Autoperfused balloon catheter for intravascular MR imaging

Intra-Aortic Balloon Pump for Patients with Cardiac Conditions: An Update on Available Techniques and Clinical Applications

In this paper, we present a review of the intra-aortic balloon pump, as well as the usage of it in the medical field today. An intra-aortic balloon pump (IABP) is a biomedical device that can assist the heart during unstable angina or after a heart attack. This pump is typically used in patients who suffer from ischemia of the heart tissue, due to an unbalanced level of myocardial oxygen supply or demand. Through counterpulsation, which is a technique to synchronize the external pumping of blood with the heart’s cycle, the device can balance the supply and demand of blood that is necessary for the heart to pump properly. The IABP is comprised of the following four components: a polyurethane balloon, a polyethylene or fiber-optic catheter, a transducer, and the intra-aortic balloon pump console. In the past, researchers have used other materials that have low biocompatibility and can cause complications within the body. This analysis will explain the complications and state changes that occurred due to them. Limitations of past designs and advantages of current designs will be acknowledged, for they can be used by researchers to enhance designs for the future. Consequently, the analysis of this device may lead to improved designs and treatment in the future for patients with cardiac conditions.

Prospective motion correction using tracking coils

Intracavity imaging coils provide higher signal-to-noise than surface coils and have the potential to provide higher spatial resolution in shorter acquisition times. However, images from these coils suffer from physiologically induced motion artifacts, as both the anatomy and the coils move during image acquisition. We developed prospective motion-correction techniques for intracavity imaging using an array of tracking coils. The system had <50 ms latency between tracking and imaging, so that the images from the intracavity coil were acquired in a frame of reference defined by the tracking array rather than by the system's gradient coils. Two-dimensional gradient-recalled and three-dimensional electrocardiogram-gated inversion-recovery-fast-gradient-echo sequences were tested with prospective motion correction using ex vivo hearts placed on a moving platform simulating both respiratory and cardiac motion. Human abdominal tests were subsequently conducted. The tracking array provided a positional accuracy of 0.7 ± 0.5 mm, 0.6 ± 0.4 mm, and 0.1 ± 0.1 mm along the X, Y, and Z directions at a rate of 20 frames-per-second. The ex vivo and human experiments showed significant image quality improvements for both in-plane and through-plane motion correction, which although not performed in intracavity imaging, demonstrates the feasibility of implementing such a motion-correction system in a future design of combined tracking and intracavity coil.

Intraoperative and interventional MRI: Recommendations for a safe environment

In this paper we report on current experience and review magnetic resonance safety protocols and literature in order to define practices surrounding MRI-guided interventional and surgical procedures. Direct experience, the American College of Radiology White paper on MR Safety, and various other sources are summarized. Additional recommendations for interventional and surgical MRI-guided procedures cover suite location/layout, accessibility, safety policy, personnel training, and MRI compatibility issues. Further information is freely available for sites to establish practices to minimize risk and ensure safety. Interventional and intraoperative MRI is emerging from its infancy, with twelve years since the advent of the field and well over 10,000 cases collectively performed. Thus, users of interventional and intraoperative MRI should adapt guidelines utilizing universal standards and terminology and establish a site-specific policy. With policy enforcement and proper training, the interventional and intraoperative MR imaging suite can be a safe and effective environment.

Imaging of the vulnerable plaque: noninvasive and invasive techniques

In a large proportion of previously asymptomatic individuals, sudden coronary death or acute myocardial infarction occurs as the first manifestation of coronary atherosclerosis. Imaging of coronary atheromatous plaques has traditionally centered on assessing the degree of luminal stenosis. The angiographic techniques that are routinely used to identify stenotic atherosclerotic lesions are unable to identify high-risk plaques; plaques prone to rupture and cause a cardiovascular event. This is partly due to the fact that the majority of culprit lesions that produce acute cardiovascular syndromes are not severely stenotic, possibly due to significant positive remodeling and reduced protective collateral circulation as well as because the risk of plaque rupture is more closely related to plaque content than plaque size. Recently, the focus of new imaging techniques is to identify the high risk plaques; the "vulnerable plaques." In this review, we will refer to the noninvasive and invasive techniques that can detect the vulnerable plaque.

Diagnosis and treatment of coronary vulnerable plaques

Thin-capped fibroatheroma is the morphology that most resembles plaque rupture. Detection of these vulnerable plaques in vivo is essential to being able to study their natural history and evaluate potential treatment modalities and, therefore, may ultimately have an important impact on the prevention of acute myocardial infarction and death. Currently, conventional grayscale intravascular ultrasound, virtual histology and palpography data are being collected with the same catheter during the same pullback. A combination of this catheter with either thermography capability or additional imaging, such as optical coherence tomography or spectroscopy, would be an exciting development. Intravascular magnetic resonance imaging also holds much promise. To date, none of the techniques described above have been sufficiently validated and, most importantly, their predictive value for adverse cardiac events remains elusive. Very rigorous and well-designed studies are compelling for defining the role of each diagnostic modality. Until we are able to detect in vivo vulnerable plaques accurately, no specific treatment is warranted.

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.

IRM et exploration du tube digestif — Aspects techniques et perspectives d’évolution

Barium enema remains the reference method for the detection of morphological intraluminal alterations of the bowel. Optimal filling of intestinal loops allows high diagnostic sensitivity and specificity. US, CT and MRI are useful diagnostic procedures in the evaluation of mural and extramural alterations. In recent years, MR-enteroclysis and MR colonography have been developed, both enable the evaluation of luminal, extraluminal and mural alterations of the bowel. While these modalities provide good imaging evaluation of the bowel, visualization of the different layers, as seen on US, is still not available. Use of high resolution endoluminal coil on MR could improve mural evaluation of bowel to differentiate inflammatory diseases and provide accurate TNM classification of tumoral lesion with minimally invasive procedure.

Evaluation of internal MRI coils using ultimate intrinsic SNR

The upper bounds of the signal-to-noise ratio (also known as the "ultimate intrinsic signal-to-noise ratio" (UISNR)) for internal and external coils were calculated. In the calculation, the body was modeled as a dielectric cylinder with a small coaxial cylindrical cavity in which internal coils could be placed. The calculated UISNR values can be used as reference solutions to evaluate the performance of internal MRI coils. As examples, we evaluated the performance of a loopless antenna and an endourethral coil design by comparing their ISNR with the UISNR.

Miniature ultrasonic probe construction for minimal access surgery

The increasing use of minimal access techniques for surgery has produced a need for imaging technologies that can be used during such interventions. Ultrasound imaging has the advantage that the probe itself can be interventional. Interventional ultrasound probes must be sufficiently small to gain access to the surgical site, and any rigid portion must be limited in length to permit adequate flexibility. In practice this means the ultrasound probes have to operate at high frequencies, and a set of design curves have been produced which relate the number of elements and the ultrasound frequency to the probe dimensions for both linear and cylindrical array configurations. Constructing high-frequency sub-miniature probes presents a number of technical challenges, in particular relating to interconnects and packaging. Solutions to these challenges are discussed using the fabrication of a 1 mm diameter intravascular probe as an example.

Active device tracking and high-resolution intravascular MRI using a novel catheter-based, opposed-solenoid phased array coil

A novel two-element, catheter-based phased array coil was designed and built for both active MR device tracking and high-resolution vessel wall imaging. The device consists of two independent solenoid coils that are wound in opposite directions, connected to separate receive channels, and mounted collinearly on an angiographic catheter. The elements were used independently or together for tracking or imaging applications, respectively. The array's dual functionality was tested on a clinical 1.5 T MRI scanner in vitro, in vivo, and in situ. During real-time catheter tracking, each element gave rise to a high-amplitude peak in the respective projection data, which enabled reliable and robust device tracking as well as automated slice positioning. In vivo microimaging with 240 microm in-plane resolution was achieved in 9 s using the device and TrueFISP imaging. Therefore, a single device was successfully implemented that met the combined requirements of intravascular device tracking and imaging.

In vivo colon wall imaging using endoluminal coils: Feasibility study on rabbits

PURPOSE

To assess in vivo distal colon wall magnetic resonance imaging (MRI) feasibility on rabbits using an endoluminal radio frequency (RF) coil on a 1.5-T clinical scanner.

MATERIALS AND METHODS

The endoluminal coil signal-to-noise ratio (SNR) was compared to a clinical four-element phased-array body coil. High-resolution (HR) MRI of rabbit colon walls was performed on six rabbits. The imaging protocol combined T1-weighted fast low-angle-shot (FLASH) sequences with and without fat saturation (FS), T2-weighted True-Fast imaging with steady state precession (Fisp), turbo spin-echo (TSE), and T1-weighted FLASH FS after contrast media injection. Images were compared to histological sections. Catheter tracking using an endoluminal coil in addition to external coils was also evaluated on two rabbits.

RESULTS

HR images allow visualization and identification of rabbit colon wall layers. Real-time tracking allows a clear visualization and a good positioning of the endoluminal coil within the rabbit.

CONCLUSION

Compared to a clinical multielement array coil, a dedicated endoluminal RF coil provides an important SNR increase at the region of interest (ROI). Very HR images of in vivo rabbit colon walls were achieved providing detailed information on the different wall layers. This technique could be considered on humans for accurate tumoral and inflammatory bowel disease diagnosis.

MRI-guided coronary catheterization and PTCA: A feasibility study on a dog model

The aim of this work was to demonstrate the feasibility of MRI-guided coronary artery catheterization and intervention in a dog model. Experiments were performed on 10 healthy dogs. A 9F introducer sheath was placed through a right carotid artery cutdown. A prototype 0.014-inch coronary MRI guidewire, a prototype 7 French MRI-guiding catheter, and two flexible surface coils were connected to a GE 1.5 T CV/i scanner for simultaneous visualization of the guidewire, guiding catheter, and chest anatomy. Images were displayed in real time on an in-room monitor. A nongated, single-slice fast gradient-echo sequence was used to obtain real-time images of the catheters and background anatomy during the intervention. Fifteen selective catheterizations were attempted in the coronary arteries, and all were successful. Selective injection of diluted gadolinium into the MRI-guiding catheter provided dynamic 2D projection coronary angiography in all cases, confirming successful catheterization. Percutaneous transluminal coronary angioplasty (PTCA) was attempted after two catheterizations, and all attempts were successful. Inflation of the balloon angioplasty catheter was performed successfully in the left anterior and circumflex arteries. Our results indicate that coronary artery catheterization and intracoronary balloon angioplasty are feasible with MRI guidance only. MRI guidance may be used as an alternative to X-ray guidance in coronary artery interventions in the future.

Klinischer Einsatz der interventionellen MRT (iMRT)

The integration of diagnostic and therapeutic procedures by MRI is based on the combination of excellent morphologic and functional imaging. The spectrum of MR-guided interventions includes biopsies, thermal ablation procedures, vascular applications, and intraoperative MRI. In all these applications, different scientific groups have obtained convincing results in basic developments as well as in clinical use. Interventional MRI (iMRI) is expected to attain an important role in interventional radiology, minimal invasive therapy, and monitoring of surgical procedures.

Interventional and intraoperative magnetic resonance imaging

The goal of the Image Guided Therapy Program, as the name implies, is to develop the use of imaging to guide minimally invasive therapy. The program combines interventional and intraoperative magnetic resonance imaging (MRI) with high-performance computing and novel therapeutic devices. In clinical practice the multidisciplinary program provides for the investigation of a wide range of interventional and surgical procedures. The Signa SP 0.5 T superconducting MRI system (GE Medical Systems, Milwaukee, WI) has a 56-cm-wide vertical gap, allowing access to the patient and permitting the execution of interactive MRI-guided procedures. This system is integrated with an optical tracking system and utilizes flexible surface coils and MRI-compatible displays to facilitate procedures. Images are obtained with routine pulse sequences. Nearly real-time imaging, with fast gradient-recalled echo sequences, may be acquired at a rate of one image every 1.5 s with interactive image plane selection. Since 1994, more than 800 of these procedures, including various percutaneous procedures and open surgeries, have been successfully performed at Brigham and Women's Hospital (Boston, MA).

Multiple field of view MR fluoroscopy

This work describes a real-time imaging and visualization technique that allows multiple field of view (FOV) imaging. A stream of images from a single receiver channel can be reconstructed at multiple FOVs within each image frame. Alternately, or in addition, when multiple receiver channels are available, image streams from each channel can be independently reconstructed at multiple FOVs. The implementation described here provides for real-time visualization of the placement of guidewires and catheters on a dynamic roadmap during interventional procedures. The loopless catheter antenna, an electrically active intravascular probe, was used for MR signal reception. In 2D projection images, the catheter and surrounding structures within its diameter of sensitivity appear as bright signal. The simplicity of the resulting images allows very-narrow-FOV imaging to decrease imaging time. Very-narrow-FOV images are acquired on MR receiver channels that collect guidewire or catheter data. These very-narrow-FOV images provide very high frame rate continuous, real-time imaging of the interventional devices (25 fps). Large-FOV images are formed from receiver channels that collect anatomical data from standard imaging surface coils, and simultaneously provide a dynamic, frequently updated roadmap. These multiple-FOV images are displayed together, improving visualization of interventional device placement.

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.

Endourethral MRI

Although high-resolution MRI with phased array pelvic, endorectal, and endovaginal coils has dramatically enhanced the ability to visualize abnormalities of the female urethra and periurethral tissues, controversy still remains about the anatomy of this region. This study introduces an endourethral approach for ultra-high-resolution MRI of the female urethra and the periurethral tissues. To this end, two different radiofrequency (RF) receiver coil designs for an endourethral insertion have been developed: a single-loop coil and a phased array/quadrature coil. Both designs feature a flexible coil circuit, small loss tuning and matching directly at the coil, active decoupling, and the integration of a lambda/4 coaxial choke to decrease unbalanced currents and limit potential RF heating effects. Effective reduction of the mutual inductance between the two coils of the phased array design was achieved by introducing a metallic "paddle" to steer the flux between the coils. The performance of the coils has been evaluated in female human cadaver studies and in an in vivo pig experiment. The novel endourethral approach enabled a dramatic increase of the signal-to-noise ratio (SNR) at the region of interest (ROI). High-resolution MR images of the female urethra have been acquired with a spatial resolution down to 78 x 78 microm. Histologic correlation was achieved for the MR images generated. The achieved high local SNR and resulting high spatial resolution will add valuable information to the discussion of female urethral anatomy. Magn Reson Med 45:138-146, 2001.

Toward MRI-guided coronary catheterization: visualization of guiding catheters, guidewires, and anatomy in real time

The success of x-ray fluoroscopy-guided coronary catheterization depends in part on the ability to obtain simultaneous and real-time visualization of the guidewire, guiding catheter, and anatomy of the chest. The hypothesis explored in this paper is that magnetic resonance imaging (MRI) could provide this ability. This hypothesis was tested with loopless antennas used as the guidewire and a guiding catheter and two surface coils, each connected to four different receiver channels of a GE 1.5-T CV/I MRI scanner. Experiments were conducted on six healthy dogs. Intravascular antennas were inserted in the right carotid artery and maneuvered in the aorta while running a fast gradient-echo sequence (TR/TE 5/1.3 msec, flip angle 7 degrees). Real-time projection images of the chest anatomy, together with the guidewire and guiding catheter, were obtained. Positioning of the MRI guiding catheter either in the descending aorta, ascending aorta, or heart was achieved easily. This study represents a step toward MRI-guided coronary catheterization.

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.

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.