Cardiac ischaemia protocol (MRI)

The cardiac MRI ischaemia or stress protocol encompasses a set of different MRI sequences for the assessment of myocardial ischaemia.

Note: This article aims to frame a general concept of a cardiac MRI protocol in the setting of vasodilator stress perfusion testing.

Protocol specifics will vary depending on additional clinical questions, differential diagnosis, MRI scanner type, specific hardware and software, radiologist and perhaps referrer preference, patient factors e.g. arrhythmia or breathing problems or implants, specific indications and time constraints.

Indications

• stable angina / chronic coronary syndrome

• known coronary artery disease with unclear significance

• symptomatic patients after percutaneous coronary intervention

1.5 vs 3 tesla

Cardiac MRI examinations can be performed on both 1.5 and 3 tesla.

Myocardial perfusion imaging benefits from the increased field strength of 3 tesla magnets. Steady-state free precession cine imaging on the other hand requires a lot of adjustments and careful shimming to avoid flow and dark banding artifacts ^1,2.

Patient preparation

Checking indications, contraindications, explanation of the examination and obtaining informed consent is obvious as in other examinations.  In addition, a cardiac MRI stress test requires consideration of precautions and contraindications concerning stress medication as adenosine, regadenoson, dipyridamole or ATP ^1,2.

Generally, patient preparation for cardiac MRI includes the following ^1,2:

• instruction how to breathe

• an electrocardiogram signal need to be acquired

Contraindications to vasodilator stress include ^1,2:

• 2nd or 3rd-degree atrioventricular block

• sinus bradycardia (≤40 bpm)

• systolic blood pressure ≤90 mmHg

• severe arterial hypertension (≥220/120 mmHg)

• active bronchoconstriction, asthma or bronchospastic disease with regular inhaler use

• known hypersensitivity to adenosine, regadenoson or dipyridamole

• acute myocardial infarction

Precautions to vasodilator stress include the following ^1,2:

• abstinence from substances, which might potentially interact with the stress medications e.g. caffeine (coffee, tea, energy drinks, chocolate and other caffeinated beverages, foods and medications), xanthine and dipyridamole for 24hrs

• no tobacco for at least 4 hours

• fasting or only very light meal are advised due to the risk of nausea and vomiting

• adenosine infusion requires two different intravenous accesses preferably in both separate arms one for adenosine and one for the contrast agent

Safety equipment

Safety precautions and equipment for stress perfusion imaging should include the following ¹:

• monitoring equipment (blood pressure monitor, ECG monitoring or pulse oximetry)

• the option of rapid removal of the patient from the scanner

• emergency resuscitation policy

• medications at hand including ß-blockers, nitroglycerin, aminophylline, bronchodilators and oxygen

• emergency cart with equipment and medications including adrenaline, atropine, ß-blockers, antiarrhythmic drugs, etc.

• automated external defibrillator (AED)

Patient positioning

A cardiac MRI is conducted in the supine position.

Technical parameters

Coil

Multiphase array coils are recommended.

• anterior surface coil, posterior coil

• cardiac coil

Scan geometry

• in-plane spatial resolution: will vary with the sequence

• field of view (FOV):  will vary, for most planes a FOV ≤320 mm is recommended

• slice thickness: varies with the sequence and is usually 6-10 mm 

Planning

The cardiac planes differ from the normal axial, coronal and sagittal body planes ^1-3:

• overview

  • angulation: strictly axial

  • volume: from thoracic inlet to the diaphragm

• horizontal long axis view or 4-chamber view (4ch)

  • angulation: along the left ventricular long axis, through apex and centre of the mitral valve

  • volume: including the anterior and inferior wall or a single slice

• left ventricular vertical long axis view or 2-chamber view (2ch)

  • angulation: along left ventricular long axis, through left ventricular apex and centre of the mitral valve

  • volume: including septum and left ventricular free wall or single slice

• sagittal left ventricular outflow tract (LVOT) or 3-chamber view (3ch)

  • angulation: through the left ventricular apex, centre of the mitral valve, left ventricular outflow tract and aortic valve

  • volume: including the anterolateral and inferoseptal left ventricular wall

• short-axis view (sax)

  • angulation: perpendicular to the left ventricular long axis

  • volume: stack usually including the atrioventricular valves and the cardiac apex or 3 single slices through basal, midventricular and apical zones

Stress medication protocol

• adenosine: infusion of 140 µg/kg for 2-4 min*

• regadenoson: 0.4 mg single injection

Indications to stop adenosine infusion include the following

• severe hypotension systolic pressure <80 mmHg or a persisting fall of 20 mmHg

• ST depression >3 mm or >2 mm with typical chest pain

• persistent 2nd or 3rd-degree atrioventricular block

• severe chest pain, nausea, vomiting, syncope, headache and dysrhythmia

• wheezing

* depending on local and institutional norms, an increase to 210 µg/kg after 2-3 min might be considered if there is no increase in heart rate by 10 bpm or drop in blood pressure by ≥10 mmHg.

Contrast injection protocol

• stress and resting perfusion with the same contrast dose

• contrast dose (Gd): 0.05-0.1 mmol/kg

• saline chasing bolus: 30 mL

• injection rate: 3-7 mL/s

• adenosine/ATP: start the first-pass perfusion during the last minute of adenosine

• regadenoson: start the first-pass perfusion approximately 45-60 sec after injection

• an additional injection of contrast medium might be required for late gadolinium enhancement after first-pass perfusion imaging (up to a total dose of 0.1-0.2 mmol/kg)

Sequences

Standard sequences

• T1 or T2 black-blood or SSFP

  • purpose: overview, depiction of the cardiac surroundings and greats vessels

  • technique: T1 black-blood, T2 black-blood, SSFP ideally over 1-2 breath holds

  • planes: axial

• cine imaging

  • purpose: left ventricular wall motion, left ventricular volumetry

  • technique: cine SSFP or spoiled GRE

  • planes: 2ch, 4ch, 3ch and short axis views

• stress perfusion (C+)

  • purpose: for the assessment of myocardial ischaemia

  • contrast dose (Gd): 0.05-0.1 mmol/kg followed by 30 mL chasing saline bolus

  • technique: saturation recovery with GRE, GRE-EPI hybrid or SSFP readout

  • planes: short axis,  long axis* optional

• resting perfusion (C+) (*might be omitted)

  • purpose: compare with stress perfusion for the assessment of myocardial ischaemia

  • timing: at least 10 min after stress perfusion

  • contrast dose (Gd): 0.05-0.1 mmol/kg followed by 30 mL chasing saline bolus

  • technique: saturation recovery with GRE, GRE-EPI hybrid or SSFP readout

  • planes: short axis

• late gadolinium enhancement

  • purpose: for the evaluation of myocardial viability (myocardial necrosis and myocardial scar tissue)

  • timing: at least 5 min after resting perfusion

  • technique: 2D and 3D IR GRE, PSIR

  • planes: 2ch, 4ch, 3ch and short axis views

  • TI as determined by TI scout (Look-Locker) or fixed TI (PSIR)

(*) indicates optional planes

Practical points

The following considerations can be made in certain conditions:

• single shot modules or free breathing with real-time image acquisition in patients with difficulties holding their breath

• abdominal bands in profound respiratory motion

• peripheral pulse gating inpatient with a weak ECG signal

• postponing the exam in patients with severe pleural effusion and related ghosting artifacts and breathing problems until after pleural drainage

• cine imaging

  • in atrial fibrillation or cardiac arrhythmia, it might be worthwhile to switch to prospective gating

  • consider the acquisition of a fair amount of cine imaging in the time period after stress-perfusion imaging e.g. long axis views after stress perfusion to save time

• perfusion imaging

  • a test run should be performed before the first perfusion scan to check image quality

  • the breath hold should start when the contrast is in the right ventricle

  • a sufficient number of images should be acquired over 50-60 heartbeats to ensure contrast has passed through the left ventricle

  • depending on experience and institutional policy, rest perfusion imaging might be omitted

• late gadolinium enhancement

  • 2D IRGRE or sequences with SFFP readout in patients with poor breath-holding capabilities

  • increase TI 10 ms every 1-2 minutes

  • acquisition in mid or late diastole to minimise motion artifacts

  • saturation bands across the spinal column and anterior chest wall can help to reduce ghosting artifacts