MR fluoroscopy in vascular and cardiac interventions (review)
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- Saeed, M., Hetts, S.W., English, J. et al. Int J Cardiovasc Imaging (2012) 28: 117. doi:10.1007/s10554-010-9774-1
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Vascular and cardiac disease remains a leading cause of morbidity and mortality in developed and emerging countries. Vascular and cardiac interventions require extensive fluoroscopic guidance to navigate endovascular catheters. X-ray fluoroscopy is considered the current modality for real time imaging. It provides excellent spatial and temporal resolution, but is limited by exposure of patients and staff to ionizing radiation, poor soft tissue characterization and lack of quantitative physiologic information. MR fluoroscopy has been introduced with substantial progress during the last decade. Clinical and experimental studies performed under MR fluoroscopy have indicated the suitability of this modality for: delivery of ASD closure, aortic valves, and endovascular stents (aortic, carotid, iliac, renal arteries, inferior vena cava). It aids in performing ablation, creation of hepatic shunts and local delivery of therapies. Development of more MR compatible equipment and devices will widen the applications of MR-guided procedures. At post-intervention, MR imaging aids in assessing the efficacy of therapies, success of interventions. It also provides information on vascular flow and cardiac morphology, function, perfusion and viability. MR fluoroscopy has the potential to form the basis for minimally invasive image–guided surgeries that offer improved patient management and cost effectiveness.
KeywordsCardiac imaging Magnetic resonance imaging Vascular imaging
The rapid evolution of minimally invasive vascular and cardiac interventions is shaping the demand for high temporal and spatial resolution imaging that offers safety, accuracy, flexibility and functionality. Recent improvements in signal processing, tissue characterization and angiographic integration allowed MR-guidance in complex interventional procedures, which require optimal spatial resolution and orientation [1, 2]. MR fluoroscopy offers rapid acquisition, reconstruction and display of 3D images. Therefore, it has been used in biopsies [3, 4, 5, 6, 7], brachytherapy [8, 9], focused ultrasound [10, 11, 12, 13], thermometry [10, 14, 15, 16, 17], functional imaging integrated into MR guided neurosurgical interventions [18, 19], local drug delivery [20, 21], endoscopy , intravascular interventions [23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34] and intra-operative imaging [19, 35, 36, 37, 38, 39, 40].
X-ray fluoroscopy is routinely used in patients to guide vascular and cardiac interventions, because of its ability for real-time imaging and easy access to patients during interventions [41, 42, 43]. X-ray fluoroscopy, however, is limited for defining soft tissue and obtaining functional information. The poor contrast between pathologic and healthy surrounding tissue hinders X-ray fluoroscopy in defining targets , which subsequently leads to blind delivery of therapies to the targets [20, 45]. Furthermore, there is a growing body of evidence that exposure to ionizing radiation from X-ray procedures is associated with an increased risk of cancer [46, 47, 48, 49, 50].
Interventional MR magnets
The advantages of hybrid XMR systems are: (1) intermodal movement is minimized because a patient will remain on a sliding table throughout the imaging session; (2) unlike single system, the XMR hybrid system permits evaluation of the impact of interventional procedures via MR monitoring; (3) it permits rapid deployment of catheters, and efficient execution of desired interventions without the obligation of using MR compatible devices; (4) it reduces radiation exposure  and (5) offers the convenience of a single visit. However, currently XMR systems are available only in few medical centers.
Devices for MR interventions
Unfortunately, endovascular catheters and devices are optimized for their mechanical properties and visibility under projection X-ray imaging. There are, therefore, substantial metallic components within the plastic sheath that may be ferrous in nature. Visualization of these commercial endovascular catheters and devices has been difficult on MR imaging due to the susceptibility artifacts derived from the ferromagnetic material, geometry and design [56, 57]. Unlike ferromagnetic material, nickel-titanium alloy (nitinol), platinum, gold, copper, nonbraided or plastic catheters cause substantially less susceptibility artifacts [57, 58, 59] and produce less radiofrequency heating in vivo [60, 61]. Nitinol stents and guide-wires are currently used for revascularization of stenosed blood vessels. Mekle et al. [62, 63] used a synthetic MR friendly polymer-based guide-wire for dilatation of an artificial stenosis in phantoms and in the carotid artery, aorta, and iliac arteries of swine . More recently, investigators manufactured a guide wire based on micropultruded fiber-reinforced material doped with iron particles to improve visibility. At the distal part of the guide-wire a nitinol wire was attached to provide flexibility to the tip .
Active tracking is another approach for tracking endovascular catheters (Figs. 2, 3). This technique relies on specially designed micro-coils, electrified wire loop and self-resonant radiofrequency circuits. The coils pick up signal during slice excitation and generate a frequency-encoded recall echo, which can be detected in 3D at a spatial resolution of approximately 1 mm. The micro coils provide robust tracking of the catheter shaft and tip that allows the user to identify its position and target (Fig. 3) [71, 72, 73, 74]. Quick et al.  used antennas for active catheter tracking and imaging of the abdominal aorta, superior mesenteric artery, renal arteries, hepatic artery and celiac trunk. In another study, they were able to simultaneously visualize vascular tree, catheter shaft and tip . The advantage of this technique is that it allows for visualization of longer portion of the catheter or guidewire when a loopless antenna is placed. On the other hand, the disadvantage includes the need for special hardware and software. Furthermore, the support patient systems, interventional devices and surgical instruments must be MR-compatible. MR-compatible equipment for anesthesia, assessment of physiologic parameters and contrast media injection are currently offered by multiple venders.
The safety of active endovascular devices is still a major concern. The conductive nature of the long metallic braid creates a safety hazard in the MR environment, as the braided shaft can interact with incident RF energy and the electric field transmitted from the RF coil [60, 61, 77]. The heat created by the active coils causes necrosis of the tissue adjacent to the catheter and blood clotting, which may lead to vascular embolization. The methods for mitigating the potential for heating include using unbraided catheters, insulating the conductive structure, limiting the RF power to which it is exposed, or altering its interaction with the RF energy source . The FDA limits the allowable power deposition via MR imaging to 8 W/kg and temperature change to 2°C. Patients with internalized devices containing long conductive structures, such as deep brain stimulators  and cardiac pacemakers [80, 81, 82], are presently scanned with MR imaging provided additional safety steps are taken. These typically include heightened patient monitoring, lower permissible specific absorption rate levels, and the use of local transmit RF coils.
Magnetic catheter steering is a new approach for tracking endovascular catheters using remote control . It relies on a small magnetic moment created by application of an electrical current to copper coils on the catheter tip, which results in alignment of the catheter in the direction of the B0 field (Fig. 4) [84, 85]. Magnetic catheter steering approach allows for more efficiency in navigating small, tortuous blood vessels, which are currently difficult to catheterize due to build-up of friction at vascular bends. In addition to improved visualization of the endovascular catheter at low power levels, this technology permits deposition of thermal energy for ablation of tissues at higher power levels. This technology is under active investigation [86, 87].
MR contrast media
MR fluoroscopy and catheter tracking can be expanded using a variety of MR contrast media with high safety profiles [88, 89, 90]. Investigators used extracellular and intravascular MR contrast media with T1-enhancing or T2-enhancing capabilities for labeling different types of cells [91, 92]. Extracellular MR contrast media have small molecular weights (<1 kDa), brief plasma half-life and are clinically used in vascular angiography and in assessing myocardial viability. On the other hand, intravascular (blood pool) MR contrast media have high molecular weights (>50 kDa), mass and T1 relaxivity with prolonged plasma half-life. Preclinical experiments showed that intravascular contrast media provide better vascular angiograms. Moreover, contrast media have been used on MR fluoroscopy to improve visualization of endovascular devices , in road mapping blood vessels [93, 94] and defining pathologic targets . Investigators also used MR contrast media for labeling different types of cells [91, 92], which assist in monitoring the distribution of the injected cells in vivo [96, 97, 98].
A study showed that high dose or repeated administration of gadolinium might be a concern, especially in patients with impaired renal function . This problem can be reduced by paying attention to a glomerular filtration rate of >30 ml/min/1.73 m2 and contrast agents with high molecular stability .
MR fluoroscopy sequences
MR fluoroscopy became possible because of the major advancements in the speed of data acquisition, data transfer, and interactive control and display. Other factors include highly uniform magnetic fields, rapidly changeable magnetic field gradients, multi-channel receivers and computing systems. MR fluoroscopy sequences achieve their high speeds by maximizing the switching rates of gradients and RF pulses. The temporal and spatial resolution in MR fluoroscopy are often complementary factors. The speed of imaging is determined by how quickly spatial encoding can be performed and how fast k-space data can be acquired. Actively shielded, strong, fast-switching gradients and fast electronics have allowed data acquisition intervals to be reduced.
Fast MR imaging techniques have been developed in recent years, allowing frame rates almost comparable to those achieved with X-ray fluoroscopy. Most modern real-time MR implementations employ balanced steady state free precession techniques because of efficient use of magnetization, high SNR, and short repetition times [101, 102, 103]. The performance of these sequences is currently in the range needed to perform MR guided procedures at >5 fps . The SSFP acquisitions have been performed using radial , and spiral  k-space trajectories. These acquisition techniques in conjunction with spiral or radial filling of the k-space are considered very reliable for high spatial and temporal resolutions. These imaging sequences also benefit from the use of multiple receiver coil elements [107, 108, 109]. Parallel imaging accelerates acquisition by using the different spatial sensitivities of the coils to correct for under-sampling of image data . Other sequences that can improve imaging speed while simultaneously balancing imaging quality include non-Cartesian k-space sampling, temporal data sharing between images, and adjusting the tradeoff between temporal and spatial resolution . The use of 32 channel receiver arrays that will perform rapid 3D cardiac imaging and parallel transmission techniques to permit more efficient parallel data collection are also under active investigation . It should be noted that MR fluoroscopy is not free of limitations. For example, the closed configuration of MR magnets >1.5T limit access to the patient and RF pulses induce heating when conductive material is applied in devices; MR imaging has relatively low spatial and temporal resolution compared with X-ray fluoroscopy; and is sensitive to magnetic field inhomogeneity, pulsatility and motion of spins and chemical shift.
In pre and post-intervention the following MR sequences were used: (a) balanced fast field echo CINE images for measuring LV volumes, ejection fraction, cardiac output, stroke volume, LV mass, wall thickness and radial strain [90, 112, 113, 114], (b) tagged gradient echo planar imaging for measuring circumferential strain and LV rotation [115, 116], (c) phase-contrast velocity-encoded gradient echo planar imaging for measuring longitudinal strain , (d) T2-weighted turbo spin echo sequence for measuring interstitial edema after ablation, (e) T2* multi-echo gradient echo sequence for measuring vascular and myocardial hemorrhage after intervention , (f) T1-weighted gradient echo (radiofrequency spoiled) perfusion imaging sequence for measuring myocardial perfusion changes after delivery of therapy, and (g) delayed contrast enhanced 3D T1-weighted gradient echo sequence for assessing tissue viability.
Applications of MR fluoroscopy
MR imaging provides detailed information on vascular layers and is able to differentiate between plaque components, such as fibrous, lipid rich and calcified tissue [119, 120]. In the last decade MR imaging has been extended from a diagnostic to a dynamic modality, which can be used to guide intravascular guidewires and catheters and to assess the success of endovascular procedures. In 1997 the first human MR-guided study was performed and showed excellent visualization of an endovascular catheter labeled with dysprosium ring markers . In this study, investigators did not use guide wires during the movement of the catheter in the cephalic vein of healthy volunteers. Later, MR-guided percutaneous transluminal angioplasty has been conducted without complications in 13 patients with iliac stenosis  and in 15 patients with femoral and popliteal artery stenosis .
MR-guided procedures (stenting and/or angioplasty) have been performed for dilatation of the aorta, pulmonary, coronary, renal iliac and femoral arteries [45, 57, 121, 122, 123, 124]. MR-guided imaging has been used for delivery of stents in major and minor blood vessels [45, 52, 57, 125, 126, 127, 128, 129, 130, 131]. Vascular stents, vena cava filters, cardioseptal occluders or prosthetic heart valves require, however, post-interventional follow-up, which are usually made under X-ray or CT. MR imaging, with its superior soft-tissue contrast, arbitrary slice orientation and flow measurement would be the preferred imaging technique; however, most conventional vascular implants made of metal create image artifacts and masked visualization of the lumen. The three main types of MR artifacts associated with metallic vascular implants are susceptibility artifacts, flow-related artifacts and RF artifacts. Active MR resonant stents provide non-invasive visualization of instant thrombosis and restenosis without the need for MR contrast media. Visualization of the lumen of vascular implants is important for a safe and reliable examination on MR-guided procedures.
Kuehne et al.  demonstrated successful implant of a self-expanding stent valve in the aorta via percutaneous access under MR fluoroscopy. Transcatheter aortic valve implantation, either retrograde through a transfemoral approach or antegrade through a transapical approach, has become a clinical reality in the treatment of critical aortic stenosis in high-risk patients. MR fluoroscopy plays an important role in transcatheter aortic valve implantation and replacement of insufficient aortic or pulmonic valves [57, 58, 136]. MR imaging enables accurate and reproducible quantifications of regurgitate fraction before and after valve placement. Under MR fluoroscopy, McVeigh et al.  used apical access to guide the placement of a prosthetic aortic valve in beating heart. MR imaging offered the visualization of both coronary ostium during stent implantation and allowed aortic flow assessment.
Percutaneous closure of atrio-septal defects and ventriculo-septal defects is increasingly performed under X-ray, which brings the disadvantages of ionizing radiation and lack of soft-tissue contrast. MR imaging is a technique that provides high-resolution 3D images of the heart. Three-dimensional MR imaging before intervention is particularly important because it improves our understanding of the anatomic basis of complex arrhythmias. Closure of such congenital defects under MR-guidance has been proved in animals , but is hampered by image artifacts produced by the materials of the closure devices and the use of fast sequences for cardiac imaging . Few studies showed that MR imaging is useful for arrhythmic substrate identification [143, 144].
MR-guided procedures have been successfully used in thermal ablation and after intervention to assess the success of ablation [145, 146, 147, 148, 149, 150]. Clinical studies showed atrial scar on contrast enhanced MR imaging that results from RF ablation [151, 152, 153]. Other studies have demonstrated the association between infarct scar, border-zone and the risk of monomorphic ventricular tachycardia [144, 154, 155]. Dong et al. found that 3D MR imaging is helpful for tailoring ablations to the variant pulmonary vein anatomy in 47% of patients with atrial fibrilation . They also noted that 3D images of the atria helped in localizing areas along the tissue ridge separating the left atrium from the pulmonary vein [156, 157]. The ability of MR fluoroscopy to visualize the needle tip in the inferior vena cava, atria, fossa ovalis, and surrounding vasculature during transseptal cardiac punctures has also been demonstrated [158, 159, 160, 161].
A recent study in 10 patients and 5 volunteers showed that MR fluoroscopy is suited to guide flow directed catheters for measurement of invasive pulmonary artery pressures . Pulmonary vascular flow was noninvasively measured using velocity-encoded cine MR imaging, while pulmonary pressure was measured invasively through a catheter guided into the pulmonary artery under MR-guidance. The results indicate that MR imaging is a promising tool for measurement of pulmonary vascular resistance in patients with different degrees and forms of pulmonary hypertension. MR fluoroscopy has also been used in connecting cardiac chambers and blood vessels in a swine model, where Arepally et al. connected the right and left atrium by puncturing the interatrial septum using an active Brockenbrough-style needle . In a clinical study in seven patients, Dick et al. conducted trans-septal puncture and balloon septostomy under MR fluoroscopy .
MR guided delivery of genes and stem cells
Vascular and cardiac disease is a major public and economic health problem leading to more than 7 million deaths world wide each year. Current treatments of this disease include pharmaceutical drugs, deployment of devices and interventional therapies. These method, however were unable to replace necrotic, apoptotic cells and damaged vessels by new cardiomyocytes or blood vessels. Clinical studies confirmed that there are an increasing number of patients who have persistent chronic angina, despite having multiple coronary revascularization procedures. Heart transplantation is the definitive therapy for these patients, but this option is limited to ~2,000 donor hearts annually. Thus, there is a mandate for alternative treatment and minimally invasive approaches, such as endovascular catheter-based techniques, for local delivery of new therapies to restore cardiomyocytes and blood vessels. Angiogenic growth factor, gene and stem cell therapy have been recently used as an alternative treatment to restore cardiomyocytes and blood vessels in end stage patients, in combination with coronary artery bypass grafting [170, 171, 172, 173, 174, 175]. Recent preclinical and clinical studies showed that percutaneous intramyocardial and intraarterial delivery of therapies is possible [176, 177, 178, 179, 180], but Hou et al. found that 11, 2.6 and 3.2% of the delivered cells are retained in the myocardium after intramyocardial, intracoronary, and interstitial retrograde coronary venous delivery .
Local delivery approaches include surgical and catheter-based delivery of various types of angiogenic proteins, genes and stem cells. Open-chest surgery, however, is impractical in end-stage patients because this strategy increases morbidity and mortality as well as limiting the feasibility of repeat administration . The advantages of catheter-based local delivery of therapies are: (1) targeting only the diseased region, (2) delivering a high local dose, (3) eliminating a high systemic dose and side effects and (4) reducing the chance of angiogenesis in hidden tumor sites especially in elderly patients [170, 171, 172, 173, 174, 175, 183].
Stem cell transplantation is another approach for treating ischemic heart disease as it improves cardiac function and revascularizes ischemic myocardium. The therapeutic effect of stem cells seems to be related to the release of angiogenic factors rather than trans-differentiation of delivered stem cells. Two predominant routes for stem cell delivery to infarcted myocardium are intracoronary infusion and direct intramyocardial injection. Each of these delivery routes attempts to maximize the retention of delivered cells to infarcted myocardium namely. Early clinical studies indicated that cell transplantation, delivered under MR fluoroscopy, is safe and feasible [97, 187, 188, 189]. MR imaging has been used not only to track stem cells in the myocardium, but also to non-invasively evaluate ventricular function, perfusion and viability . Cell tracking on MR imaging is based on labeling injected cells with US FDA approved super paramagnetic iron oxide particles [189, 191]. It has been shown that iron labeled cells maintain their viability, proliferation and differentiation . The cluster of iron labeled cells appear dark on T2* and T2 MR images [189, 191, 192]. Several factors affect the detection of labeled cells, which include the (1) magnetic field, (2) labeling efficiency, (3) type of cells and (4) time of imaging after delivery. Investigators found that the duration of MR detection varies between cells; up to 5 weeks for embolic stem cells  and up to 16 weeks for skeletal myoblasts . Investigators also found hypo-intense tiny regions far from the site of injection, indicative of migration of stem cells within the infarction several weeks after delivery. MR imaging was used to evaluate changes in LV remodeling following the delivery of cellular therapy [98, 195, 196, 197, 198, 199]. Amado et al.  were able to identify a time-dependent recovery of local contractility associated with the appearance of new tissue resulting from transplantation of allogeneic stem cells in a pig model of myocardial infarct.
Recent randomized clinical trials demonstrated the safety of bone marrow mononuclear treatment after intracoronary injection [201, 202]. Promising clinical results from intracoronary delivery of autologous bone marrow derived stem cells and progenitor cells showed improved myocardial function [201, 203, 204, 205, 206]. The proposed mechanisms of protection by stem cells include angiogenesis via the release of angiogenic factors, myogenesis, cytoprotection via the release of paracrine factors, recruitment of stem cells and suppression of inflammation . More recent 5 year follow-up studies showed that cell therapy causes no significant improvement in LV ejection fraction compared to placebo [208, 209]. Major limitations of intracoronary delivery include: (1) no delivery access to infarct related to permanent coronary artery occlusion; (2) inadequate cellular migration into the interstitial space during the first pass transit; (3) microembolization ; (4) systemic delivery to non-cardiac tissue  and (5) possibility of intimal dissection . It has been shown that approximately 2% of intracoronary delivered bone marrow mononuclear cells were retained by infarcted myocardium in humans, but when the investigators used enriched bone marrow mononuclear cells the retention increased to 14–39% .
Ripa et al.  used MR imaging to monitor the changes in LV function after subcutaneous granulocytes colony stimulating factor (G-CSF) injection in patients with ST-elevated infarct. They found that G-CSF caused no improvement in LV function. Investigators used different types of cells, such as progenitor cells, myocytes, adipocytes, fibroblasts, and smooth muscle cells, in patients with ischemic heart disease [202, 203, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223]. Three randomized controlled studies have been published using bone marrow cells for promoting angiogenesis [202, 203, 218]. In the BOOST study 60 patients were enrolled to evaluate the effect of intracoronary autologous bone marrow cells after myocardial infarction . MR imaging showed a significant increase in ejection fraction from 50 to 57% in treated patients versus 51–52% in untreated patients . A more recent update from the BOOST study found the beneficial effects of bone marrow cells were sustained at 18 months . Several complications have been reported after local delivery of growth factor and cell therapies including hemangioma , in-stent stenosis and hyperplasia  as well as arrhythmia .
During the last decade medical imaging and minimally invasive cardiovascular interventions have made substantial progress. Improvements in temporal resolution, tissue component characterization and angiographic integration have allowed guidance in complex interventional procedures. MR imaging provides 3D datasets, excellent soft-tissue contrast, multi-planar views, dynamic imaging in a single imaging session and guidance of interventional vascular and cardiac procedures. MR imaging allows monitoring of treatment success after intervention that is not available on X-ray fluoroscopy. These advantages of MR imaging are complementary to its potential advantage against the harmful effects of X-ray guided procedures. In recent studies, balloon dilation, stent placement, valvar replacement, atrial septal defect closure, radiofrequency ablation and local gene and cell delivery have been shown to be feasible. In addition, MR-guided procedures involving gene or stem cell therapy represent a new discipline whose systematic development will foster minimally invasive interventional procedures and will hasten the identification and deployment of effective new therapies for revascularization and myogenesis.
At present, cardiovascular interventions are addressed by multimodality imaging using computed tomography, invasive angiography and transesophageal echocardiography. Whether MR is suited to obviate the need for multimodality imaging is currently unclear and needs to be further evaluated. Furthermore, the availability of safe MR compatible devices will guide future minimally invasive cardiovascular procedures. MR-guided percutaneous transluminal angioplasty and vascular implants placements (such as stents, vena cava filters, heart valves) are examples of future clinical applications.
Catheter-based MR guidance enables a substantially reduced level of invasiveness compared with open-chest surgery, potentially resulting in treatment on an outpatient basis, rapid patient recovery, eliminate radiation exposure and cost savings to the health care system. It should be noted, however, that translation of MR-guided interventions to clinical use has been very slow due to limited availability of MR-friendly catheters, wires, devices and financial funding by National Institute of Health and venders.
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