ACR practice guideline for the performance and interpretation of cardiac magnetic resonance imaging (MRI)
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Editor in Chief: Evan Appelbaum, MD [1]
Adapted from the American College of Radiology's Practice Guideline for the Performance and Interpretation of Cardiac Magnetic Resonance Imaging (MRI), 2006, Effective 10/01/06acrcmr
Introduction
Important Attributes and Capabilities of CMR
- High natural contrast exists between the intracardiac/intravascular blood pools and the surrounding cardiac and vascular structures because of the lack of signal from the blood on "dark-blood" spin echo MRI pulse sequences or the enhanced signal intensity from blood on "bright-blood" gradient-echo sequences.
- Internal cardiovascular structures (valves, endocardial borders of cardiac chambers) can be distinguished from the blood pools within the heart and great vessels.
- Contrast agents are not routinely required for discrimination of the blood pool, although contrast administration has become a key component in state-of-the-art time resolved MR tissue perfusion and delay-enhanced viability techniques
- Excellent soft tissue differentiation capabilities of MRI also permit delineation of cardiac structures (e.g., ventricular myocardium) and paracardiac structures related to the heart and great vessels (e.g., pericardium and mediastinum).
- MRI is a three-dimensional and/or multiplanar imaging modality that provides the capability for precise and reproducible (intraobserver, interobserver, and/or inter-examination) quantification of cardiac chamber cavity size or wall mass.
- When sequential tomographic images or true-volume sets entirely encompassing the heart in the same cardiac phase are acquired, the resulting three-dimensional data series permits direct measurement of cardiac volumes or mass without the use of any assumed formulas or geometric models.
- Cine MRI techniques can be used to assess routine measures of cardiac function such as global and regional ventricular systolic function (e.g., ejection fraction), ventricular diastolic function (e.g., filling rates), shunt quantity (e.g., interatrial defect shunt volume) and valve regurgitation quantity (e.g., mitral regurgitant volume).
- Measurements depend on cavity volume changes over the cardiac cycle or differences in stroke volume between the two ventricles.
- Cine MRI techniques with high temporal resolution of the cardiac cycle (preferably less than or equal to 50 msec) including standard cine and tagged (e.g., spatial modulation of magnetization [SPAMM]) gradient echo imaging, allow the assessment of regional ventricular function (e.g., systolic wall thickening or systolic circumferential strain).
- Studies can be performed at rest or during the intravenous administration of a pharmacologic stress agent, such as dobutamine
- Velocity-encoded techniques permit measurement of blood flow from the standpoint of flow velocity or flow volume
- Practical uses include: stroke volume determination, valvular insufficiency quantification (e.g., diastolic retrograde flow volume/systolic antegrade flow volume in ascending aorta for determining aortic regurgitant fraction), assessment of stenoses (e.g., measurement of peak systolic velocity beyond stenotic aortic valve for transvalvular pressure gradient determination by modified Bernoulli equation, or by velocity-time intregral methods, and shunt calculation (e.g., ascending aortic flow voluime/pulmonary artery flow volume to determine Qp/Qs)
- First pass perfusion, utilizing near-real-time or real-time monitoring of the appearance of a rapidly administered MRI contrast agent (e.g., gadolinium chelate), can be used to evaluate the adequacy of delivery of blood (i.e., perfusion) to the myocardial tissue based on patterns of tissue enhancement; time-intensity curves may be analyzed to quantify the degree of underperfusion in ischemic or infarcted myocardium (fixed perfusion defect)
- This can be performed both at rest and during intravenous administration of a pharmacologic stress agent such as adenosine
- Delayed contrast-enhanced viability MRI methods can be used to evaluate the steady-state distribution of the agent, most importantly to detect the presence of necrotic myocardium
- Method can be used alone or with cine imaging to assess the extent of myocardial infarction (transmural vs. subendocardial) to predict wall motion recovery after revascularization, or in combination with first pass stress perfusion to ass ischemic vs. nonviable myocardial tissue.
- Delayed contrast-enhanced technique has been show to be useful in assessing cardiomyopathies, myocarditis, and myocardial infiltrative processes.
- Multiple noncontrast and contrast-medium-based MRI techniques are available to characterize cardiovascular tissue as follows:
- Cystic vs. Solid
- Transudative vs. Exudative
- Simple vs. Complex
- Hypervascular vs. Hypovascular
- Necrotic vs. Viable
- Edematous vs. Normal water content
- Calcified vs. Noncalcified
- Fibrotic vs. Nonfibrotic
- Fat vs. Soft tissue
- Angiographic techniques (i.e., MRA) are often discussed separately; nonetheless, they are essential to many comprehensive cardiovascular MRI examinations, especially those of the coronary arteries and great vessels.
CMR: Only for a Valid Medical Reason
- Cardiovascular MRI (CMR) should only be performed for a valid medical reason
- It is not always possible to detect all abnormalities by using CMR, adherence to guidelines listed below will enhance the probability and accurace of their detection
Qualifications and Responsibilities of Personnel
Physician
- Physician should have responsibility for all aspects of the study, including (but not limited to):
- Reviewing all indications for the examination
- Specifying the pulse sequences to be performed
- Specifying the imaging planes
- Specifying the use and dosage of contrast agents
- Interpreting images
- Generating an official interpretation
- Assuring the quality of the images and the interpretation
Physician With Prior Qualifications in General MRI
- Radiologist or other physician who meats the qualifications for the ACR Practice Guideline for Performing and Interpreting Magnetic Resonance Imaging (MRI) for all anatomic areas will have substantial knowledge of the following:
- Physics of MRI
- Principles of MR image acquisition and postprocessing, including use of diagnostic workstations
- Design of MR protocols, including pulse sequences
- Rate and timing of contrast administration
- Physician should also have substantial experience in MRI interpretation, including MRI of extracardiac thoracic structures that will be included on the cardiac MRI examination and MRA
- Physician may also have substantial experience in other methods of cardiac imaging, assessing cardiac function, and/or experience specifically in cardiac MRI
- Many physicians will require additional education in cardiac anatomy, physiology, pathology, and/or cardiac MRI in order to achieve competency in all aspects of cardiac MRI
Supervising and Interpreting Physician
- Should meeting one of the following requirements:
- Training in cardiac MRI in an Accreditation Council for Graduate Medical Education (ACGME) or an American Osteopathic Association (AOA) approved training program to include:
- Education in cardiac anatomy, physiology, pathology, and cardiac MRI for a time equivalent to at least 30 hours of CME
- The interpretation, reporting and/or supervised review of at least 50 cardiac MRI examinations in the last 36 months
- Completion of at least 30 hours of Category I CMR in cardiac imaging, including:
- Cardiac MRI, anatomy, physiology, and/or pathology, or documented equivalent supervised experience in a center actively performing cardiac MRI
- The interpretation, reporting, and/or supervised review of at least 50 cardiac MRI examinations in the last 36 months
- Training in cardiac MRI in an Accreditation Council for Graduate Medical Education (ACGME) or an American Osteopathic Association (AOA) approved training program to include:
Physician Without Prior Qualifications in General MRI
- The radiologist or other physician who does not meet qualifications of the ACR Practice Guideline for Performing and Interpreting Magnetic Resonance Imaging (MRI) for all anatomic areas requires more extensive training and experience in MRI with an emphasis on cardiac MRI
- In addition to specific training in imaging interpretation, training must also include:
- Physics of MRI
- MRI safety
- Principles of MRI acquisition and postprocessing
- Use of diagnostic workstations
- Design of MRI protocols, including pulse sequences and the rate and timing of contrast administration
- Some physicians will also require additional education in cardiac anatomy, physiology, and pathology
Supervising and Interpreting Physician
- Should meet the following requirements:
- Completion of an ACGME approved training program in the specialty practiced, plus 200 hours of Category I CMR in MRI to include (but not limited to):
- MRI physics
- Recognition of MRI artifacts
- Safety
- Instrumentation
- Clinical applications of MRI in cardiac and thoracic MRI
- Supervision, interpretation, and reporting of at least 150 MRI cases in the past 36 months in a supervised situation with an emphasis on thoracic MRI and cardiac MRI to include the interpretation, reporting, and/or supervised review of at least 50 cardiac MRI examinations in the last 36 months
- Completion of an ACGME approved training program in the specialty practiced, plus 200 hours of Category I CMR in MRI to include (but not limited to):
Pharmacologic Stress Testing and Administration of Other Pharmacologic Agents
- Physicians performing pharmacologic stress testing or administering other pharmacologic agents as part of cardiac MR imaging should be knowledgeable about the administration, risks, and contraindications of the pharmacologic agents used, and should be monitoring the patient throughout the procedure
- Personnel monitoring stress-induced studies should have current Advanced Cardiac Life Support (ACLS) certification
Maintenance of Competence
- All physicians performing cardiac MRI examinations should demonstrate evidence of continuing competence in the interpretation and reporting of those examinations
- If competence is assured primarily on the basis of continuing experience, performance and interpretation of a minimum of 75 examinations every 3 years is recommended in order to maintain physicians' skills
Continuing Medical Education
- The physician's continuing medical education should be in accordance with the ACR Practice Guideline for Continuing Medical Education (CME) of 150 hours of approved education every 3 years, and should include CME in cardiac MRI as is appropriate to the physician's practice needs
Medical Physicist/MR Scientist
- Personnel qualified to carry out acceptance testing and monitoring of MRI equipment for the purposes of this guideline include:
- Qualified Medical Physicist OR
- Qualified MR Scientist
- Qualified Medical Physicist
- Individual who is competent to practice independently one or more subfields in medical physics
- ACR considers certification and continuing education in the appropriate subfields to demonstrate that an individual is competent to practice in one or more subfields in medical physics, and to be a Qualified Medical Physicist
- ACR recommends that the individual be certified in the appropriate subfield(s) by the American Board of Radiology (ABR) or for MRI, by the American Board of Medical Physics (ABMO) in magnetic resonance imaging physics.
- Appropriate subfields of medical physics for this guideline are Diagnostic Radiological Physics and Radiological Physics.
- Qualified MR Scientist
- Individual who has obtained a graduate degree in physical science involving nuclear magnetic resonance (NMR) or MRI, by the American Board of Medical Physics (ABMP) in magnetic imaging physics
- These individuals should have 3 years of documented experience in a clinical MR environment
- Continuing education of a Qualified Medical Physicist/MR Scientist should be in accordance with the ACR Practice Guideline for Continuing Medical Education (CME)
- Qualified Medical Physicist/MR Scientist must be familiar with the principles of MRI safety for patients, personnel, and the public; the Food and Drug Administration guidance for MRI diagnostic devices; and other regulations pertaining to the performance of the equipment being monitored.
- Qualified Medical Physicist/MR Scientist shall be knowledgeable in the field of nuclear MR physics and familiar with MRI technology, including:
- Function
- Clinical uses
- Performance specifications of MRI
- Calibration processes and limitations of the performance testing hardware, procedures, and algorithms
- Qualified Medical Physicist/MR Scientist should have a working understanding of clinical imaging protocols and methods of their optimization
- Proficiency should be maintained by participation in continuing education programs of sufficient frequency to ensure familiarity with current concepts, equipment, and procedures.
- The Qualified Medical Physicist/MR Scientist may be assisted in obtaining test data for performance monitoring by other properly trained individuals
- Individuals must be properly trained and approved by the Qualified Medical Physicist/MR Scientist in the techniques of:
- Performing the tests
- Function and limitations of the imaging equipment and test instruments
- Reason for tests
- Importance of test results
- The Qualified Medical Physicist/MR Scientist must review and approve all measurements.
Radiologist Assistant
- Radiologist assistant is an advanced level radiographer who is certified and registered as a radiologist assistant by the American Registry of Radiologic Technologists (ARRT) after having successfully completed an advanced academic program encompassing an ACR/ASRT (American Society of Radiologic Technologists) radiololgist assistant curriculum and a radiologist-directed clinical preceptorship.
- Under radiologist supervision, the radiologist assistant may perform patient assessment, patient management and selected examinations as patient management and selected examinations as delineated in the Joint Policy Statement of the ACR and the ASRT titled "Radiologist Assistant: Roles and Responsibilities" and as allowed by state law.
- Radiologist assistant transmits to the supervising radiologists those observations that have a bearing on diagnosis.
- Performance of diagnostic interpretations remains outside the scope of practice of the radiologist assistant.
Radiologic Technologist
- The technologist should participate in assuring patient comfort and safety in preparing and positioning the patient for the MRI examination, including:
- Proper positioning of electrocardiogram (ECG) leads
- Obtaining the MRI data in a manner suitable for interpretation by the physician
- The technologist performing cardiac MRI should be certified by the American Registry of Radiologic Technologists (ARRT) or the Canadian Association of Medical Radiation Technologists (CAMRT)
- It is recommended that the technologist performing cardiac MRI have advanced certification in MR
- Each technologist should have supervised experience in the performance of cardiac MRI examinations and in the intravenous administration of conventional MR contrast agent
- If intravenous contrast material is to be administered, qualifications for technologists performing intravenous injections should be in compliance with current ACR policy statements and existing operating procedures or manuals at the imaging facility
- The technologist's continuing education credits should include continuing education in cardiac MRI as is appropriate to the technologist's practice needs
- Basic life support (BLS) and automatic defribillator (AED) training is recommended
- Any technologist practicing MRI scanning should be licensed in the jurisdiction in which he/she practices, if state licensure exists
- To assure competance, all technologists must be evaluated by the supervising physician
Indications
Acquired Heart Disease
Dynamic Cardiac Anatomy and Ventricular Function
- In general, echocardiography is a reasonable first test for left ventricular (LV) function.
- MRI because of its three-dimensional data acquisition, is considered to be more accurate and reproducible
- Qualitative assessment of regional ventricular wall-motion abnormalities (WMAs) and quantitative assessment of LV function are appropriate in most MRI examinations of the heart
- Qualitative assessment of regional WMA should use the standard 17-segment model and following terms:
- Hypokinetic
- Akinetic
- Dyskinetic
- LV quantitative function should be performed using short axis views from base to apex
- To provide complete qualitative analysis, the following should also be performed:
- LV two-chamber
- LV four-chamber
- Three-chamber long axis views
- Parameters recommended to be routinely reported in a functional MRI examination include:
- LV end-diastolic volume (LVEDV) or indexed LVEDV
- LV end-systolic volume (LVESV) or indexed LVESV
- LV stroke volume
- LV ejection fraction (LVEF)
- LV mass index
- LV end diastolic and end systolic diameter
- Routine use of Simpson's Rule for calculating LVEF is recommended, although in patients without significant regional wall motion abnormalities, the area-length method may be a satisfactory alternative
- Diastolic dysfunction may also be assessed using flow quantification methods in order to assess E/A ratios (early [E] and late, or atrial [A] phases of LV filling)
- Specific indications for assessment of regional or global LV function include indeterminate or discrepant echocardiography results or situations where serial assessment of regional change in LV function is important (e.g., following patients after myocardial infarction, left ventricular hypertrophy, valvular regurgitation or atrial septal defects)
- Right ventricular (RV) size as well as global and regional wall motion may be assessed qualitatively and reported
- MRI is the recommended first-line diagnostic test for assessing RV function (RVEF and RVEDV) by applying Simpson's rule to short axis slices
- The most common indication for RV assessment is to evaluate patients for suspected arrhythmogenic RV cardiomyopathy (ARVC), where global and regional RV WMAs constitute diagnostic criteria for disease
Assessment of Cardiomyopathies, Myocardial Fibrosis, and Infarction
- Assessment of regional and global myocardial thickness may provide adjunctive value to echocardiography in patients with suspected myocardial infarction, myocarditis, or cardiomyopathy
- In particular, patients with atypical hypertrophic cardiomyopathy, such as apical hypertrophy, may be better assessed with MRI than echocardiography
- MRI is considered the gold standard in the assessment of myocardial mass because it is more accurate and reproducible than echocardiography
- In hemochromatosis, MRI may be used for qualitative and/or quantitative assessment of myocardial iron overload
- MRI can also be used to assess fatty infiltration of the heart is suspected ARVC
- The optimal scanning approach as well as the sensitivity and specificity of MRI for detecting intramural fat in this condition have not been established
- Besides iron and fat, MRI rarely provides tissue-specific information relevant to infiltrative diseases of the heart, but it may provide a comprehensive pattern of wall thickness and wall motion of all four cardiac chambers
- Myocardial delayed hyper-enhancement (MDH) is a specific feature of cardiac MRI that may be extremely useful in detecting areas of myocardial damage and fibrosis
- A subendocardial or transmural pattern of enhancement distinguishes ischemic scar from other causes of enhancement such as myocarditis and scarring in nonischemic cardiomyopathy
- Cardiac MRI with evaluation of global/regional function and MDH is indicated in the evaluation of dilated cardiomyopathy to exclude ischemia as the cause and obviate the need for cardiac catheterization in many patients
- MDH may also be helpful in the diagnosis of chronic or acute myocarditis
- In chronic ischemic cardiomyopathy, the evaluation of regional wall thickness, regional WMAs, and delayed hyperenhancement may be used to evaluate the likelihood of functional recovery after percutaneous or surgical revascularization
- Can also assist in surgical planning for ischemic aneurysms of the heart and be used to identify ventricular thrombus in association with ischemic scar
Myocardial Ischemia and Viability Assessed Through Use of Pharmacologic Agents
- MRI perfusion imaging during gadolinium infusion can be used to detect areas of perfusion abnormality at rest or during pharmacologically induced stress
- Diagnosis of perfusion abnormalities can be performed qualitatively although use of semiquantitative parametric imaging uses features related to the upslope of the perfusion curve may improve accuracy of diagnosis
- MRI is capable of quantifying perfusion and perfusion reserve, but the tools to do this are not yet widely available
- Resting perfusion imaging may provide adjunctive information in chronic ischemia to differentiate among normal, ischemic but viable (hibernating), and nonviable myocardium
- The major indication for perfusion MRI is in conjunction with vasodilator stress agents such as adenosine to detec inducible ischemia
- Precautions and contraindications specific to the chosen vasodilatory agent as described in the package insert and in the literature should be followed
- The relative merits of perfusion MRI in clinical practice have not been definitively established
- High dose dobutamine stress MRI may also be performed to detect ischemia as inducible wall motion abnormalities
- High dose dobutamine should be administered at a maximum of four stress levels, if starting at a dose of 10 μg/kg/min, and a maximum of five stress levels if starting at a dose of 5 μg/kg/min, at 3-5 minutes per level
- Dosing should not be above 40 μg/kg/min
- No more than 1 mg of atropine at the highest dobutamine dose should be administered to achieve a submaximal heart rate
- Dobutamine stress may be performed in the MRI environment safely:
- For administration of dobutamine (>10 μg/kg/min), a separate satellite monitor/workstation in addition and adjacent to the scanning console in the control room is suggested
- Images should be rigorously monitored by a physician and assessed for induced wall motion abnormality at each increment of dobutamine as the images are acquired
- The physician should observe regional wall motion in the long and short axis at each stress level and the examination stopped if new regional WMAs are seen
- The physician should be prepared to treat any induced ischemia with medications, including beta blockers and nitrates
- An external cardiodefibrillator should also be available
- Perfusion MRI with gadolinium can be performed at peak dobutamine stress and may provide additional diagnostic information
- Lower dose dobutamine (at levels of 5 and then 10 μg/kg/min) can be administered to determine myocardial variability through qualitative and quantitative assessment of myocardial thickening and improvement in wall motion
- With administration of all stress agents, patients should be hemodynamically monitored (blood pressure, heart rate, SaO2, and rhythm assessment) throughout the MR examination
- A 12-lead EKG should be obtained prior to and after the examination and compared for differences suggestive of induced ischemia or infarction
- As with vasodilatory agents, all precautions and contraindications specific to dobutamine administration as described in the vendor's package insert and in the literature should be observed
- Functional MRI, MDH, and perfusion MRI may be used to diagnose segments with regional ischemia and acute myocardial infarction in acute coronary syndromes (ACS)
- Serial ECG and enzyme assessment remain the diagnostic standard for ACS, but cardiac MRI may be helpful in cases where clinical exam, ECG, and enzymes are indeterminate
Characterization of Cardiac Masses
- Most cardiac masses are initially identified on echocardiography, MRI is indicated to evaluate tumors with regard to:
- Specific tissue characterization (fat-containing, cystic, fibrotic, etc.)
- Origin
- Relationship to chambers and valves
- Myocardial-extracardial extension
- MRI features such as susceptibility effects, enhancement pattern, and extension from central venous thrombosis can be helpful in differentiating thrombus from tumor
- MRI is the optimal imaging method for evaluating paracardiac masses as it allows evaluation of mediastinal, pericardial, and myocardial involvement in a single study
Pericardial Disease
- Cardiac MRI can be used to evaluate:
- The size and location of pericardial effusions
- Help differentiate simple from complex or loculated fluid collections
- Assess for pericardial thickening
- MRI tissue characterization can also help determine the etiology of effusions (e.g., transudative, exudative, hemorrhagic, or neoplastic)
- Tamponade and constrictive pericarditis can be detected by evaluating anatomic and functional characteristics
- Major characteristic of tamponade is diastolic collapse of the right ventricular outflow tract
- Characteristics of constrictive pericarditis include
- Conical deformation of the ventricles
- Atrial and caval dilatation
- Abnormal motion of the interventricular septum
- Assessment of effusions can also be coupled with delayed contrast-enhancement assessment of the myocardium to assess for myocarditis
Valvular Disease
- Using phase contrast techniques and functional assessment, cardiac MRI has the capability to evaluate congenital or acquired cardiac valve stenosis and/or insufficiency
- Aortic and pulmonic valve stenoses can be assessed by phase contrast determination of peak systolic velocity combined with the modified Bernoulli equation
- Direct planimetry of the aortic valve on high resolution cine images can also be performed
- Aortic and mitral valvular regurgitation fractions may be measured quantitatively by calculations based on aortic root phase contrast flow assessment and LV stroke volume
- Pulmonic valvular regurgitation fractions may be measured quantitatively by calculations based on pulmonary outflow tract flow assessment
- Anatomic and blood flow characteristics can determine the type and degree of valve abnormality and the subsequent functional impact on adjacent cardiac chambers
Coronary Artery Disease
- Although MRI can depict acquired proximal disease of the coronary arteries, the clinical application is limited at this time
- Stenotic disease and aneurysms can be detected, and such findings could be of clinical importance in some patients
- However, this is not indicated in the routine evaluation of coronary artery disease
- Characterization of atherosclerotic plaque and determination of coronary blood flow are research applications that may become clinically valuable
- Cardiac MRI can be used to evaluate the patency of and, indirectly, the presence of stenoses of coronary artery bypass grafts
Congenital Heart Disease
Congenital Shunts
- MRI may be used to quantify and follow right and left ventricular volumes and function as well as pulmonary to aortic flow ratios over time
- Specific forms of atrial or ventricular septal defects that are difficult to identify or characterize on echo may benefit from MRI assessment and Qp/Qs quantification
- MRA of the chest can be used to identify shunts due to anomalies of pulmonary venous return and can also assess the aorta and pulmonary arteries if desired
Complex Congenital Anomalies
- MRI with MRA and/or flow measurement may be helpful in cases of complex congenital anomalies to assist:
- In situs determination chamber identification
- Chamber size
- Global function
- Atrioventricular and ventricular-arterial relationships
- Intracardiac or extracardiac shunts
Pericardial Anomalies
- Congenital pericardial defects can be evaluated for size and location, and complete absence of the pericardium can be differentiated from partial defects
- Complications such as entrapment of the left atrial appendage can be detected
Congenital Valve Disease
- Cardiac MRI has the capability to evaluate for congenital cardiac valve stenosis and/or insufficiency (e.g., bicuspid aortic valves, cleft mitral valve, Ebstein's anomaly of the tricuspid valve, etc.)
- Anatomic and blood flow characteristics can determine the type and degree of valve abnormality, and the subsequent functional impact on adjacent cardiac chambers
Coronary Artery Anomalies
- MR imaging can be useful in detecting anomalous origins of the coronary arteries
- Significant anomalies such as abnormal positioning of a coronary artery between the aorta and right ventricular outflow tract can be determined
- Extracardiac anomalous coronary artery origin (e.g., Bland-White-Garland syndrome) can also be determined
- Other indications include assessment of aneurysms and/or stenoses of the native coronary arteries such as may occur in Kawasaki's disease or Takayasu's arteritis
Safety Guidelines and Possible Contraindications
- The cardiac MRI physician should have thorough knowledge of patient safety:
- Specific absorption rate (SAR) limits
- Possible neurologic effects
- Tissue heat deposition
- Contraindications to MRI such as implantable devices
- With regard to administration of IV contrast media, the physician should supervise patient selection to identify those patients for whom IV contrast media administration may present an increased risk or be contraindicated
- Contrast reactions occur less frequently with gadolinium-based contrast agents in comparison to iodinated agents, some patients may require pretreatment to allow safe contrast administration
- The physician should also be available to treat adverse reactions to IV contrast media as described in the ACR Practice Guideline for the Use of Intravascular Contrast Media and ACR Manual on Contrast Media
- When exercise or pharmacologic stress is performed or hemodynamically unstable patients are studied, a physician must always be present
- Life support instruments, medications and ACLS-trained personnelmust be available in the immediate vicinity of the stress laboratory
- Baseline blood pressure measurement and electrocardiographic tracing should be obtained before performing pharmacologic stress
- Heart rhythm and blood pressure monitoring must be performed during stress and recovery
- During dobutamine administration, a second (satellite) viewing station is suggested to permit direct comparison of wall motion at various dobutamine dose levels
- Workstation is in addition to console used by the MR technologist for scanning purposes
- Reader should see the ACR Practice Guideline for Performing and Interpreting Magnetic Resonance Imaging (MRI) and the ACR White Paper on Magnetic Resonance Safety
- Peer-reviewed literature pertaining to MRI safety should be reviewed on a regular basis
Specifications of the Examination
Overview
- The written or electronic request for cardiac MRI should provde sufficient information to demonstrate the medical necessity of the examination and allow for the proper performance and interpretation of the examination
- Documentation that satisfies medical necessity includes:
- Signs and symptoms
- Relevant medical history (including known diagnoses)
- The provision of additional information regarding the specific reason for the examination or a provisional diagnosis would be helpful and may at times be needed to allow for the proper performance and interpretation of the examination
- Request for examination must be originated by a physician or other appropriately licensed health care provider
- The accompanying clinical information should be provided by a physician or other appropriately licensed health care provided familiar with the patient's clinical problem or question and consistent with the state scope of practice requirements
- The supervising physician must have complete understanding of the indications, risks, and benefits of the examination, as well as alternative imaging procedures
- Physician must be familiar with potential hazards associated with MRI, including potential adverse reactions to contrast media
- The physician should be familiar with relevant ancillary studies that the patient may have undergone
- The physician performing MRI interpretation must have a clear understanding and knowledge of the anatomy and pathophysiology relevant to the MRI examination
- The supervising physician must also understand the pulse sequences to be employed and their effect on the appearance of the images, including the potential generation of image artifacts
- Standard imaging protocols may be established and varied on a case-by-case basis when necessary
- These protocols should be reviewed and updated periodically
Patient Selection
- Physician responsible for the examination shall supervise patient selection and preparation and be available in person or by phone for consultation
- Patients should be screened and interviewed prior to the examination to exclude individuals who may be at risk by exposure to the MR environment
- Certain indications require administration of intravenous (IV) contrast media
- IV contrast enhancement should be performed using appropriate injection protocols and in accordance with the institution's policy on IV contrast utilization
- Patients suffering from anxiety or claustrophobia may require sedation or additional assistance
- Administration of moderate or "conscious" sedation may enable achievement of the examination
Facility Requirements
- An appropriately equipped emergency cart must be immediately available to treat adverse reactions associated with administered medications
- The cart should be monitored for inventory and drug expiration dates on a regular basis and comply with institutional policies
Examination Technique
- A phased array surface coil should be used, unless precluded by patient body habitus
- The heart is a small structure, so the field of view should be reduced to maintain adequate spatial resolution
- An adaquete signal to noise ration should also be maintained
- MRI techniques must be optimized for the wide range of indications for cardiac imaging and may be highly variable due to advances in MRI scanner software and hardware
- Most examinations will include short axis and long axis cine images of the heart obtained for ventricular function
Left Ventricular Function
- Images in the true short axis plane of the heart should be obtained from just above the mitral valve plane to the apex of the heart at approximately 1 cm intervals
- Depending on the pulse sequence used, this could be accomplished, for example, using 8 mm thick slices and 2 mm thick gap between the slices for a two-dimensional acquisition. Similarly, 10 mm thick slices may be used, with no interslice gap
- In addition, horizontal and long axis cine images of the left ventricle are routinely acquired
Cine Imaging
- On most MRI systems, cine image acquisition should be gated to the R wave of the electrocardiogram and will involve suspended respiration typically at resting lung volume during the acquisition
- Acquired temporal resolution, preferably, should be less than or equal to 50 msec; interpolation methods (e.g., view sharing) are desirable to display reconstructed cine images at less than the acquired temporal resolution
- Segmented fast gradient echo images with flow compensation have traditionally been used for cine imaging
- More recently, steady state free precession gradient echo imaging has been demonstrated to result in faster high quality cine images of the heart and is now preferred if this sequence is available
Assessment of Cardiac Morphology
- For cardiac indications that require assessment of cardiac morphology, T1- and/or T2-weighted images of the heart may be helpful
- The imaging planes should be tailored to the pathology that is present, but transaxial images are often suitable
- Images should be gated to the R wave of the electrocardiogram
- Conventional spin echo or fast/turbo spin echo images have been traditionally been used to obtain T1- or T2-weighted images
- These sequences can be used in combination with inflow saturation bands to produce dark blood images
- More recently, double inversion recovery fast/turbo spin echo techniques have been implemented
- Since these images are usually obtained in a breathold, there is excellent reduction of motion artifacts as well as good suppression of the blood pool resulting in high quality black blood images
- Echo train lengths (ETLs) with this sequence are usually less than 40; even shorter ETLs (<10) may be required for short effective-TE scans; very high echo train lengths associated with single shot techniques results in excessive blurring of intracardiac detail and, if possible should be avoided
Use of Gadolinium
- Administration of intraveneous gadolinium chelates (0.1-0.2 mmol/kg) for myocardial enhancement may be required for certain cardiac indications, including but not limited to evaluation of:
- Masses/cysts
- Pericardium
- Myocardial perfusion
- Inflammation or infarction
- Myocardial perfusion evaluation additionally requires rapid bolus administration (3-5 ml/sec) if the gadolinium chelate
- Postgadolinium images of the heart are T1 weighted images acquired using spin echo, fast/turbo spin echo, double inversion recovery fast/turbo spin echo or gradient echo techniques
- Evaluation of myocardial infarction/scar or fibrosis is optimally performed using an inversion prepared gradient echo technique
- The inversion time is optimized to supress normal myocardium (T1 typically 175-250 msec) during the washout phase (e.g. 5-30 minutes) of gadolinium chelate distribution
- Precise T1 is dependent upon gadolinium chelate dose time after administration and individual patient pharmacokinetics and must be determined for each individual being scanned
Phase Contrast Imaging
- Phase contrast imaging of the heart may be used for a variety of indications related to quantification of flow
- The velocity encoding gradient should be set to a value higher than the maximum expected linear flow rate of blood
- Phase contrast images are acquired either parallel or perpendicular to the direction of flow, depending upon the indication
- MR angiongraphy using gadolinium-enhanced techniques is frequently used in conjunction with other cardiac methods
- MR angiography may provide additional useful information regarding the status of the aorta, pulmonary artery, pulmonary veins, coronary arteries, and vena cava
Tagging
- MRI tagging is a technique in which radiofrequency vands are applied to the heart at end diastole
- Cine images are then acquired and the motion of the bands, or tags, is observed
- MRI tagging may provide additional visual indication of focal wall motion abnormalities in selected cases
- MRI tagging lines applied perpendicular to the free wall of the right ventricle may be useful to determine the relative motion of the pericardium compared to the myocardium in patients with suspected constrictive pericarditis
- When available, techniques such as parallel imaging and partial Fourier methods may be used to shorten patient breatholds
Analysis
- The analysis of cardiac MRI examinations is optimally performed using a separate imaging workstation
- Separate cardiac imaging software is usually required for evaluation of cardiac function, blood flow (from phase contrast images), and three-dimensional MR angiography
Documentation
- Reporting should be in accordance with the ACR Practice Guideline for Communication of Diagnostic Imaging Findings
- When reporting information regarding myocardial function, perfusion, viability or infarction, the 17-segment model should be used
- Wall motion abnormalities should be described using conventional terminology such as:
- Hyperkinetic
- Hypokinetic
- Akinetic
- Dyskinetic
- Images should be labeled with the patient identification, facility identification, examination date, and the side (right or left) of the anatomic site imaged
Equipment Specifications
- Scanners for clinical cardiac MRI should be accredited by the ACR and equipment performance monitoring should be in accordance with the ACR Technical Standard for Diagnostic Medical Physics Performance Monitoring of MRI Equipment
- MRI equipment specifications and performance shall meet all state and federal requirements
- Requirements include, but are not limited to:
- Specifications of maximum static magnetic strength
- Maximum rate of change of the magnetic field strength (dB/dt)
- Maximum radiofrequency power deposition (specific absorption rate)
- Maximum acoustic noise levels
- MRI scanners used for cardiac MRI performance should be 1.0 Tesla field strength or above and have a slew rate of at least 70 mT/meter/sec
- At time of writing, the best proven field strength for performance of cardiac MRI is 1.5 Tesla
- It may be that in the future, cardiac imaging can be routinely carried out at 3.0 Tesla, but at this current time, substantial challenges exist for performing certain pulse sequences
- MRI scanners should be equipped with a localized multi-channel radiofrequency surface coil and ECG-gating
- Ideally, ECG-gating capabilities would include prospective triggering, retrospective gating and triggered retrograting
- Vectorcardiographic gatng is desirable but not essential
- An MRI-compatible power injector is required for performing myocardial perfusion MR imaging or any MR angiographic methods
- A power-injector is not required for delayed contrast-enhanced studies
- MRI scanner should be capable of fast 3D gradient echo imaging, steady state imaging and delayed contrast-enhanced myocardial imaging
- Parallel imaging and half-Fourier capabilities are desirable to permit shortened breath-hold requirements
- Commercial, FDA-approved software for processing data (calculation of ejection fractions, reformatting angiographic data) should be available either as part of the MR system, or available on a separate workstation
- Postprocessing should be performed or supervised by the cardiac MRI physician
Quality Control and Improvement, Safety, Infection Control, and Patient Education Concerns
- Policies and procedures related to quality, patient education, infection control, and safety should be developed and implemented in accordance with the ACR Policy on Quality Control, and Patient Education Concerns appearing elsewhere in the ACR Practice Guidelines and Technical Standards book
- Specific policies and procedures related to MRI safety should be in place with documentation that is updated annually and compiled under the supervision and direction of the supervising MRI physician
- Guidelines should be provided that deal with potential hazards associated with the MRI examination of the patient as well as to others in the immediate area
- Screening forms must also be provided to detect those patients who may be at risk for adverse events associated with the MRI examination or with any contrast agent or pharmaceutical to be administered
- Equipment monitoring should be in accordance with the ACR Technical Standard for Diagnostic Medical Physics Performance Monitoring of Magnetic Resonance Imaging (MRI) Equipment
Reference
- PMID 17412147