CMR perfusion is increasingly used in cardiac imaging to detect inducible myocardial ischemia and has been well validated against other imaging modalities such as invasive angiography or FFR. Several recent large-scale studies have demonstrated noninferiority or superiority of SPECT imaging. It is increasingly established as a prognostic marker in patients with coronary artery disease.

sign

There are two main reasons for conducting this test:

1. Evaluate the significance of stenosis (narrowing) of one or more coronary arteries previously identified by standard coronary angiography or CT coronary angiography. This is often used by cardiologists to determine whether coronary artery narrowing should be treated with angioplasty or coronary artery bypass surgery.
2. Screen patients with chest pain and risk factors for coronary artery disease to evaluate for ischemia that may be caused by a narrowing of one of the coronary arteries. It can then (if it shows ischemia) be studied further with another imaging modality to image the coronary arteries directly, such as invasive coronary angiography.

In contrast to nuclear imaging modalities (PET and SPECT), CMR perfusion does not involve the use of ionizing radiation and therefore can be used multiple times without risking patient exposure to radiation.

It is a non-invasive test that is generally considered a safe procedure and is well tolerated by patients.

mechanism

Most scans are performed using stress/rest, a protocol that uses adenosine as a stressor to induce myocardial ischemia through the coronary "steal" phenomenon. Some centers use the cardiotonic drug dobutamine to put pressure on the heart, and the images are interpreted in a manner similar to a dobutamine stress echocardiogram.

adenosine stress

Hyperemia was induced by intravenous infusion of adenosine at 140 µg/Kg/min for 3 minutes while heart rate and blood pressure were continuously recorded. Following this, adenosine was injected with 0.05 mmol/kg of gadolinium chelate via the antecubital fossa vein in the contralateral arm.

scanning

Typically, three short-axis slices, each 10 mm thick, were acquired per cardiac cycle at the level of the base, middle papilla, and apex of the left ventricle. Using a single-shot prospectively gated balanced TFE sequence, typical resolution is 2.5 x 2.5mm. The patient is then allowed to rest until the hemodynamic effects of adenosine cease. Then perform the same scan while at rest.

image analysis

Images are stored as video files and analyzed on a dedicated workstation. Most clinical scans were analyzed qualitatively by visually comparing stress and resting scans side-by-side. In normal scans, in stress scans and in resting scans, the myocardium can be seen changing from black to medium gray evenly throughout the left ventricle on the first pass into the myocardium. In abnormal scans, areas of the heart muscle will turn gray slower than surrounding tissue because the coronary arteries supplying the blood are narrowed and blood, and therefore gadolinium, enters more slowly. This is called a perfusion defect and usually represents myocardial ischemia. It may be seen on both resting and stress scans, in which case it is called a matched perfusion defect and may be due to an area or scar from a previous myocardial infarction. If it is visible only on stress scans, it is called an inducible perfusion defect area. The AHA 17-segment model was used to describe the location of perfusion defects in the left ventricle.

limit

Stress CMR cannot be performed in all patients due to the relative or absolute contraindications listed below, which is a problem especially in patients with pacemakers or severe renal failure.
The acquisition of images is very sensitive to heart rhythm, and scans of patients with atrial fibrillation, dilogy, or trilogy are sometimes of poor quality and may not be interpretable.
Due to the high contrast between the blood pool and the myocardium, what appears to be a thin area of ​​subendocardial ischemia is often present, known as Gibbs artifact, however, this is not possible in newer technologies that allow higher resolution imaging. Too common.
In patients with a previous myocardial infarction or coronary artery bypass surgery, the images may be difficult to interpret, in which case scan analysis needs to be supplemented by another imaging modality.

Taboo

• Any patient with contraindications to MRI scanning, especially those with a pacemaker
• People with severe asthma, as adenosine may trigger asthma attacks
• In patients with severe renal insufficiency, gadolinium contrast agents are contraindicated when eGFR is less than 30 because the risk of nephrogenic systemic fibrosis (NSF) is very small.
• Patients who have heart block on an EKG before testing, as adenosine may make the condition worse.
• Patients with severe claustrophobia because the MRI scanner is enclosed

adverse events

After receiving an adenosine infusion, patients often experience mild symptoms such as feeling hot, sweating, shortness of breath, nausea, and notice their heartbeat is racing. These, if they occur, resolve rapidly (usually within 60 seconds) after stopping the adenosine infusion.

There are also a number of more serious but less common side effects, including a 1 in 10,000 risk of transient heart block, bronchoconstriction and allergic reactions caused by gadolinium contrast agents. These can always be treated successfully with no long-term side effects.

Adenosine infusions are associated with some very rare but serious side effects, including acute pulmonary edema and cardiac arrest (occurring in approximately 1 in 1,000 patients)

12 hours before scan

You must not have any caffeinated drinks, food or medicines in the 12 hours before your scan appointment. This includes:

• tea/coffee
• hot chocolate
• Ovaltine or Horlicks
• soda
• Caffeinated pain relievers or cold medicines
• chocolate

During scanning

During the first 15 minutes, your heart will be scanned to see how well it is working. After this, your heart will be "stressed" by a drug called adenosine, which makes your heart work harder like a workout.

You will only receive this medicine for about 3 minutes.
For the first 30 seconds to a minute, you probably won't notice anything. After a minute you may start to feel a little breathless, have chest tightness, and blush.

After 3 minutes, more scans will be done and you will receive an injection of contrast material to show the blood supply to your heart.
This scan takes approximately 1 minute, during which time the adenosine injection is turned off and any symptoms you have will disappear within seconds.

After the pressure portion of the scan is complete, there will be another 20 minutes of scanning.

When your heart is stressed you will be contacted by staff and will be closely monitored

After scanning

There is nothing special to do after the examination, you can eat and take your medications normally.

Comparing CTA scans and MRA scans

The importance of addressing the coronary artery disease (CAD) epidemic is well known, and the tools at our disposal to accurately identify and manage it are evolving in very positive ways.

Computed tomography (CT) and magnetic resonance imaging (MRI), two important diagnostic tools, play a crucial role in the assessment of coronary atherosclerosis. These modalities enable clinicians to perform noninvasive diagnostic tests using coronary CT angiography (CTA) and magnetic resonance angiography (MRA), thereby reducing patient trauma and lowering health care costs.

However, resistance to noninvasive approaches to diagnosing CAD, coupled with today's challenging economy, threatens to hinder progress in advancing this technology and its clinical applications.

valuable diagnosis

Since the 1960s, we have relied on direct coronary angiography to visualize coronary artery disease and guide care. While the procedure is safe, there are pros and cons. It is invasive, which is stressful for the patient and expensive. Many believe that despite its proven value, patients are on a slippery slope in receiving intervention, and often no significant stenosis is found, the result of a false-positive pre-angiographic test. Until recently, the tools that revealed the location, extent, and stability of coronary plaques were invasive: angiography and intravascular ultrasound.

However, over the past decade, there has been growing hope that MRA and CTA scans can become noninvasive alternatives for plaque detection in clinical situations. These include outpatient chest pain assessment, or screening applications for preclinical identification of atherosclerosis. The results could mean better care through more accurate diagnoses and savings on healthcare costs.

But there are other problems. Despite advances in analyzing patients' clinical risk, an ongoing reality is that more than half of all illnesses are detected as emergencies. Furthermore, more than half of the people who presented to our emergency department with acute coronary syndrome had not even been eligible for medications aimed at primary prevention the day before. Moreover, we know that most culprit plaques in acute coronary syndromes are not stenotic plaques. If this seems complicated, that's because it is.

Finally, the imaging world is not without bias, and the flow of healthcare funding greatly influences changes in "conventional" or "perceived" medical wisdom. Popular headlines, even from peer-reviewed journals, can get us all excited about something new and important. Learning a new vocabulary that includes terms like “spiral,” “gating,” “T1 weighting,” “phase encoding,” and “dose modulation” and understanding statistics are essential. The list is almost endless and confusing. Let's try to clarify something here.

The science behind CTA scans and MRA scans

A CTA scan is an X-ray study that uses an iodinated intravenous contrast agent to change the radiation absorption properties of coronary blood and heart muscle. Despite the small caliber and constant motion of arteries, technology now can resolve these vessels and successfully create images that are actually maps contrasting the attenuating properties of tissue.

MRA scans are usually performed without the use of intravenous contrast material. It relies on the magnetic and quantum spin properties of hydrogen protons (mainly in water). The heart and its blood supply sit in the central hole of the magnet, with its trillions and trillions of hydrogen nuclei aligned in a very strong magnetic field. They are then pulsed with radiofrequency energy. The return of this energy is used to create a radiofrequency map, a high-contrast image "slice" of soft tissue.

With both techniques, the image is a slice of the heart, with picture elements created through complex computer processing. Image slices have 3-D properties that determine spatial resolution (CT is better than MRI), and contrast sensitivity that allows tissue characterization (MRI is better than CT). Like all images, both tools suffer from artifacts that limit information content. Furthermore, the images arrived at just the right time and had high operational overhead. Furthermore, both tools are best suited for selective clinical situations in which the pretest probability of significant findings is moderate. So, let’s explore the differences and indications for each type of coronary imaging.

Clearer view of CTA scans

The best way to explore the differences and indications for each type of coronary imaging is in real-life rather than research situations.

There is probably not a single metropolitan area in the United States today that does not have a 64-slice CT scanner. Over the past five years, 64-slice CT has been the workhorse scanner for high-resolution X-ray-derived slices through the heart. There are four excellent competitors: GE, Philips, Siemens, and Toshiba. There have been numerous trials demonstrating high accuracy in detecting stenotic coronary segments and quantifying plaque volume in clinically available cardiac catheterization populations.

There are also outcome studies showing that CTA scans are a safe and effective triage tool in low- and intermediate-risk populations in the emergency department. But this comes at the expense of ionizing radiation, intravenous contrast media, the need to control heart rate and regularity, and the special imaging difficulties of large amounts of coronary artery calcium. Added to these issues are reimbursement uncertainty and serious “turf” wars among imaging specialties, with the ensuing chaos that ensues.

The main selling point and leading advantage of CTA scans over MRA is spatial resolution. A conventional scanner can quickly acquire 0.5 mm on any plane resolution image during a breath-hold stroke of approximately 15 seconds through the CT scanner aperture. That is, the tissue volume displayed on each image is 0.5 x 0.5 x 0.5 mm when viewed in any oblique plane. This resolution is sufficient to study all important parts of the coronary arteries. Ideally, calcium-containing and calcium-free plaques can be quantified as none, mild, moderate, and severe.

The best use of a cardiac CTA scan is when little or no coronary artery disease is expected and the cardiac examination can be shortened. Therefore, patients with a clinical profile of low to moderate risk do have clear indications for CTA scanning. In the emergency department, in patients with negative cardiac enzymes and an average electrocardiogram (ECG), a normal or near-normal CTA can effectively rule out significant stenosis because of its high negative predictive value, almost 100%. This may avoid the need for further studies, particularly invasive angiography.

In the outpatient setting, CTA is an option for acute and chronic chest pain cases—again, low to moderate risk. This is especially true for inconclusive stress test data or if the patient is unable to exercise. CTA scanning may be chosen as an alternative to stress testing with or without perfusion information. This information is entirely structural rather than functional, however, a negative high-quality scan can put an end to the cardiac aspects of the chest pain workup.

The use of CTA scans as a screening test for asymptomatic individuals has not been officially endorsed. However, since we know that most acute coronary syndromes occur in asymptomatic low- to moderate-risk individuals, there is an obvious dilemma. Public opposition to CTA screening in moderate-risk patients with worrisome family histories would be reduced if the scans were of good quality, low cost and the radiation dose was less than 3 millisieverts (mSv).

What about a CTA scan for patients who have stents or have undergone coronary artery bypass grafting (CABG)? Although there is literature on high sensitivity for detecting in-stent restenosis and saphenous vein graft patency, most active centers find the application of CTA more challenging. This is primarily due to the prevalence of metal artifacts, which reduce the diagnostic accuracy of the test and often reduce the size of the target coronary artery. Studying these patients with CTA is of limited value.

Useful but troublesome

In CT, we talk about acquisition protocols. In MRI, we talk about imaging sequences. Understanding the physical principles and quantum theory that underlie this amazing technology is a daunting task. However, it is necessary to know the correct MRI sequence to answer clinical questions.

Additionally, the MRA exam is very time-consuming—it can take anywhere from one to two hours. Images are highly desirable and depict cardiac structure, myocardial viability, vasodilator pressure perfusion, and dobutamine pressure wall motion. However, in reality, a very expensive machine is relied upon by each patient in the long term. This makes routine use of MRA for coronary atherosclerosis studies difficult.

That said, there are certain clinical situations where information on coronary anatomy without radiation exposure is a priority, especially in younger adults. Cardiac MRA is used to diagnose suspected coronary artery abnormalities. The reduced spatial resolution of MRA compared with CTA scans is not an issue in a well-conducted study. The study can be done without intravenous contrast material. Signal differences between blood and surrounding soft tissue allow good identification of coronary artery origins and proximal left and right arterial segments, in fact, better than direct cannulation.

Kawasaki disease is an early childhood vasculitis that can cause coronary artery aneurysms or significant occlusions. This is an excellent role of coronary MRA as a preliminary study and can be continuously repeated without ionizing radiation. In the general adult population, left main or three-vessel coronary artery disease can be successfully detected, but with lower accuracy compared with CTA scans, at least in the studies published to date.