in an MR environment have been developed, including both passive and active techniques. Passive techniques depend on contrast agents or susceptibility artifacts that enhance the appearance of the catheter in the image itself, whereas active techniques rely on supplemental hardware built into the catheter, such as a radiofrequency (RF) coil. Additionally, the ability to introduce an RF coil mounted on a catheter presents the opportunity to obtain high-resolution images of the vessel wall. These images can provide the capability to distinguish and identify various plaque components. The additional capabilities of MRI could potentially open up new applications within the purview of vascular interventions beyond those currently performed under X-ray fluoroscopic guidance.

Abciximab in carotid stenting for postsurgical carotid restenosis: intermediate results

PURPOSE

To report intermediate results of a pilot study in which the glycoprotein IIb/IIIa receptor antagonist abciximab was given to patients during percutaneous carotid stenting for recurrent internal carotid artery (ICA) stenosis. The objective was to prevent procedure-related cerebral embolic events and decrease the incidence of recurrent stenosis.

METHODS

Sixteen patients (9 women; mean age 66.5 years, range 39-78) with severe ICA recurrent stenosis (>80%) underwent balloon angioplasty and stenting. Before the procedure, abciximab was administered intravenously as a bolus (0.25 mg/kg) followed by a 12-hour continuous infusion (10 microg/min).

RESULTS

Fifteen patients received stents (14 Wallstent and 1 Strecker); 1 vessel was dilated with only 50% improvement in luminal diameter. Two stented arteries had residual stenosis (<30%) but satisfactory luminal diameter was achieved in the remaining 13 (81%) arteries. There were no neurological ischemic events during or following the procedure, nor were there any bleeding or peripheral vascular complications. Duplex surveillance studies up to 12 months revealed no significant recurrent stenosis in the treated vessels.

CONCLUSIONS

The administration of abciximab in conjunction with percutaneous revascularization procedures for postsurgical carotid artery stenosis may reduce cerebral ischemic episodes. It may also attenuate restenosis in the treated artery.

Techniques characterizing the coronary atherosclerotic plaque: influence on clinical decision making?

The composition of the atherosclerotic lesion rather than the degree of stenosis is currently considered to be the most important determinant for acute clinical events. Modalities capable of characterizing the atherosclerotic lesion may be helpful in understanding its natural history and detecting lesions with high risk for acute events. Speaking grossly, three histologic features of the vulnerable plaque have been reported: size of the atheroma, thickness of the fibrous cap, and inflammation. Imaging techniques are currently being deployed and are under development to aid visualization of the vulnerable coronary plaque. Most of these diagnostic modalities have the potential to detect locally one or more of the three histologically defined features of vulnerable plaque. This review will focus on imaging techniques that have been developed to characterize the atherosclerotic lesion. Most catheter-based visualization techniques will provide insight into components of the local atherosclerotic plaque which may limit their predictive value for the occurrence of a clinical event. Therefore, the clinical relevance of these imaging tools will be discussed.

Imaging techniques to predict cardiovascular risk

Conventional cardiovascular imaging, with a focus on identifying flow-limiting stenoses, does not directly image the atherosclerotic lesion. Recent clinical and pathobiologic data indicate that stenosis severity does not dictate cardiovascular risk and that there are functional, structural, and biologic features of atherosclerosis that are associated with cardiovascular events. Imaging technologies, such as ultrasound, light, x-ray, magnetic resonance, and targeted contrast agents, have been developed to characterize directly the atherosclerotic vessel wall. They provide promising approaches to predict cardiovascular risk and facilitate further study of the mechanisms of atherosclerosis progression and its response to therapy.

Visualization, tracking, and navigation of instruments for magnetic resonance imaging-guided endovascular procedures

Interventional procedures using magnetic resonance imaging (MRI) guidance have increased in interest during the last few years. Central to the success and safety of MRI-guided procedures is the accurate visualization of the interventional instruments relative to the surrounding anatomy. A variety of methodologies for visualizing and automatically tracking instruments, including needles, radiofrequency and laser ablation devices, endoscopes, catheters, and guidewires have been developed and introduced to help the interventionalist to safely guide the device toward the target region. This article describes and compares characteristics of the four most commonly used localization and tracking systems used for MRI-guided interventional procedures: those based on the susceptibility artifact of the device, those that intentionally create field inhomogeneity along the device, those that rely on an optical tracking system, and active tracking systems using micro receive coils.

MRI of acute and chronic aortic pathology: pre-operative and postoperative evaluation

With recent advances in the understanding of aortic diseases, both power and versatility have put magnetic resonance imaging (MRI) in the focus of diagnostic work-up in the entire spectrum of clinical aortic pathology. Technical refinements, from classic anatomic imaging to three-dimensional gadolinium-enhanced MR angiography and tissue characterization, have rendered MRI ideal for assessment of acquired disease such as aortic dissection, intramural hematoma, and aneurysm, along with postoperative follow-up evaluation, with better reliability and safety than other imaging modalities. Moreover, congenital pathology of the aorta, including aortic arch anomalies and coarctation, can be non-invasively evaluated by MRI. With spectroscopy and the advent of high-resolution intravascular coils, MRI may even provide histopathologic and potentially prognostic information unparalleled by any other method. J. Magn. Reson. Imaging 1999;10:741-750.

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.