Stress perfusion imaging
Stress perfusion CMR (Stress Perfusion CMR) aims to identify areas of ischemia that correspond to the blood supply territories of stenosed coronary arteries.
The analysis is based on capturing the first pass of the contrast agent compared to the resting state. A decrease in the slope of the contrast accumulation curve indicates pathology. In hypoperfused areas, myocardial perfusion reserve (MPR) decreases accordingly. Quantitative assessment of absolute blood flow, similar to PET studies, is also possible.
The method has significant advantages over SPECT (SPECT). High spatial resolution allows the detection of tiny subendocardial defects that other methods miss. In addition, MRI lacks soft tissue and attenuation artifacts typical of isotope methods. Cardiac CT (CCT) can serve as an alternative, showing anatomical plaques, but its main disadvantage is the significant radiation dose to the patient.
Assessment of myocardial function
Stress function CMR (Stress Function CMR) is an equivalent of classical stress echocardiography, but performed within a magnetic field. Dobutamine, a $\beta$-agonist that increases heart rate and contractility and thereby increases myocardial oxygen demand, is used as the stress agent.
Normally, in response to the drug, wall contractility progressively increases. If there is a significant stenosis, the compensatory blood inflow is insufficient, and ischemia develops. This leads to regional wall motion abnormalities (Wall Motion Abnormalities, WMA).
Dobutamine stress test protocol:
- Imaging is performed using cine MRI (cine CMR) along the short and long axes of the left ventricle at rest and then at each stress stage (each lasting 3 minutes).
- The starting dose is 5 mcg/kg/min.
- The dose is increased stepwise (10, 20, and up to a maximum of 40 mcg/kg/min).
- If the target heart rate is not reached at the maximum dose, atropine is administered intravenously in fractionated doses (up to 1–2 mg).
- Images at each stage are acquired during three consecutive breath-holds.
Diagnostic value of different doses
The choice of dobutamine dose depends on the clinical objective:
- Low-dose stress MRI (up to 20 mcg/kg/min). Used in patients with recent myocardial infarction or chronic coronary artery disease. The goal is to differentiate dysfunctional but viable ("stunned" / stunned) myocardium from irreversibly dead tissue.
- High-dose stress MRI (up to 40 mcg/kg/min + atropine). Aimed at detecting obstructive coronary artery disease. A positive test is considered the onset of angina pectoris or the detection of new (or worsening of old) WMAs.
In addition to visual wall motion assessment, new technologies are developing, such as SENC-CMR (myocardial strain imaging — strain). They allow the detection of signs of coronary artery disease at even lower stress stages.
Safety and test termination criteria
Despite the use of high doses of stimulants, the procedure is safe in experienced hands. However, strict monitoring of blood pressure and heart rate, the availability of a resuscitation kit, and a team ready to quickly evacuate the patient from the magnet are required.
The test is immediately stopped in case of:
- Achievement of the target heart rate.
- Appearance of new or worsening of old WMAs.
- Onset of chest pain or severe arrhythmias.
- Patient request.
- Critical blood pressure changes: a drop in systolic BP by more than 20 mm Hg from baseline, or by more than 40 mm Hg from the previous stage, as well as a spike exceeding 240/120 mm Hg.
Tissue characterization and myocardial edema
MRI is unique in its ability to directly characterize tissue in vivo. Prolonged ischemia leads to the accumulation of free water (edema), which prolongs T1 and T2 relaxation times. Edema is noticeable as early as 30 minutes after the onset of ischemia.
On T2-weighted images (with "dark" or "bright" blood), the ischemic zone appears abnormally bright (hyperintense) against the background of the gray healthy muscle. However, T2 images have limitations: suboptimal quality, difficulties in separating the myocardium from stagnant blood in the ventricular cavity, and, most importantly, the inability to distinguish reversibly injured myocardium from irreversible necrosis based solely on a bright signal. A comprehensive approach is always required for the final conclusion.
Briefly: answers to topical questions
What are the main indications and contraindications for cardiac stress MRI?
The main indications for stress MRI are the detection of significant coronary artery disease, evaluation of recurrent angina after surgical intervention, and differentiation of viable and non-viable myocardium. Contraindications include general magnetic resonance imaging limitations:
- Ferromagnetic clips — represent an absolute or relative contraindication.
- Pacemakers — certain models are incompatible with the magnetic field.
- Metal devices — recent implants and other metal structures.
The study may also be difficult in patients with claustrophobia and anxiety.
What are the changes in MR signal on T1- and T2-weighted images during ischemia and myocardial edema?
During myocardial ischemia, due to an increase in free water content, T1 and T2 relaxation times are prolonged. On T2-weighted images, the ischemic myocardium appears abnormally bright (hyperintense) against the background of normal gray myocardium. Myocardial edema is visualized as early as 30 minutes after the onset of ischemia. It is impossible to distinguish irreversibly damaged myocardium from reversibly damaged (viable) myocardium based on a bright signal on T2-weighted images.
What complications can occur during stress MRI and what is the management for dobutamine and atropine side effects?
Severe arrhythmias, chest pain or angina attack, as well as critical blood pressure changes can occur during stress MRI: a drop in systolic BP by more than 20 mm Hg below the baseline level, a drop of more than 40 mm Hg compared to the previous stage, or an increase in BP exceeding 240/120 mm Hg. Side effects of dobutamine include tachycardia, ventricular and supraventricular arrhythmias. Specific complications of atropine and the medical management of their reversal are not described in the sources. When termination criteria are met, the test is stopped. To ensure safety, careful monitoring of heart rate and BP, the presence of resuscitation equipment, a trained team, and the ability to quickly evacuate the patient from the magnet are necessary.
The "20-40-240" rule for stopping the test due to blood pressure: a 20 mmHg drop in SBP from rest, a 40 mmHg drop from the previous stage, an increase above 240 mmHg.
Class and exam questions
What is the main advantage of perfusion MRI over SPECT?
MRI has higher spatial resolution, which allows the detection of small subendocardial perfusion defects. In addition, MRI is not subject to soft tissue signal attenuation artifacts.
Why is atropine used in the protocol?
Atropine is added at the maximum dobutamine dose (40 mcg/kg/min) if the patient's heart rate still has not reached the calculated target value.
Can myocardial viability be determined from T2-weighted images?
No. A bright signal on T2 indicates the presence of myocardial edema, but does not reliably distinguish reversibly damaged (viable) tissue from irreversibly dead tissue.
What else to study on this topic
- Methods for calculating myocardial perfusion reserve (MPR).
- Principles of T1 and T2 mapping in the diagnosis of ischemia.
- SENC-CMR technology and strain imaging.
- Comparison of MRI with dual-energy CT in acute myocardial infarction.
- Late gadolinium enhancement (LGE) for assessing scar tissue.
Other topics in the «Chest Radiology» section
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.