Intravascular MR imaging-guided balloon angioplasty with an MR imaging guide wire: feasibility study in rabbits

Current State of MRI-Guided Endovascular Arterial Interventions: A Systematic Review of Preclinical and Clinical Studies

BACKGROUND

MRI guidance of arterial endovascular interventions could be beneficial as it does not require radiation exposure, allows intrinsic blood-tissue contrast, and enables three-dimensional and functional imaging, however, clinical applications are still limited.

PURPOSE

To review the current state of MRI-guided arterial endovascular interventions and to identify the most commonly reported challenges.

STUDY TYPE

Systematic review.

POPULATION

Pubmed, Embase, Web of Science, and The Cochrane Library were systematically searched to find relevant articles. The search strategy combined synonyms for vascular pathology, endovascular therapy, and real-time MRI guidance.

FIELD STRENGTH/SEQUENCE

No field strength or sequence restrictions were applied.

ASSESSMENT

Two reviewers independently identified and reviewed the original articles and extracted relevant data.

STATISTICAL TESTS

Results of the included original articles are reported.

RESULTS

A total of 24,809 studies were identified for screening. Eighty-eight studies were assessed for eligibility, after which data were extracted from 43 articles (6 phantom, 33 animal, and 4 human studies). Reported technical success rates for animal and human studies ranged between 42% to 100%, and the average complication rate was 5.8% (animal studies) and 8.8% (human studies). Main identified challenges were related to spatial and temporal resolution as well as safety, design, and scarcity of current MRI-compatible endovascular devices.

DATA CONCLUSION

MRI guidance of endovascular arterial interventions seems feasible, however, included articles included mostly small single-center case series. Several hurdles remain to be overcome before larger trials can be undertaken. Main areas of research should focus on adequate imaging protocols with integrated tracking of dedicated endovascular devices.

High-resolution intravascular magnetic resonance imaging of the coronary artery wall at 3.0 Tesla: toward evaluation of atherosclerotic plaque vulnerability

Background

To validate the feasibility of generating high-resolution intravascular 3.0 Tesla (T) magnetic resonance imaging of the coronary artery wall to further plaque imaging.

Methods

A receive-only 0.014-inch diameter magnetic resonance imaging guidewire (MRIG) was manufactured for intravascular imaging within a phantom experiment and the coronary artery wall of the swine. For coronary artery wall imaging, both high-resolution images and conventional resolution images were acquired. A 16-channel commercial surface coil for magnetic resonance imaging was employed for the control group.

Results

For the phantom experiment, the MRIG showed a higher signal-to-noise ratio than the surface coil. The peak signal-to-noise ratio of the MRIG and the surface coil-generated imaging were 213.6 and 19.8, respectively. The signal-to-noise ratio decreased rapidly as the distance from the MRIG increased. For the coronary artery wall experiment, the vessel wall imaging by the MRIG could be identified clearly, whereas the vessel wall imaging by the surface coil was blurred. The average signal-to-noise ratio of the artery wall was 21.1±5.40 by the MRIG compared to 8.4±2.19 by the surface coil, where the resolution was set at 0.2 mm × 0.2 mm × 2 mm. As expected, the high-resolution sequence clearly showed more details than the conventional resolution sequence set at 0.7 mm × 0.7 mm × 2.0 mm. Histological examination showed no evidence of mechanical injuries in the target vessel walls.

Conclusions

The study validated the feasibility of generating magnetic resonance imaging (MRI) at 0.2 mm × 0.2 mm × 2 mm for the coronary artery wall using a 0.014 inch MRIG.

MR-guided Cardiac Interventions

Diagnostic and interventional cardiac catheterization is routinely used in the diagnosis and treatment of congenital heart disease. There are well-established concerns regarding the risk of radiation exposure to patients and staff, particularly in children given the cumulative effects of repeat exposure. Magnetic resonance imaging (MRI) offers the advantage of being able to provide better soft tissue visualization, tissue characterization, and quantification of ventricular volumes and vascular flow. Initial work using MRI catheterization employed fusion of x-ray and MRI techniques, with x-ray fluoroscopy to guide catheter placement and subsequent MRI assessment for anatomical and hemodynamic assessment. Image overlay of 3D previously acquired MRI datasets with live fluoroscopic imaging has also been used to guide catheter procedures.Hybrid x-ray and MRI-guided catheterization paved the way for clinical application and validation of this technique in the assessment of pulmonary vascular resistance and pharmacological stress studies. Purely MRI-guided catheterization also proved possible with passive catheter tracking. First-in-man MRI-guided cardiac catheter interventions were possible due to the development of MRI-compatible guidewires, but halted due to guidewire limitations.More recent developments in passive and active catheter tracking have led to improved visualization of catheters for MRI-guided catheterization. Improvements in hardware and software have also increased image quality and scanning times with better interactive tools for the operator in the MRI catheter suite to navigate through the anatomy as required in real time. This has expanded to MRI-guided electrophysiology studies and radiofrequency ablation in humans. Animal studies show promise for the utility of MRI-guided interventional catheterization. Ongoing investment and development of MRI-compatible guidewires will pave the way for MRI-guided diagnostic and interventional catheterization coming into the mainstream.

MRI use for atrial tissue characterization in arrhythmias and for EP procedure guidance

We review the utilization of magnetic resonance imaging methods for classifying atrial tissue properties that act as a substrate for common cardiac arrhythmias, such as atrial fibrillation. We then review state-of-the-art methods for mapping this substrate as a predicate for treatment, as well as methods used to ablate the electrical pathways that cause arrhythmia and restore patients to sinus rhythm.

Magnetic Resonance-guided Active Catheter Tracking

Several advantages of MR imaging compared with other imaging modalities have provided the rationale for increased attention to MR-guided interventions, including its excellent soft tissue contrast, its capability to show both anatomic and functional information, and no use of ionizing radiation. An important aspect of MR-guided intervention is to provide visualization and navigation of interventional devices relative to the surrounding tissues. This article focuses on the methods for MR-guided active tracking in catheter-based interventions. Practical issues about implementation of active catheter tracking in a clinical setting are discussed and several current application examples are highlighted.

Intrabiliary RF heat-enhanced local chemotherapy of a cholangiocarcinoma cell line: monitoring with dual-modality imaging--preclinical study

PURPOSE

To determine whether magnetic resonance (MR) imaging heating guidewire-mediated radiofrequency (RF) hyperthermia could enhance the therapeutic effect of gemcitabine and 5-fluorouracil (5-FU) in a cholangiocarcinoma cell line and local deposit doses of chemotherapeutic drugs in swine common bile duct (CBD) walls.

MATERIALS AND METHODS

The animal protocol was approved by the institutional animal care and use committee. Green fluorescent protein-labeled human cholangiocarcinoma cells and cholangiocarcinomas in 24 mice were treated with (a) combination therapy with chemotherapy (gemcitabine and 5-FU) plus RF hyperthermia, (b) chemotherapy only, (c) RF hyperthermia only, or (d) phosphate-buffered saline. Cell proliferation was quantified, and tumor changes over time were monitored with 14.0-T MR imaging and optical imaging. To enable further validation of technical feasibility, intrabiliary local delivery of gemcitabine and 5-FU was performed by using a microporous balloon with (eight pigs) or without (eight pigs) RF hyperthermia. Chemotherapy deposit doses in the bile duct walls were quantified by means of high-pressure liquid chromatography. The nonparametric Mann-Whitney U test and the paired-sample Wilcoxon signed rank test were used for data analysis.

RESULTS

Combination therapy induced lower mean levels of cell proliferation than chemotherapy only and RF hyperthermia only (0.39 ± 0.13 [standard deviation] vs 0.87 ± 0.10 and 1.03 ± 0.13, P < .001). Combination therapy resulted in smaller relative tumor volume than chemotherapy only and RF hyperthermia only (0.65 ± 0.03 vs 1.30 ± 0.021 and 1.37 ± 0.05, P = .001). Only in the combination therapy group did both MR imaging and optical imaging show substantial decreases in apparent diffusion coefficients and fluorescent signals in tumor masses immediately after the treatments. Chemotherapy quantification showed a higher average drug deposit dose in swine CBD walls with intrabiliary RF hyperthermia than without it (gemcitabine: 0.32 mg/g of tissue ± 0.033 vs 0.260 mg/g ± 0.030 and 5-FU: 0.660 mg/g ± 0.060 vs 0.52 mg/g ± 0.050, P < .05).

CONCLUSION

The use of intrabiliary MR imaging heating guidewire-mediated RF hyperthermia can enhance the chemotherapeutic effect on a human cholangiocarcinoma cell line and local drug deposition in swine CBD tissues.

Interventional loopless antenna at 7 T

The loopless antenna magnetic resonance imaging detector is comprised of a tuned coaxial cable with an extended central conductor that can be fabricated at submillimeter diameters for interventional use in guidewires, catheters, or needles. Prior work up to 4.7 T suggests a near-quadratic gain in signal-to-noise ratio with field strength and safe operation at 3 T. Here, for the first time, the signal-to-noise ratio performance and radiofrequency safety of the loopless antenna are investigated both theoretically, using the electromagnetic method-of-moments, and experimentally in a standard 7 T human scanner. The results are compared with equivalent 3 T devices. An absolute signal-to-noise ratio gain of 5.7 ± 1.5-fold was realized at 7 T vs. 3 T: more than 20-fold higher than at 1.5 T. The effective field-of-view area also increased approximately 10-fold compared with 3 T. Testing in a saline gel phantom suggested that safe operation is possible with maximum local 1-g average specific absorption rates of <12 W kg(-1) and temperature increases of <1.9°C, normalized to a 4 W kg(-1) radiofrequency field exposure at 7 T. The antenna did not affect the power applied to the scanner's transmit coil. The signal-to-noise ratio gain enabled magnetic resonance imaging microscopy at 40-50 μm resolution in diseased human arterial specimens, offering the potential of high-resolution large-field-of-view or endoscopic magnetic resonance imaging for targeted intervention in focal disease.

Interventional MRI: tapering improves the distal sensitivity of the loopless antenna

The "loopless antenna" is an interventional MRI detector consisting of a tuned coaxial cable and an extended inner conductor or "whip". A limitation is the poor sensitivity afforded at, and immediately proximal to, its distal end, which is exacerbated by the extended whip length when the whip is uniformly insulated. It is shown here that tapered insulation dramatically improves the distal sensitivity of the loopless antenna by pushing the current sensitivity toward the tip. The absolute signal-to-noise ratio is numerically computed by the electromagnetic method-of-moments for three resonant 3-T antennae with no insulation, uniform insulation, and with linearly tapered insulation. The analysis shows that tapered insulation provides an approximately 400% increase in signal-to-noise ratio in trans-axial planes 1 cm from the tip and a 16-fold increase in the sensitive area as compared to an equivalent, uniformly insulated antenna. These findings are directly confirmed by phantom experiments and by MRI of an aorta specimen. The results demonstrate that numerical electromagnetic signal-to-noise ratio analysis can accurately predict the loopless detector's signal-to-noise ratio and play a central role in optimizing its design. The manifold improvement in distal signal-to-noise ratio afforded by redistributing the insulation should improve the loopless antenna's utility for interventional MRI.

Can Magnetic Resonance Imaging Be the Key Technique to Visualize and Investigate Endovascular Biomaterials?

OBJECTIVE

Magnetic resonance imaging (MRI) is an established modality in clinical use but may be potentially underutilized to visualize and investigate biomaterials. As its use is totally contraindicated only for ferromagnetic devices, it was employed to visualize deployment, biofonctionality, healing, and biodurability of a commercially available endovascular device, namely the Medtronic-AVE AneuRx. The quality of the observations coupled with the absence of ionizing radiations are likely to make this technique an attractive imaging modality in the future.

METHOD

The potential benefits of the MRI technique were investigated in a GE Vectra-MR 0.5T MRI for the Medtronic-AVE AneuRx endovascular prosthesis, under different conditions: undeployed i.e., inserted in the delivery cartridge as received from the manufacturer (step 1), deployed in a mock glass-aneurysm tube (step 2), and as a pathological explant harvested at the autopsy of a patient (step 3). The device was submitted to X-rays for examination in addition to MRI. At step 3, the device was further investigated with light microscopy and scanning electron microscopy (SEM) together with X-ray diffraction.

RESULTS

The device which was inserted and pleated in the delivery cartridge did not demonstrate any significant observation either in MRI or in X-rays. When it was deployed in the mock aneurysmal glass tube, light artefacts were associated with the T2 weighed FSE images around the Nitinol whereas X-rays gave images of indisputable interest. Similar results were noted using the explanted device. Very high contrasts were obtained with T1 whereas T2 images were almost defect free. The X-rays allowed to accurate imaging of the Nitinol skeleton but were poor to discriminate between the different tissues. Pathology observations using light microscopy were not really challenged, as the magnetic resonance imaging was performed using a 0.5T machine.

DISCUSSION

The benefits of magnetic resonance imaging as a quality control technique to examine an endovascular device within its cartridge remains ill defined. Similarly, the role of conventional X-rays is unknown. The observation of devices fully deployed in a mock aneurysmal glass-tube under MRI are potentially useful but X-rays images allowed better definition. The MRI examination of the explanted device does permit observations related to the healing of the device that might be obtained in vivo and, thus offers new avenues for the follow-up of implanted devices. The pathological investigations brought additional informations about the tissues and the corrosion of the Nitinol. However, it is unlikely that MRI will permit detailed analysis of the biomaterials and in particular the corrosion process of the stents.

CONCLUSION

These early observations of the follow-up of devices using MRI warrant further investigation. The absence of ionizing radiation with MRI makes this technique particularly attractive. As there is no emission of ionizing radiation associated with magnetic resonance, it is recommended that further investigation using this environment friendly technique for the follow-up of devices made of biomaterials that are MRI compatible.

The performance of interventional loopless MRI antennae at higher magnetic field strengths

Interventional, "loopless antenna" MRI detectors are currently limited to 1.5 T. This study investigates whether loopless antennae offer signal-to-noise ratio (SNR) and field-of-view (FOV) advantages at higher fields, and whether device heating can be controlled within safe limits. The absolute SNR performance of loopless antennae from 0.5 to 5 T is investigated both analytically, using electromagnetic (EM) dipole antenna theory, and numerically with the EM method of moments, and found to vary almost quadratically with field strength depending on the medium's electrical properties, the noise being dominated by direct sample conduction losses. The prediction is confirmed by measurements of the absolute SNR of low-loss loopless antennae fabricated for 1.5, 3, and 4.7 T, immersed in physiologically comparable saline. Gains of 3.8 +/- 0.2- and 9.7 +/- 0.3-fold in SNR, and approximately 10- and 50-fold gains in the useful FOV area are observed at 3 and 4.7 T, respectively, compared to 1.5 T. Heat testing of a 3 T biocompatible nitinol-antenna fabricated with a redesigned decoupling circuit shows maximum heating of approximately 1 degrees C for MRI operating at high MRI exposure levels. Experiments in the rabbit aorta confirm the SNR and FOV advantages of the 3 T antenna versus an equivalent commercial 1.5 T device in vivo. This work is the first to study the performance of experimental internal MRI detectors above 1.5 T. The large SNR and FOV gains realized present a major opportunity for high-resolution imaging of vascular pathology and MRI-guided intervention.

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.

Interventional cardiovascular magnetic resonance imaging

Magnetic resonance imaging provides structural and functional cardiovascular information with excellent soft tissue contrast. Real-time magnetic resonance imaging can guide transcatheter cardiovascular interventions in large animal models and may prove superior to x-ray and adjunct modalities for peripheral vascular, structural heart, and cardiac electrophysiology applications. We describe technical considerations, preclinical work, and early clinical studies in this emerging field.

The future of real-time cardiac magnetic resonance imaging

Dynamic changes in cardiac structure and function are usually examined by real-time imaging techniques such as angiography or echocardiography. MRI has many advantages compared with these established cardiac imaging modalities. However, system hardware and software limitations have limited cardiac MRI to gated acquisitions that are lengthy and often result in failed acquisitions and examinations. Recently, MRI has evolved into a technique capable of imaging dynamic processes in real time. Improvements in hardware, pulse sequences, and image reconstruction algorithms have enabled real-time cardiac MRI with high spatial resolution, high temporal resolution, and various types of image contrast without requiring cardiac gating or breath-holding. This article provides an overview of current capability and highlights key technical and clinical advances. The future prospects of real-time cardiac MRI will depend on 1) the development of techniques that further improve signal to noise ratio, contrast, spatial resolution, and temporal resolution, without introducing artifacts; 2) the development of software infrastructure that facilitates rapid interactive examination; and 3) the development and validation of several new clinical assessments.

Magnetic resonance imaging-guided balloon angioplasty of coarctation of the aorta: a pilot study

BACKGROUND

MRI guidance of percutaneous transluminal balloon angioplasty (PTA) of aortic coarctation (CoA) would be desirable for continuous visualization of anatomy and to eliminate x-ray exposure. The aim of this study was (1) to determine the suitability of MRI-controlled PTA using the iron oxide-based contrast medium Resovist (ferucarbotran) for catheter visualization and (2) to subsequently apply this technique in a pilot study with patients with CoA.

METHODS AND RESULTS

The MRI contrast-to-noise ratio and artifact behavior of Resovist-treated balloon catheters was optimized in in vitro and animal experiments (pigs). In 5 patients, anatomy of the CoA was evaluated before and after intervention with high-resolution respiratory-navigated 3D MRI and multiphase cine MRI. Position monitoring of Resovist-treated catheters was realized with interactive real-time MRI. Aortic pressures were continuously recorded. Conventional catheterization was performed before and after MRI to confirm interventional success. During MRI, catheters filled with 25 micromol of iron particles per milliliter of Resovist produced good signal contrast between catheters and their background anatomy but no image distortion due to susceptibility artifacts. All MRI procedures were performed successfully in the patient study. There was excellent agreement between the diameters of CoA and pressure gradients as measured during MRI and conventional catheterization. In 4 patients, PTA resulted in substantial widening of the CoA and a decrease in pressure gradients. In 1 patient, PTA was ineffective.

CONCLUSIONS

The MRI method described represents a potential alternative to conventional x-ray fluoroscopy for catheter-based treatment of patients with CoA.

High-resolution real-time spiral MRI for guiding vascular interventions in a rabbit model at 1.5T

PURPOSE

To study the feasibility of a combined high spatial and temporal resolution real-time spiral MRI sequence for guiding coronary-sized vascular interventions.

MATERIALS AND METHODS

Eight New Zealand White rabbits (four normal and four with a surgically-created stenosis in the abdominal aorta) were studied. A real-time interactive spiral MRI sequence combining 1.1 x 1.1 mm(2) in-plane resolution and 189-msec total image acquisition time was used to image all phases of an interventional procedure (i.e., guidewire placement, balloon angioplasty, and stenting) in the rabbit aorta using coronary-sized devices on a 1.5 T MRI system.

RESULTS

Real-time spiral MRI identified all rabbit aortic stenoses and provided high-temporal-resolution visualization of guide-wires crossing the stenoses in all animals. Angioplasty balloon dilatation and deployment of coronary-sized copper stents in the rabbit aorta were also successfully imaged by real-time spiral MRI.

CONCLUSION

Combining high spatial and temporal resolution with spiral MRI allows real-time MR-guided vascular intervention using coronary-sized devices in a rabbit model. This is a promising approach for guiding coronary interventions.

MR-guided endovascular interventions: device visualization, tracking, navigation, clinical applications, and safety aspects

Reliable visualization and tracking are essential for guiding endovascular devices within blood vessels. The most commonly used methods are susceptibility artifact-based tracking that relies on the artifact created within the image by the device and microcoil- or antenna-based tracking that uses the high signal generated by small MR endovascular receive coils when the transmit coil emits a nonselective radiofrequency pulse. To date, the use of endovascular MR guidance techniques has primarily been confined to animal experiments. There are only a few reports on MR-guided endovascular applications in patients. Therefore, access to the patient within the scanner, dedicated devices, and safety issues remain major challenges. To face these challenges, attention from all radiologists, especially interventional radiologists, is required to make MR-guided endovascular procedures a clinical reality.

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.

MR imaging-guided percutaneous angioplasty and stent placement in a swine model comparison of open- and closed-bore scanners

RATIONALE AND OBJECTIVES

The purpose of this study is to compare the feasibility and precision of renal artery angioplasty and stent placement using two different MR scanners.

MATERIALS AND METHODS

MR imaging-guided angioplasty and stent placements were performed on seven pigs using 0.2 and 1.5 T scanners (Magnetom Open and Magnetom Sonata, Siemens Medical Solutions, Erlangen, Germany). For guidance of catheters, guide wires and stents susceptibility artifact-based tracking was used. The end point of each intervention was to position a stent in the renal artery with its proximal end at the level of the aortic wall. Procedure time and stent position were evaluated.

RESULTS

Catheterization, angioplasty, and stent placement were feasible using MRI guidance at both 0.2 and 1,5 Tesla. At 1.5 T all catheter manipulations and interventions were performed in less than 30 minutes. At 0.2 T the interventions took up to 90 minutes. No significant difference in the stent deviation was noted between the two scanners.

CONCLUSION

The use of a high-performance 1.5 T scanner helped to reduce the procedure time to half of that of a low-field system. Since no difference in stent placement precision was noted, a dedicated MR-stent might be mandatory for more precise stent placement.

Radiofrequency-enhanced vascular gene transduction and expression for intravascular MR imaging-guided therapy: feasibility study in pigs

PURPOSE

To evaluate the feasibility of radiofrequency (RF)-enhanced vascular gene transduction and expression by using a magnetic resonance (MR) imaging-heating guidewire as an intravascular heating vehicle during MR imaging-guided therapy.

MATERIALS AND METHODS

The institutional committee for animal care and use approved the experimental protocol. The study included in vitro evaluation of the use of RF energy to enhance gene transduction and expression in vascular cells, as well as in vivo validation of the feasibility of intravascular MR imaging-guided RF-enhanced vascular gene transduction and expression in pig arteries. For in vitro experiments, approximately 10(4) vascular smooth muscle cells were seeded in each of four chambers of a cell culture plate. Next, 1 mL of a green fluorescent protein gene (gfp)-bearing lentivirus was added to each chamber. Chamber 4 was heated at approximately 41 degrees C for 15 minutes by using an MR imaging-heating guidewire connected to a custom RF generator. At day 6 after transduction, the four chambers were examined and compared at confocal microscopy to determine the efficiency of gfp transduction and expression. For the in vivo experiments, a lentivirus vector bearing a therapeutic gene, vascular endothelial growth factor 165 (VEGF-165), was transferred by using a gene delivery balloon catheter in 18 femoral-iliac arteries (nine artery pairs) in domestic pigs and Yucatan pigs with atherosclerosis. During gene infusion, one femoral-iliac artery in each pig was heated to approximately 41 degrees C with RF energy transferred via the intravascular MR imaging-heating guidewire, while the contralateral artery was not heated (control condition). At day 6, the 18 arteries were harvested for quantitative Western blot analysis to compare VEGF-165 transduction and expression efficiency between RF-heated and nonheated arterial groups.

RESULTS

Confocal microscopy showed gfp expression in chamber 4 that was 293% the level of expression in chamber 1 (49.6% +/- 25.8 vs 16.8% +/- 8.0). Results of Western blot analysis showed VEGF-165 expression for normal arteries in the RF-heated group that was 300% the level of expression in the nonheated group (70.4 arbitrary units [au] +/- 107.1 vs 23.5 au +/- 29.8), and, for atherosclerotic arteries in the RF-heated group, 986% the level in the nonheated group (129.2 au +/- 100.3 vs 13.1 au +/- 4.9).

CONCLUSION

Simultaneous monitoring and enhancement of vascular gene delivery and expression is feasible with the MR imaging-heating guidewire.

MR-guided percutaneous intramyocardial injection with an MR-compatible catheter: feasibility and changes in T1 values after injection of extracellular contrast medium in pigs

PURPOSE

To assess the feasibility of percutaneous magnetic resonance (MR)-guided intramyocardial injection of gadodiamide by using real-time imaging and to quantify T1 values and the size of the enhanced region for different concentrations of contrast agent for 30 minutes after injection.

MATERIALS AND METHODS

Animal care committee approval was obtained. A catheter with a needle tip was advanced into the left ventricle in seven pigs by using real-time imaging with radial steady-state free precession. After intramyocardial injection of 2 mL of solution at concentrations of 0.05 or 0.10 mmol/mL gadodiamide, local changes in T1 values and size of the contrast material-enhanced region were sequentially measured at 3, 15, and 30 minutes after injection by using the Look-Locker sequence. Two-tailed paired Student t tests were used for statistical analysis.

RESULTS

Catheter guidance and visualization of contrast agent distribution were feasible in all animals. Regional changes in T1 values were significantly different for different contrast agent concentrations (for 0.05 mmol/mL, 456 msec +/- 5 [+/- standard error of the mean]; for 0.10 mmol/mL, 228 msec +/- 4; P < .001) measured 3 minutes after injection. T1 values increased significantly (P < .05) to 720 msec +/- 7 for 0.05 mmol/mL gadodiamide and 445 msec +/- 6 for 0.10 mmol/mL gadodiamide 30 minutes after injection but remained significantly lower than those of remote myocardium (879 msec +/- 8). The size of the contrast-enhanced region increased from 13 mm(2) +/- 2 at 3 minutes to 30 mm(2) +/- 3 at 30 minutes (P < .05).

CONCLUSION

Catheter MR-guided percutaneous intramyocardial injection is feasible; after intramyocardial injection of gadodiamide at concentrations of 0.05 and 0.10 mmol/mL, T1 values decreased over the observation time.

Preliminary ex vivo 3D microscopy of coronary arteries using a standard 1.5 T MRI scanner and a superconducting RF coil

This paper presents the feasibility of three-dimensional (3D) magnetic resonance (MR) histology of atheromatous coronary lesions in the entire human heart ex vivo using a standard 1.5 T scanner and a 12 mm high-temperature superconducting (HTS) surface coil. The HTS coil was a five-turn transmission-line resonator operated at 77 K, affording a signal-to-noise ratio (SNR) gain of about ninefold as compared to a similar, room-temperature copper coil. Local microscopy at the surface of an explanted, entire heart was achieved by a 3D spoiled gradient echo sequence and assessed by comparison with conventional histology. One hundred and twenty four adjacent cross sections of the coronary artery, with voxels of 59 x 59 x 100 microm3 and an SNR of about 20, were obtained in 25 min. Consecutive data sets were combined to reconstruct extended views along the artery. Compared to histology, MR microscopy allowed precise nondestructive 3D depiction of the architecture of the atheromatous plaques. This is the first report of microscopic details (less than 10(-3) mm3 voxels) of diseased arteries obtained in an entire human heart preserving the arterial integrity and the spatial geometry of atheroma. This noninvasive microscopy approach using a HTS surface coil might be applied in vivo to study the architecture and components of superficial human structures, using routine MR scanners.

Development of a 0.014-inch magnetic resonance imaging guidewire

The purpose of this study was to develop a standard 0.014-inch intravascular magnetic resonance imaging guidewire (MRIG), a coaxial cable with an extension of the inner conductor, specifically designed for use in the small vessels. After a theoretical analysis, the 0.014-inch MRIG was built by plating/cladding highly electrically conductive materials, silver or gold, over the inside and outside of the coaxial conductors. The conductors were made of superelastic, nonmagnetic, biocompatible materials, Nitinol or MP35N. Then, in comparison with a previously designed 0.032-inch MRIG, the performance of the new 0.014-inch MRIG in vitro and in vivo was successfully evaluated. This study represents the initial work to confirm the critical role of highly conductive and superelastic materials in building such small-size MRIGs, which are expected to generate high-resolution MR imaging of vessel walls/plaques and guide endovascular interventional procedures in the small vessels, such as the coronary arteries.

Renal Embolization: Feasibility of Magnetic Resonance-Guidance Using Active Catheter Tracking and Intraarterial Magnetic Resonance Angiography

RATIONALE AND OBJECTIVES

Magnetic resonance (MR)-guidance of endovascular interventions offers various advantages, including the absence of ionizing radiation, excellent soft tissue contrast, and multiplanar and functional imaging capabilities. The objective of this study was to assess the feasibility of MR-guided renal embolization using active catheter tracking with automatic slice positioning and intraarterial contrast-enhanced MR angiography (MRA).

MATERIALS AND METHODS

MR-guided embolization of 16 kidneys was attempted in 15 pigs using real-time tracking of active 5-Fr. catheters. Embolization was monitored by selective intraarterial projection MRA. Intraarterial three-dimensional (3D) MRA was used for the assessment of embolization results. Additional pathologic correlation was available in 2 animals. The image quality of intraarterial 3D contrast-enhanced-MRA was rated by an independent radiologist who was not involved in the animal experiments.

RESULTS

Active catheter tracking with automatic slice positioning allowed reliable catheter guidance and catheterization of the renal artery in all animals. Embolization was successful in all kidneys (11 left, 5 right), as verified by intraarterial 3D contrast-enhanced MRA (ce-MRA) and/or pathology. The image quality of intraarterial 3D ce-MRA was rated excellent in 10 animals, moderate in 4 animals, and poor in 1 animal.

CONCLUSION

Renal embolization using active catheter tracking and intraarterial ce-MRA is feasible. Selective intraarterial ce-MRA allows the assessment of blood supply and organ perfusion before, during, and after therapeutic interventions, thereby complementing MR-guided endovascular interventions.

Magnetic resonance imaging-guided coronary interventions

Magnetic resonance imaging (MRI) guidance for coronary interventions offers potential advantages over conventional x-ray angiography. Advantages include the use of nonionizing radiation, combined assessment of anatomy and function, and three-dimensional assessment of the coronary arteries leading to the myocardium. These advantages have prompted a series of recent studies in this field. Real-time coronary MR angiography, with low-dose catheter-directed intraarterial (IA) infusion of contrast media, has achieved in-plane spatial resolution as low as 0.8 x 0.8 mm2 and temporal resolution as short as 130 msec per image. Catheter-based IA injection of contrast agent has proven useful in the collection of multislice and three-dimensional images, not only for coronary intervention guidance, but also in the assessment of regional myocardial perfusion fed by the affected vessel. Actively visible guidewires and guiding catheters, based on the loopless antenna concept, have been effectively used to negotiate tortuous coronary vessels during catheterization, permitting placement of coronary angioplasty balloon catheters. Passive tracking approaches have been used to image contrast agent-filled coronary catheters and to place susceptibility-based endovascular stents. Although the field is in its infancy, these early results demonstrate the feasibility for performing MRI-guided coronary interventions. Although further methodological and technical developments are required before these methods become clinically applicable, we anticipate that MRI someday will be included in the armamentarium of techniques used to diagnose and treat coronary artery disease.

Catheter-directed contrast-enhanced coronary MR angiography in swine using magnetization-prepared True-FISP

Contrast-enhanced (CE) coronary magnetic resonance angiography (MRA) following intraarterial (IA) injection of contrast agent was compared using two sequences in swine: magnetization-prepared fast imaging with steady-state precession (True-FISP), and magnetization-prepared fast low-angle shot (FLASH). Thick-slice projection images were acquired with submillimeter in-plane spatial resolution (0.9 x 0.8 mm(2)). The magnetization-preparation scheme provided a clear delineation of the major coronary arteries with excellent background suppression. The True-FISP acquisition resulted in an increase in signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) by approximately a factor of 2 over FLASH (P < 0.05). Magnetization-prepared True-FISP is a promising technique for catheter-directed CE thick-slice projection coronary MRA.

Intravascular extended sensitivity (IVES) MRI antennas

The design and application of an intravascular extended sensitivity (IVES) MRI antenna is described. The device is a loopless antenna design that incorporates both an insulating, dielectric coating and a winding of the antenna whip into a helical shape. Because this antenna produces a broad region of high SNR and also allows for imaging near the tip of the device, it is useful for imaging long, luminal structures. To elucidate the design and function of this device, the effects of both insulation and antenna winding were characterized by theoretical and experimental studies. Insulation broadens the longitudinal region over which images can be collected (i.e., along the lumen of a vessel) by increasing the resonant pole length. Antenna winding, conversely, allows for imaging closer to the tip of the antenna by decreasing the resonant pole length. Over a longitudinal region of 20 cm, the IVES imaging antenna described here produces a system SNR of approximately 40,000/r (mL(-1)Hz(1/2)), where r is the radial distance from the antenna axis in centimeters. As opposed to microcoil antenna designs, these antennas do not require exact positioning and allow for imaging over broad tissue regions. While focusing on the design of the IVES antenna, this work also serves to enhance our overall understanding of the properties and behavior of the loopless antenna design.

Endovascular interventional magnetic resonance imaging

Minimally invasive interventional radiological procedures, such as balloon angioplasty, stent placement or coiling of aneurysms, play an increasingly important role in the treatment of patients suffering from vascular disease. The non-destructive nature of magnetic resonance imaging (MRI), its ability to combine the acquisition of high quality anatomical images and functional information, such as blood flow velocities, perfusion and diffusion, together with its inherent three dimensionality and tomographic imaging capacities, have been advocated as advantages of using the MRI technique for guidance of endovascular radiological interventions. Within this light, endovascular interventional MRI has emerged as an interesting and promising new branch of interventional radiology. In this review article, the authors will give an overview of the most important issues related to this field. In this context, we will focus on the prerequisites for endovascular interventional MRI to come to maturity. In particular, the various approaches for device tracking that were proposed will be discussed and categorized. Furthermore, dedicated MRI systems, safety and compatibility issues and promising applications that could become clinical practice in the future will be discussed.

Imaging of vascular gene therapy

Gene therapy is an exciting frontier in medicine today. Many genes have been shown to be useful for treatment of various vascular diseases, including chronic cardiac and limb ischemia syndromes, vasculoproliferative disorder, hypercholesterolemia, atherosclerosis, thrombosis, and hypertension. Precise delivery of genes into target vessels, efficient transfer of genes into vascular cells of the target, and prompt assessment of gene expression over time are three challenging tasks for successful vascular gene therapy. Thus, in vivo imaging methods that can be used to monitor gene delivery and localize gene expression are needed. Modern imaging techniques provide an opportunity to monitor and direct vascular gene therapy. Radiologists play a key role not only in developing and mastering endovascular genetic interventions but also in assessing the success of vascular gene therapy and directing further refinement of vascular gene therapy technology. This article provides an overview of the current status of imaging of vascular gene therapy.

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.

Vascular MRI in the diagnosis and therapy of the high risk atherosclerotic plaque

Disruption of a high risk plaque is known as the primary cause of cardiovascular events. Characterization of arterial wall components has become an essential adjunct in the identification of patients with plaques prone to rupture. Magnetic Resonance Imaging (MRI) has been revealed as one of the noninvasive tools possibly capable of identifying and characterizing high risk atherosclerotic plaque. MRI may facilitate diagnosis, and guide and serially monitor interventional and pharmacological treatment of atherosclerotic disease. In addition, it permits the simultaneous assessment of the anatomy, morphology, and hemodynamics for the study of flow-induced atherogenesis. It possibly will identify asymptomatic patients with subclinical atherosclerosis. This has potential significance for the improvement of strategies in primary and secondary prevention.

Endovascular stents in pulmonary valve and artery in swine: feasibility study of MR imaging-guided deployment and postinterventional assessment

PURPOSE

To assess the feasibility of using magnetic resonance (MR) imaging to guide stent deployment in the pulmonary valve and artery and evaluate, after stent deployment, the position and morphology of and blood flow through the stent.

MATERIALS AND METHODS

Angiography and 1.5-T MR imaging were performed in a dual-imaging suite. Nitinol stents were placed in the pulmonary valve and main pulmonary artery in five pigs by using MR imaging guidance. For interactive MR imaging monitoring of catheter manipulation and stent delivery, balanced fast field-echo and T1-weighted turbo field-echo sequences were used. Visualization of the delivery system was based on T2* (with air as the contrast material) or T1 (with gadodiamide as the contrast material). After stent deployment, the position and morphology of and flow through the stent were verified with multiphase multisection balanced fast field-echo and velocity-encoded cine MR imaging. Findings at angiography and postmortem examination also helped verify stent placement. The paired Student t test was used for data analysis.

RESULTS

The stent was successfully deployed in all animals. The stent was placed distal to the pulmonary valve in four animals and across the pulmonary valve in one animal. The position and morphology of the stent were clearly depicted on balanced fast field-echo images. In the animal with the stent placed across the pulmonary valve, the pulmonary regurgitant fraction was 37%; this was not seen in the animals with stents placed distal to the pulmonary valve. No complication (eg, stent migration, intramural injury, or vascular perforation) was noted during the intervention. Findings at angiography and postmortem examination confirmed the position of the stents.

CONCLUSION

MR imaging has the potential to guide stent placement in the pulmonary valve or artery and to evaluate flow volume within the stent lumen after the intervention.

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.

Use of internal coils for independent and direct MR imaging-guided endovascular device tracking

PURPOSE

To test the hypotheses that a single internal guide wire coil (i) permits independent and direct depiction of guide wires and catheters and (ii) improves catheter-tracking accuracy and depiction compared to external receiver coils.

MATERIALS AND METHODS

Standard 5-6-F angiographic catheters were filled with dilute 4% gadolinium chelate. A single 0.030-inch-diameter internal guide wire coil was placed inside the catheter. True fast imaging with steady-state precession was used to directly visualize the guide wire. Inversion recovery-prepared fast low-angle shot technique was used to track catheters over a thick slice. In phantom experiments, we compared catheter signal-to-noise ratios (SNRs) with the internal coil and a phased-array surface coil with use of the Wilcoxon signed-rank test. Tip-tracking accuracy was assessed with use of linear regression. In pigs (n = 7), catheters and guide wires were independently tracked in real time.

RESULTS

In phantoms, catheter SNR with the internal coil (12.0) was significantly greater than that with the surface coil (4.0; P =.001). Tip-tracking accuracy was also improved with use of the internal coil (R(2) = 0.94 vs 0.50). In swine vasculature, catheters and guide wires could be directly and independently tracked at 1.7-2.0 frames per second. Catheters were clearly visualized with use of the internal coil, with a typical catheter background contrast-to-noise ratio of 6.6. Catheters were not visible with use of the external coil because of the small catheter size compared to the slice thickness.

CONCLUSION

Internal guide wire coils permit independent and direct depiction of guide wires and catheters in vivo for MR imaging-guided endovascular interventions. They also improve catheter tracking accuracy and depiction compared to external coils.

Simultaneous real-time visualization of the catheter tip and vascular anatomy for MR-guided PTA of iliac arteries in an animal model

PURPOSE

To examine the feasibility of simultaneous MR real-time active tip tracking and near real-time depiction of the vascular anatomy for percutaneous angioplasty of iliac arteries under MR guidance.

MATERIALS AND METHODS

Nine surgically created stenoses of external iliac arteries in pigs were dilated with MR-compatible balloon catheters (Cordis, Roden, The Netherlands). These catheters were equipped with a microcoil for active tracking of the catheter tip with an in-plane update rate of 10 positions per second. The procedures were performed on an interventional 1.5 T Gyroscan ACS-NT scanner (Philips, Best, The Netherlands). Real-time calculation of images acquired by radial k-space filling was performed on a specially designed backprojector exploiting the sliding window reconstruction technique (Philips Research Laboratories, Hamburg, Germany). The image update rate was 20 frames per second using a radial gradient-echo technique (TR = 12 msec, TE = 3.3 msec, 300 radials). MR angiography and X-ray digital subtraction angiography on the X-ray system positioned in-line next to the interventional MR system served as control for the angioplasty results.

RESULTS

Real-time guidance and positioning of the balloon catheters was possible. The actual position of the catheter tip was indicated in the MR images without any time delay for the reconstruction of the anatomical MR images, which were updated with a rate of 20 frames per second. This yielded a combination of a roadmap and fluoroscopy image, in which the catheter position and the anatomical background image both were continuously updated in real time. Six out of nine stenoses were successfully dilated. The effects of the angioplasty could be visualized by the real-time MR technique, as was proven by X-ray digital subtraction angiography.

CONCLUSION

Active tip tracking simultaneous with MRI of the vascular anatomy-both in real time-is possible with the applied technique, enabling MR-guided percutaneous dilatation (PTA) of iliac arteries.

Catheter-directed gadolinium-enhanced MR angiography

In the setting of MRI-guided endovascular interventions, catheter-directed Gd-enhanced MRA offers many of the same capabilities as conventional x-ray DSA. Local injections permit rapid depiction of blood vessels and help guide interventions. The primary benefit of IA injections is significant reduction of administered contrast-agent dose compared with conventional IV injections. Another major benefit is facilitated background suppression, including that of adjacent vascular beds. As MRI guidance methods improve, catheter-based Gd injections should gain expanded use in clinical practice.

MRI and characterization of atherosclerotic plaque: emerging applications and molecular imaging

Noninvasive high-resolution magnetic resonance has the potential to image atherosclerotic plaque and to determine its composition and microanatomy. This review summarizes the rationale for plaque imaging and describes the characteristics of plaque by use of existing MRI techniques. The use of MRI in human disease and in animal models, particularly in rabbits and mice, is presented. Present and future applications of MRI, including real-time vascular intervention, new contrast agents, and molecular imaging, are also discussed.

Magnetic resonance imaging permits in vivo monitoring of catheter-based vascular gene delivery

BACKGROUND

Gene therapy is an exciting frontier in modern medicine. To date, most investigations about the imaging of gene therapy have primarily focused on noncardiovascular systems, and no in vivo imaging modalities are currently available for monitoring vascular gene therapy. The purpose of this study was to develop an in vivo imaging tool to monitor a catheter-based vascular gene delivery procedure.

METHODS AND RESULTS

We produced gadolinium/blue dye and gadolinium/gene-vector media by mixing Magnevist with a trypan-blue or a lentiviral vector carrying a green fluorescent protein (GFP) gene. The gadolinium was used as an imaging marker for magnetic resonance (MR) imaging to visualize vessel wall enhancement, and the blue dye/GFP was used as a tissue stain marker for histology/immunohistochemistry to confirm the success of the transfer. Using Remedy gene delivery catheters, we transferred the gadolinium/blue dye (n=8) or gadolinium/GFP lentivirus (n=4) into the arteries of 12 pigs, that were monitored under high-resolution MR imaging. The results showed, in all 12 pigs, the gadolinium enhancement of the target vessel walls on MR imaging and the blue/GFP staining of the target vessel tissues with histology/immunohistochemistry. This study shows the potential of using MR imaging to dynamically visualize (1) where the gadolinium/genes are delivered; (2) how the target portion is marked; and (3) whether the gene transfer procedure causes complications.

CONCLUSIONS

We present a technical development that uses high-resolution MR imaging as an in vivo imaging tool to monitor catheter-based vascular gene delivery.

Magnetic resonance imaging in coronary heart disease

Modern level of cardiac magnetic resonance imaging (MRI) development already allows its routine use (with proper indications) in coronary heart disease patients for studies of heart morphology and functions, performance of stress tests for evaluation of myocardial perfusion and contractile function. Coronary MRA and some other new MR techniques are close to its wide-scale clinical application. It has been shown that cardiac MRI is a valuable tool for detection of postinfarction scars, aneurysms, pseudoaneurysms, septal defects, mural thrombi and valvular regurgitations. Due to intrinsic advantages of the method it is of special value when these pathological conditions cannot be fully confirmed or excluded with echocardiography. MRI is recognized as the best imaging method for quantification of myocardial thickness, myocardial mass, systolic myocardial thickening, chamber volumes, ejection fraction and other parameters of global and regional systolic and diastolic function. MRI is used in studies of cardiac remodeling in postinfarction patients. The most attractive areas for cardiovascular applications of MRI are assessment of myocardial perfusion and non-invasive coronary angiography. Substantial progress has been achieved in these directions. There are some other new developments in studies of coronary artery disease with MRI. High-resolution MR is used for imaging and quantification of atherosclerotic plaque composition in vivo. Intravascular MR devices suitable for performing imaging-guided balloon angioplasty are created. But before MRI will be widely accepted by the medical community as a important cardiovascular imaging modality several important problems have to be solved. Further technical advances are necessary for clinical implementation of all major diagnostic capabilities of cardiac MRI. The subjective obstacles for growth of clinical applications of cardiac MRI are lack of understanding of its possibilities and benefits both by clinicians and radiologists themselves. So proper training of specialists and promotion of this promising modality among the medical community are necessary.

A Green's function approach to local rf heating in interventional MRI

Current safety regulations for local radiofrequency (rf) heating, developed for externally positioned rf coils, may not be suitable for internal rf coils that are being increasingly used in interventional MRI. This work presents a two-step model for rf heating in an interventional MRI setting: (1) the spatial distribution of power in the sample from the rf pulse (Maxwell's equations); and (2) the transformation of that power to temperature change according to thermal conduction and tissue perfusion (tissue bioheat equation). The tissue bioheat equation is approximated as a linear, shift-invariant system in the case of local rf heating and is fully characterized by its Green's function. Expected temperature distributions are calculated by convolving (averaging) transmit coil specific absorption rate (SAR) distributions with the Green's function. When the input SAR distribution is relatively slowly varying in space, as is the case with excitation by external rf coils, the choice of averaging methods makes virtually no difference on the expected heating as measured by temperature change (deltaT). However, for highly localized SAR distributions, such as those encountered with internal coils in interventional MRI, the Green's function method predicts heating that is significantly different from the averaging method in current regulations. In our opinion, the Green's function method is a better predictor since it is based on a physiological model. The Green's function also elicits a time constant and scaling factor between SAR and deltaT that are both functions of the tissue perfusion rate. This emphasizes the critical importance of perfusion in the heating model. The assumptions made in this model are only valid for local rf heating and should not be applied to whole body heating.