MRI: Now and in Future

28 February 2006 (Last Updated February 28th, 2006 18:30)

Dr Jens Bremerich and Professor Peter Buser report on the growing use of MRI in the diagnosis of cardiovascular disease. They look forward to the development of MRI-guided cardiac interventions.

In 1906, X-ray flouroscopy was the only way to image the heart. A hundred years later, we can image the heart's structure in three dimensions using MRI and CT.

Cardiac MRI is indicated in many cardiac conditions such as masses, cardiomyopathy, congenital heart disease, pericardial disease, valvular disease and Coronary Artery Disease (CAD) to assess myocardial structure, function, perfusion and viability. The imaging of coronary arteries using MRI would also be desirable, but is not yet a robust application.

"MRI can provide most of the data required to diagnose coronorary artery disease."

However, CT is now used for coronary angiography in selected patients. MRI is becoming more and more popular for non-invasive diagnostic management in cardiology. Many well-informed patients request MRI because it is non-invasive, it is highly sensitive and specific, it can be made available to outpatients and it is radiation free.

MRI MANAGEMENT

In the past decade, cardiac imaging was exclusively in the hands of practitioners of cardiology and nuclear medicine. The role of the radiologist was confined to conventional chest X-rays. Today, things have changed. Specific expertise is needed for MRI, since image acquisition and analysis are technically demanding, and image interpretation requires a profound knowledge of cardiac pathophysiology.

Moreover, the reading of a magnetic resonance scan must include a careful review of extra-cardiac structures. It is the responsibility of the reading physician to report significant extra-cardiac findings, such as pulmonary embolism and the infiltration or extra-cardiac masses. The complexity of cardiac imaging is properly addressed and best results achieved when cardiologist, radiologist, nuclear cardiologist and physicists work together.

"MRI provides an attractive one-stop diagnostic option in cardiovascular medicine."

The reimbursement of cardiac MRI is still not well defined in most European countries. Reimbursement should reflect the ECG gated image acquisition effort, long room time, the post-processing and analysis of data, the setting up of pharmacological stress testing and also the interdisciplinary contribution of all the partners involved.

Quality standards have to be established in cardiac MRI, and physicians need specific training to meet these standards. A licence for cardiac imaging could be issued to guarantee a certain level of training. Such a licence must be available for all clinical partners with a specific interest in cardiac imaging.

CAD DIAGNOSIS

CAD is the most frequent cardiac disease and the leading cause of death in Europe. It is a substantial burden on healthcare systems. Recent progress in the diagnosis and therapeutic management of CAD has improved the life expectancy and quality for patients with CAD.

The cornerstones of diagnostic management of CAD are stress echocardiography, perfusion scintigraphy and catheter angiography. However, comprehensive diagnostic work requires diagnostic studies across various departments, and it is time-consuming. MRI can provide most of the diagnostic information required in one operation. This is convenient for the patient and saves time, making MRI cost-efficient.

Invasive angiography remains the gold standard for the imaging of coronary arteries, because no other modality provides such high spatial and temporal resolution. Only invasive imaging enables the treatment of coronary lesions using angioplasty or stent implantation.

However invasive angiography can be complicated by haemorrhage, emboli or even death, while invasive catheter facility capacity is limited in many European countries, resulting in waiting lists.

There is now a substantial interest in referring selected patients to CT rather than to invasive angiography, because CT is non-invasive, it can be done in outpatient departments and it allows interventional cardiologists to focus on therapeutic interventions.

"The success of MRI-based cardiovascular procedures depends on a high degree of cooperation."

ONE-STOP OPTION

Magnetic resonance angiography of coronary arteries is not yet available. However, MRI provides perfusion images with better spatial resolution than scintigraphy, and without radiation exposure. Today, MRI is the gold standard for the imaging of wall motion and mass.

MRI enables the imaging of myocardial viability at better resolution than PET. It is also cheaper than PET and radiation free. Spatial resolution is particularly important in distinguishing transmural and nontransmural infarction and in predicting the recovery of wall motion after therapy.

Sending a patient for an MRI rather than to perfusion scintigraphy, wall-motion echocardiography and viability PET, saves time and money and avoids radiation exposure.

Triage is important to reduce unnecessary testing for CAD. However, no single diagnostic test suits all patients. CT is a good test to exclude CAD in a middle-aged man with a low pre-test probability, because CT has a high negative predictive value.

But CT is probably the wrong test in a 75-year-old man with diabetes and effort-induced chest pain. In such a case, CT is probably non-diagnostic, because severe calcified arteriosclerosis produces artefacts, and this patient will have to undergo catheter angiography anyway. The most important factors for triage are risk stratification and age.

MOVING FORWARD

Close cooperation between cardiology and radiology requires the consensus reading and reporting of studies. Consensus reading enables the synergy of radiological and cardiological expertise, quality control and continuous technical improvement, and it enhances the confidence of referring physicians. Close cooperation between cardiology and radiology is the key to establishing a successful cardiac imaging centre.

In future, the acquisition of cardiac MR will become easier. ECG and respiratory gated sequences will allow high resolution imaging during free breathing. Three-dimensional time-resolved images of the heart will provide images in any desired plane.

ECG gating without leads might became a reality, based on the automatic detection of cardiac motion. Also, perfusion imaging will probably replace nuclear methods as the gold standard. It will still take a while for coronary angiography to become a robust application.

Vascular MRI has matured into a powerful and robust tool that has replaced invasive magnetic resonance angiography in most applications. It is easy to perform and well established in many institutions.

Magnetic resonance angiography is now entering the arena of image-guided therapy and intervention. This development is driven by the desire to avoid radiation exposure in a procedure to visualise vessel wall, monitor vascular flow and limb perfusion or to monitor the success of an intervention.

"Invasive angiography remains the gold standard for the imaging of coronary arteries."

Open or wide-bore scanners are now available, and MR compatible catheters, guide wires and balloons are currently under development. The feasibility of angioplasty, cardiac valve placement, stenting, embolisation and cardiac septum defect occlusion has been demonstrated in animal studies, while the first reports on magnetic resonance-guided interventions in humans have just been published.

Cardiac imaging is still technically quite demanding and requires special expertise in both cardiac pathophysiology and MRI technology. Cooperation between radiologists, cardiologists and physicists remains the key to success.

Today MRI is the gold standard for cardiac volume, function, mass and viability testing. Vascular imaging based on MRI is a well-established and robust tool in many institutions, and has replaced catheter angiography in most vascular territories. MRI-guided interventions will become available in the future that can visualise morphology and monitor changes in flow and limb perfusion during therapeutic interventions.