Magnetic resonance coronary angiography
Currently the gold standard for assessment of coronary artery disease is coronary angiography using x-rays. Since conventional coronary angiography is an invasive procedure with a small risk of significant complications, other modalities of imaging are being evaluated. Though multi-slice CT is a less invasive method of evaluating the coronary arteries, it still involves radiation exposure and the use of iodinated contrast material, with its attendant risks. Coronary magnetic resonance angiography (MRA) is being considered as a potential noninvasive alternative for assessing the coronary anatomy.
Steady-state free precession (SSFP) whole-heart coronary MRA has become the method of choice for coronary imaging with 1.5 Tesla magentic resonance imaging systems. The intrinsically high blood signal intensity allows whole heart coronary MRA with SSFP sequence without using MR contrast medium. Myocardial and venous blood signals can be suppressed using T2 preparation. Diaphragmatic movements can be reduced by a tight fitting abdominal belt so that imaging can be done during free breathing.
A paper by Kato S and associates [J Am Coll Cardiol, 2010; 56:983-991] had a negative predictive value of 88% with whole-heart coronary MRA which is useful in avoiding unnecessary X-ray coronary angiography. There is also a 99% negative predictive value for detecting left main stenosis or three vessel disease so that a 1.5 Tesla whole-heart coronary magnetic resonance angiography reliably rules out siginificant left main disease or three vessel disease. Total study time including the pre-scan was less than 30 minutes, while it was 70 minutes for the previous target volume coronary MRA, which had also a poor specificity of 42% when all vessels were analyzed. Diagnostic images can be obtained even in patients with heart rates above 70 beats per minute without administration of beta blockers, in contrast from multidetector computed tomography (MDCT). High density calcium does not cause beam-hardening artifacts in MRA, which is one of the reasons for excluding patients with heavy calcification from MDCT evaluation studies.
Intra coronary thrombus detection by cardiac magnetic resonance (CMR) imaging
Intra coronary thrombus is usually detected by conventional coronary angiography as a negative shadow with haziness. Direct imaging of the carotid thrombus and hemorrhage within the plaque has been done using non contrast enhanced T1 weighted magnetic resonance imaging. This technique takes advantage of the short T1 of methemoglobin present in the acute thrombus and intraplaque hemorrhage. Christian Jansen and associates from the St. Thomas’ Hospital, King’s College London [Jansen CH et al. Detection of intracoronary thrombus by magnetic resonance imaging in patients with acute myocardial infarction. Circulation. 2011;124:416-24], have evaluated the use of non contrast enhanced magnetic resonance for direct thrombus imaging in patients with acute myocardial infarction. Non contrast enhanced magnetic resonance for direct thrombus imaging was done with a T1 weighted, three dimensional, inversion recovery black blood gradient echo sequence. Ten of the eighteen patients imaged were found to have intra coronary thrombus by conventional coronary angiography. Nine of these could be identified by magnetic resonance imaging as well. One thrombus in posterior descending artery was not detected by CMR (sensitivity 91%). Seven of the eight in the control group (without thrombus) were correctly classified by CMR (specificity 88%).
Intravascular magnetic resonance imaging and special contrast agents
Magnetic resonance imaging can assess the finer details of atherosclerotic lesions in blood vessels – i.e., plaque component characterization. But currently this is limited to the evaluation of large or superficial vessels like aorta and carotid arteries because of the drop in signal to noise ratio while imaging deeper vessels using an external coil. Hence intra vascular MRI probes are being developed. Systems based on 0.030 inch and 0.014 inch guide wires have been developed. Most of these systems need an intravascular probe as well as an external MRI scanner. The devices have been made using nitinol tubing and the intravascular probe has an MRI receiver coil. Potential concerns exist about tissue heating in a radio frequency field in the presence of metallic structures. Newer probes with both magnets and coils within the probe are also being developed, allowing stand alone catheter based imaging without the aid of an external MRI scanner. The advantage would be obvious as an MRI scanner makes cathlab environment complicated as every equipment there needs to be MRI compatible. The prototype of this device can provide colour coded tissue component map. These may have a role in identifying the vulnerable plaque, especially along with targeting contrast agents like supermagnetic iron oxide (SPIO). Catheter tip magnet producing 0.2 Tesla magnetic field has been tested. A preliminary study presented at the Transcatheter Cardiovascular Therapeutics (TCT) 2007 had evaluated 104 patients from ten centres in United States of America, Europe and Israel. The study was designed to assess the device safety and functional performance of the intravascular MRI system. There were no deaths or perforations and the major adverse cardiac event rate was 0.96% at twenty four hours and 1.9% at one month. Success of the procedural performance was 88% and at least one intravascular magnetic resonance imaging measurement could be obtained in 95% of patients.
Water diffusion coefficients are different for lipid rich core and collagenous cap of the atherosclerotic plaque. Intravascular MRI catheter makes use of this difference in determining the extend and location of vascular lipid infiltration. It is well known that a thin fibrous cap makes a plaque vulnerable while high extracellular matrix content or smooth muscle rich tissue within the plaque makes it stable. Intravascular MRI probe can provide a radial resolution of 250 micrometer [J Am Coll Cardiol, 2006; 47:48-56].
Ultra small particles of SPIO given intravenously are retained in the macrophages within the plaques. While this uptake is 75% for ruptured or vulnerable plaques, it is only 7% for stable plaques. Use of labeled LDL helps in the detection of LDL receptors within the liver. Studies are underway to see if LDL labeled particles can be used determine the presence of LDL rich lesions and whether these can be used to deliver targeted therapy. Gadolinium-labeled human HDL nanoparticles are also being evaluated as novel contrast agents as they can enter the atherosclerotic plaques without any additional targeting mechanism.
MR angio in aortic bifurcation disease
Magnetic resonance angiography (MR angiography) is useful in the non invasive assessment of aortic bifurcation disease. It gives an overall view of the aorta and its branches with precise delineation of the obstructive lesions and collateral circulation.
MR angio in aortic bifurcation disease
MR angio in aortic bifurcation disease showing the descending aorta and renal arteries in the upper region and the aortic bifurcation with severe disease in the lower region.
MR angio of femoral and popliteal arteries
MR angio of femoral and popliteal arteries
Magnetic resonance (MR) angiography of both femoral arteries showing severe disease on both sides with reformation of the vessels lower down as a good sized popliteal arteries. It may be noted that the superficial femoral branch of the common femoral artery continues as the popliteal artery across the knee joint after it exits from the adductor canal (subsartorial canal or Hunter’s canal).
MR angio of anterior and posterior tibial arteries
MR angio of anterior and posterior tibial arteries
Popliteal artery terminates below the knee joint by bifurcating into anterior and posterior tibial arteries which supply the leg. Anterior tibial artery supplies the anterior compartment of the leg and the dorsal surface of the foot while the posterior tibial artery supplies the posterior compartment of the leg and the plantar surface of the foot. Anterior tibial artery becomes the dorsalis pedis artery when it crosses the ankle joint.
Role of cardiac magnetic resonance imaging (CMR) in the evaluation of GUCH (grown up congenital heart disease)
Cardiac magnetic resonance imaging (CMR) has the advantage that it is less operator dependent than echocardiography regarding the acquisition of images. But when it comes to interpretation, experience does matter a lot in delineating the potential permutations and combinations possible in congenital heart disease. Three dimensional reconstruction enables better visualisation of the cardiac anatomy. Unlike echocardiography, CMR is not restricted by the echo window, which is often a limitation for the acquisition of echocardiographic data. But echocardiography is superior to CMR in estimating gradients and in detecting small highly mobile structures like vegetations. Quantification of ventricular volumes, ejection fractions and valvular regurgitations can be done by CMR. Evaluation of right ventricular volumes and ejection fraction are better done with CMR than echo because of the complex shape of the right ventricle. CMR is useful in the evaluation of right ventricle to pulmonary artery conduits, branch pulmonary arteries, aorta, systemic and pulmonary veins and collaterals. Detection and quantification of myocardial fibrosis with late gadolinium enhancement is another advantage of CMR. Tissue characterisation for fat and iron is also feasible with CMR. Though coronary anomalies can be detected by CMR, computerised tomographic (CT) angiography is superior for this purpose. Intracardiac and extra cardiac masses can be delineated well by CMR.