Abstract
The critical care provider when faced with complicated clinical problems often asks the imager, “Which test is best?” to which the imager should reply, “For what question?” With the multiple imaging modalities of echocardiography, cardiac magnetic resonance, computed tomography, and cardiac catheterization, it can be confusing for the provider and can result in unnecessary or ineffective imaging. This chapter is organized to help the provider to understand better how to use imaging and to do so in an efficient and cost-effective manner.
Key Words
cardiac magnetic resonance (CMR), cardiac computed tomography, radiation, imaging, temporal resolution, spatial resolution
Defining Imaging Modalities
Strengths and Weaknesses of the Various Imaging Modalities
| Modality | Strengths | Weaknesses |
|---|---|---|
| Echocardiography | Spatial and temporal resolution Logistically simple Intracardiac visualization | Acoustic windows limited in some patients |
| Cardiac magnetic resonance | Biventricular function Tissue characterization | Logistically difficult Sedation requirements |
| Cardiac computed tomography | Spatial resolution Coronary imaging Extracardiac visualization | Radiation |
| Cardiac catheterization | Hemodynamic measurements Interventional options | Logistics and radiation |
Spatial resolution is how close together in space two points can be differentiated from one another and is critical in imaging small and closely adjacent structures. The best spatial resolution is typically submillimeter.
Temporal resolution is how fast a moving structure can be imaged through time and is critical when structures are moving at high speeds (e.g., flail valve leaflet). Temporal resolution is typically less than 30 ms but can be as good as 10 ms. With improved temporal resolution one typically trades reduced spatial resolution.
Magnetic Resonance Imaging
The role of cardiac magnetic resonance (CMR) imaging in congenital heart disease (CHD) has expanded considerably in recent years. CMR is now routinely used for assessment of anatomy (especially in those with limited acoustic windows or to limit radiation exposure), quantification of ventricular function, valvular shunt fraction, and tissue characterization. Advantages of CMR include lack of ionizing radiation, wider field of view including extracardiac vascular structures, and noninvasive assessment of flow dynamics and cardiac output, obvious advantages for a pediatric population with complex cardiac anatomy and need for serial assessments over time. This section aims to provide a practical guide for the pediatric cardiac intensivist by including a basic description of commonly used CMR pulse sequences and their application to frequently encountered clinical case scenarios.
Basic Cardiac Magnetic Resonance Pulse Sequences
Conventional Spin Echo or “Black Blood” Imaging.
In this sequence, flowing blood is suppressed and appears black while surrounding stationary structures appear gray or white to provide image contrast ( Fig. 36.1 ). Images are acquired at a single phase in the cardiac cycle and are typically static. This sequence is especially useful for anatomic detail in the presence of magnetic field inhomogeneities, including metallic implants. Breath-holds are required to limit blurring from cardiac motion.
Application: Anatomic definition, tissue characterization, cardiac tumors.
Gradient Echo.
Gradient echo (GRE) sequences generate images in which blood appears white, surrounding structures gray ( Fig. 36.2 , ), and any turbulence within the blood pool (indicating valve obstruction or regurgitation, shunt lesions) appears void of signal or dark. Images are acquired throughout the cardiac cycle and displayed in a “movie,” or cine, format to illustrate dynamic cardiac motion. Two types of GRE imaging are typically used: (1) steady-state free precession (SSFP) is widely used due to excellent contrast between the blood pool and surrounding structures, or (2) standard spoiled GRE sequences are used for phase contrast (PC), late gadolinium enhancement (LGE), and three-dimensional contrast-enhanced magnetic resonance angiography (CE-MRA) as defined below. GRE sequences are also used in the presence of magnetic field inhomogeneity (endovascular metallic implants) because SSFP images are vulnerable to artifact.
Application: Quantification of ventricular volume and function, assessment of ventricular mass, visualization of cardiac motion (e.g., ventricular wall motion abnormalities), surgical conduits, or patches, as well as valve morphology.
Phase Contrast Imaging.
This modified GRE sequence is used primarily for quantification of flow volume (in milliliters) and velocity (in meters per second) in vessels or across valves and to estimate cardiac shunts. When protons in motion in the blood pool are placed in a specific magnetic field gradient, they acquire a phase shift that is proportional to the velocity of flowing blood. A weighted average of the velocity through all the pixels within a designated slice in a vessel lumen is measured. The product of the velocity encoded and the cross-sectional area of the vessel or valve yields the flow at that instant in the cardiac cycle, which is integrated over time for flow rate over the entire cardiac cycle. Similar to echocardiography, peak flow can be estimated by applying the Bernoulli equation: maximum instantaneous gradient = 4V 2 , where V 2 is the maximum instantaneous velocity.
Application: Quantification of valvular regurgitant fraction (e.g., aortic valve regurgitant fraction), intracardiac and extracardiac shunt flow (e.g., ratio of pulmonary blood flow [Q p ] to systemic blood flow [Q s ], or Q p :Q s , for ventricular septal defects, patent ductus arteriosus), flow across vessel stenosis, quantification of collateral flow, and cardiac output.
Late Gadolinium Enhancement.
Late gadolinium enhancement (LGE) is used to delineate focal regions of myocardial fibrosis based on the principle that contrast washout is delayed in fibrosed or infarcted myocardium compared with normal myocardium. These are static images in which normal myocardium is black or nulled, whereas fibrosed or scarred regions appear bright or white. Myocardial scar formation can be used for prognosis as well as anticipation of arrhythmia.
Application: Myocardial assessment for focal scar formation, cardiac tumor and thrombus assessment.
Three-Dimensional Contrast-Enhanced Magnetic Resonance Angiography.
Three-dimensional contrast-enhanced magnetic resonance angiography (CE-MRA) is a fast spoiled GRE sequence performed using a single breath-hold to obtain a high-resolution three-dimensional data set of the heart. Data are displayed as two-dimensional images that can be visualized in any plane or volume rendered to a three-dimensional image ( Fig. 36.3 ). Gadolinium is typically used as the contrast agent (dose 0.1 to 0.2 mmol/kg). If multiple data sets are acquired over shorter acquisition time, dynamic contrast flow can be demonstrated in vascular structures, resembling contrast angiography via cardiac catheterization (“time-resolved” MRA). This is useful when visualization of contrast passage in all phases of the cardiac cycle is important; however, spatial resolution is typically less than with CE-MRA, and artifact susceptibility is higher ( Fig. 36.4 , ). Use of CE-MRA or time-resolved MRA can replace cardiac catheterization for some cases when delineation of the anatomy is the primary question.
Application: Definition of extracardiac vascular structures (e.g., pulmonary arteries, anomalous pulmonary veins, aortic arch abnormalities, systemic veins, collateral vessels, surgical shunts, and conduits).
First-Pass Perfusion Imaging.
First-pass perfusion imaging is used to visualize the passage of contrast through the myocardium. Poorly perfused areas are darker, whereas normally perfused regions are brighter with contrast. Images can be acquired at rest or during stress (via administration of adenosine or similar coronary vasodilator).
Application: Evaluate coronary perfusion if concern for coronary obstruction following coronary reimplantation surgery (arterial switch operation or anomalous coronary repair), Kawasaki disease.
Parametric Mapping.
T1 and T2 parametric mapping sequences are gaining importance in myocardial tissue characterization. T1 maps are useful for quantification of diffuse fibrosis (compared with regional scarring as depicted by LGE), whereas T2 maps can help distinguish areas of edema or hyperemia. Parametric mapping sequences provide a quantitative assessment of myocardial involvement, compared with existing sequences such as T1- or T2-weighted imaging, which rely on visual estimates of signal intensity. Application of these techniques are evolving for monitoring of disease progression, as well as therapy.
New Sequences
Four-Dimensional Flow Magnetic Resonance Imaging.
Two-dimensional PC, as described previously, measures volume of blood flow in a single direction assigned perpendicular to direction of flow. An emerging technique in flow measurement encodes velocity in three spatial dimensions over time in the cardiac cycle. Although it is not in broad clinical use currently, advantages include more comprehensive flow characterization, including those of complex flow patterns; characterization of beat-to-beat variability; and retrospective assessment of blood flow in any vessel within an acquired three-dimensional volume data set. Additional derived-flow parameters include wall shear stress, pulse wave velocity, and energy loss. Multicenter studies are necessary to validate and assess reproducibility of this technique for clinical use, as well as to continue to develop methodologic modifications for shorter scan times.
Frequently Asked Questions
- 1.
When is contrast needed?
Contrast is required for specific pulse sequences as discussed earlier. In general, CE-MRA (for delineation of extracardiac vascular structures, including pulmonary arteries, veins, aortic arch, surgical shunts, and conduits), tissue characterization by LGE, and myocardial perfusion studies require contrast. Assessment of ventricular volumes, cardiac output, and quantification of valvar regurgitation do not require contrast administration. In some cases, use of three-dimensional SSFP with respiratory and cardiac gating can provide anatomic data without the use of a contrast agent.
- 2.
My patient had a contrast reaction during computed tomography (CT). Can this patient receive contrast with magnetic resonance imaging (MRI)?
Yes. MRI requires use of gadolinium-based contrast agents (GBCAs), which are different than iodinated CT contrast agents, so allergy to one does not preclude use of the other. In general, most common reactions to GBCAs include nausea, cold sensation at the injection site, vomiting, or itching. Anaphylaxis is rare. The incidence of adverse events related to gadolinium is very low. Patients with acute or chronic kidney disease are at risk for nephrogenic systemic fibrosis with highest risk in those with glomerular filtration rate at less than 30 mL/min/1.73 m 2 . Recent studies have shown preliminary effects of gadolinium deposition in the brain; however, long-term implications are unknown.
- 3.
When is anesthesia needed?
A successful CMR scan may require breath-holds to limit motion artifact, though new free-breathing sequences are available. Patients also have to lie still and to understand and comply with multiple breath-holding instructions over a 40- to 60-minute scan. In general, children 8 years and older without developmental delay do not require sedation if adequate preparation is performed before the scan. In some cases, infants may be fed and swaddled before the scan, and high-priority imaging can be accomplished quickly without sedation using free-breathing sequences with both cardiac and respiratory gating. When required, sedation is typically administered in the form of general anesthesia at the discretion of an experienced pediatric anesthesiologist to comply with breath-holding instructions. Complete cardiorespiratory monitoring is undertaken, with ongoing monitoring throughout the scan for adverse effects, including concerns for hypothermia or hyperthermia, rhythm abnormalities, or other anesthesia-related events. The electrocardiographic (ECG) tracing can be used to calculate the patient’s heart rate and demonstrate changes in cardiac rhythm, but it cannot be used for diagnostic purposes because the MR signal alters the ECG signal. When need for sedation is unclear, goals of imaging should be discussed with the provider responsible for performance and interpretation of the MRI.
Case Examples
Case 1.
A previously healthy teenager presents with a 3-day history of dyspnea on exertion associated with chest pain at rest. This follows a recent viral illness. The echocardiogram shows poor ventricular function and pericardial effusion.
Onset of cardiac symptoms and ventricular dysfunction following a recent acute viral illness are most consistent with a diagnosis of acute myocarditis. However, the differential diagnosis for ventricular dysfunction is broad and includes other cardiomyopathies. Dilated cardiomyopathy (DCM) and hypertrophic cardiomyopathy (HCM) are most common ; however, left ventricular noncompaction cardiomyopathy (LVNC), restrictive cardiomyopathy, arrhythmogenic right ventricular dysplasia (ARVD), and important coronary anomalies are also part of the differential and must be ruled out. CMR is an important noninvasive diagnostic tool in these cases.
CMR has a sensitivity of 82% in diagnosing pediatric myocarditis. In acute myocarditis CMR can be used to detect inflammation and edema (T2-weighted imaging) in ventricular myocardium. Fibrosis (LGE) may be seen, especially in a patchy subepicardial noncoronary distribution ( Fig. 36.5 ). SSFP imaging provides information about wall motion abnormalities. Right and left ventricular ejection fraction can be quantified in both acute and chronic myocarditis. Newer techniques such as parametric mapping may help quantify myocardial involvement without relying on visual estimation of signal intensity.
DCM may be due to genetic or metabolic disease, cardiotoxic chemotherapy, associated with known neuromuscular disease, or idiopathic. Quantification of ventricular function and mass and serial assessments over time are useful. LGE, if present, is diagnostically useful because it has a characteristic midwall location and does not follow a coronary distribution. In some specific disease states such as Duchenne muscular dystrophy, diffuse fibrosis characterized by parametric mapping may allow for earlier detection and treatment of disease. Acute myocarditis may be ruled out as described in association with clinical history.
For HCM, CMR identifies the pattern of myocardial hypertrophy (septal, concentric, apical, midventricular) with accurate quantification of left ventricular (LV) mass. LGE is usually localized to regions of hypertrophy, and its presence may predict development of arrhythmia ( Fig. 36.6 ). Atrial enlargement (in the setting of ventricular diastolic dysfunction) can also be measured and serially assessed.
Diagnosis of other causes of myocardial dysfunction such as LVNC and ARVD are more challenging in children because criteria are less well defined. LVNC is a cardiomyopathy with presumed arrest of myocardial compaction. Myocardial trabeculae are well seen by CMR, especially when they exist in the apex; however, LGE may not be as useful. For ARVD, right ventricular (RV) wall motion abnormalities, including visualization of segmental thinning and quantification of RV function, are useful by CMR because this imaging can be limited on echocardiogram. RV fatty infiltration into normal myocardium can be assessed on directed T1-weighted spin echo imaging. Given that ARVD typically manifests in the second to fifth decade, criteria may not be sensitive in the pediatric population, and progressive follow-up scans may be recommended to evaluate for onset of disease.
Case 2.
An adolescent who underwent transannular patch repair for tetralogy of Fallot (TOF) as an infant has had increasing exertional intolerance over the past year.
CMR is ideally suited for postsurgical assessment in TOF in either the presence or absence of symptoms. Despite excellent long-term survival in repaired patients, long-term morbidity due to hemodynamic and cardiac rhythm disruptions still exists. Chronic pulmonary insufficiency (due to disruption of pulmonary valve annulus at the time of surgery) can lead to RV volume overload with resultant dilation and dysfunction. Follow-up of adult repaired TOF patients showed that severe RV or LV dysfunction (RV ejection fraction < 45%; LV ejection fraction < 55%) and RV dilation ( z score > 7) as measured by CMR were independent predictors of major adverse clinical outcomes, including death and ventricular tachycardia. Other long-term morbidities include LV dysfunction, arrhythmia, heart failure, presence of residual ventricular or atrial shunts, RV outflow tract (RVOT) aneurysms and diffuse myocardial fibrosis.
Given these known morbidities, data from CMR are invaluable in assessing the cause of new-onset symptoms and determining criteria for further intervention. PC sequences are used for quantification of pulmonary valvar regurgitant fraction, as well as differential pulmonary blood flow to each lung ( Fig. 36.7 , and ). Pulmonary regurgitation is generally classified as mild (<20%), moderate (20% to 40%), or severe (>40%).
Proposed parameters for pulmonary valve replacement: RV diastolic volume index above 150 to 160 mL/m 2 or RV end-systolic volume index above 80 mL/m 2 .
Morphology of branch pulmonary arteries, RVOT anatomic abnormalities, RV wall motion abnormalities, or abnormal RV ventricular geometry are well seen using customized imaging planes ( Fig. 36.8 , and ). This is especially relevant to the older patient with limited echocardiographic windows. Location and hemodynamic impact of intracardiac shunts can be quantified (using Q p :Q s ).
RV dimensions, function, and pulmonary regurgitant fraction are critical to decision making regarding surgical pulmonary valve replacement. Delayed gadolinium enhancement is useful in assessment of the adult with repaired TOF for prediction of arrhythmia burden, heart failure and exercise intolerance.
Increasing interest in use of percutaneous pulmonary valves in a conduit or dynamic native RVOT underscores the importance of sizing the pulmonary valve annulus and high-resolution imaging of the dynamic RVOT, coronary arteries, and the branch pulmonary arteries.
Case 3.
An 8 year old was found to have hypertension on physical examination before sports participation.
Initial cardiovascular assessment of hypertension in the older child warrants evaluation of aortic coarctation, a localized narrowing in aortic lumen generally at the aortic isthmus ( Fig. 36.9 ). In general, echocardiography is the first choice for imaging and may be sufficient to determine the degree of coarctation and additional aortic arch anatomy. However, in the older child, imaging may be suboptimal or inadequate to delineate precise anatomy and presence and extent of collateralization.
Additional imaging choices include CMR or CT, which may be determined by the ability of this child to undergo a CMR study or clinical status. Both techniques should provide excellent data sets to resolve the anatomy. The specific advantage of CMR over CT is quantification of peak velocity at the level of the coarctation, as well as collateral burden, if suspected. Peak velocity at the level of stenosis can be used to estimate the hemodynamic gradient. Collateral flow is estimated by measuring the difference in flow at the level of the diaphragm and the aortic arch just distal to the obstruction. Anatomic imaging of collaterals is well defined by MRA after gadolinium administration. Additional assessments relevant to aortic coarctation, including cardiac output, LV function and mass, and aortic valve stenosis and regurgitation, can be quantified by CMR.
Following intervention (surgery or balloon or stent angioplasty), CMR is performed to identify sequelae, including residual arch hypoplasia or coarctation and presence of aneurysms and dissections.
Case 4.
Over the past year, a 10-year-old status post non-fenestrated extracardiac Fontan operation at age 4 years has developed worsening cyanosis with an oxygen saturation of 82%.
The total cavopulmonary connection (Fontan) operation is the final step in the three-step staged palliation for the functionally univentricular heart. The Fontan procedure involves redirection of systemic venous flow directly to the pulmonary arteries while ventricular output is pumped to the body, creating a circulation in series. Variations in type of Fontan connection exist, and knowledge of these should inform assessment in the unknown adult patient with CHD ( Fig. 36.10 ).
CMR is particularly suited to imaging complex postsurgical anatomy, especially in the case of single-ventricle anatomy with variable ventricular geometry.
The specific goals of CMR imaging:
- 1.
Assessment of anatomy and flow in the Fontan cavopulmonary connection: Obstructions or baffle leaks can be seen on SSFP cine imaging. PC can be used to quantify blood flow into the superior and inferior vena cava, pulmonary veins, and branch pulmonary arteries, as well as to estimate Q p :Q s .
- 2.
Ventricular function and cardiac output: SSFP cine stacks can accurately quantify right or left ventricular volumes and function with superior reproducibility compared with echocardiography. Systemic cardiac output can be obtained by PC at the level of the systemic semilunar valves (or across both semilunar valves in a Damus-Kaye-Stansel type of arch reconstruction).
- 3.
Quantification of collateral burden: Aortopulmonary (connection between systemic and pulmonary circulation) or venoatrial collaterals (connection between systemic venous and pulmonary venous circulation) can impose a volume load or cyanosis, respectively. Quantification of the collateral burden has been proposed by flow mapping in CMR by two separate methods with good agreement.
- a.
Aortopulmonary collateral: Flow in ascending aorta − flow in systemic cavae (inferior vena cava and superior vena cava)
- b.
Aortopulmonary collateral: Total pulmonary venous flow − total flow in branch pulmonary arteries
- a.
- 4.
Presence of thrombi: Thrombi may be present in up to 30% of the Fontan population with highest risk in the immediate postoperative period ; thrombi can be demonstrated by SSFP cine or other specialized black blood imaging.
- 5.
Three-dimensional MRA with contrast or three-dimensional SSFP (noncontrast) sequences define the pulmonary arteries, including distortions or stenosis, aorta, and systemic venous connections.
- 6.
LGE for myocardial fibrosis has been reported to be as high as 28% in the Fontan population and is associated with lower ejection fraction and higher frequency of arrhythmia.
Case 5.
A 2 year old is found to have a large RV mass as an incidental finding on echocardiogram.
Cardiac tumors are rare in children. Approximately 90% of pediatric cardiac tumors are benign. Rhabdomyoma is the most common type, followed by fibroma. The most common malignant cardiac tumor is a sarcoma. If adequately imaged, fetal and transthoracic echocardiograms are typically sufficient to make a diagnosis of cardiac tumor; however, the tumor type is not easily characterized. Characterization of tumors by fat content, vascularity, fluid composition, and presence of fibrosis allows for noninvasive diagnosis.
Additional information includes tumor location (including obstruction to blood flow), size, and extent of tumor burden. LGE can also assist with diagnosis of intracardiac thrombi, a common differential in the workup of a cardiac mass. In a multicenter study, if a complete CMR was performed with good image quality, diagnosis of tumor type or a correct differential occurred in 97% of cases (confirmed by pathology).
Typical assessment of tumor type by CMR involves use of specific sequences as outlined by Beroukhim et al.:
- 1.
GRE cine imaging for size, location, and extent of cardiac mass, assessment of ventricular function and size
- 2.
Spin echo (black blood sequences) with T1 and T2 weighting to bring out the properties of the tumor and use of fat suppression to increase diagnostic accuracy
- 3.
Myocardial first-pass perfusion imaging to characterize vascular properties
- 4.
Delayed enhancement imaging to improve recognition of thrombi
Of note, differentiation of malignant versus nonmalignant tumors remains limited using CMR. This is specifically true for vascular tumors in which vascularity can be classified; however, malignancy cannot be ruled out.
Noninvasive Calculation of Cardiac Output, Q p /Q s , and Valvar Regurgitated Fraction
Measurement of flow and velocity are integral to assessment and decision making in CHD. The principles outlined below can be applied to the breadth of congenital cardiac disease for simple or complex lesions.
Specific quantitative data provided by CMR include:
- 1.
Stroke volume (milliliters per cardiac cycle) and cardiac output (liters per minute)
- 2.
Shunt quantification (Q p :Q s ): ratio of pulmonary blood flow (Q p ) to systemic blood flow (Q s )
- 3.
Valve regurgitant fraction (%)
- 4.
Quantification of collateral flow
Cardiac Output and Stroke Volume.
To assess flow by PC a cross-sectional region is defined and blood flowing perpendicular to the defined plane is quantified. In the case of cardiac output a plane perpendicular to blood flow is defined in the ascending aorta. Regions of interest are drawn around the vessel in systole and diastole to obtain a flow volume over time. This provides the stroke volume over one cardiac cycle and can be used to calculate cardiac output.
Estimation of Shunt Fraction (Q p :Q s ).
In the absence of shunts, pulmonary and systemic blood flow are equal (i.e., Q p :Q s = 1). In the presence of shunts, in right-to-left shunt, Q p :Q s is less than 1; in left-to-right shunt, Q p :Q s is greater than 1.
In general, flow measurements in the ascending aorta are quantified in this way (Q s ). A similar measurement is made in the main pulmonary artery (Q p ), including other sources of pulmonary blood flow if present (e.g., shunts). In the absence of significant semilunar or atrioventricular valve regurgitation or intracardiac shunts, RV and LV stroke volumes derived from short-axis cine images should be the same and may be used for confirmation and internal consistency. Quantification of shunt fraction by CMR has been shown to correlate well with angiographically derived measurements.
Valve Regurgitant Fraction or Stenosis
Semilunar Valve Regurgitant Fraction.
Blood flow is directly measured just distal to the pulmonary or aortic valve annulus and displayed using a flow curve representing antegrade and retrograde flow in the main pulmonary artery over time. The example shown for pulmonary regurgitant fraction (PRF) below is commonly used for surgical or interventional decision making in TOF (see Fig. 36.7 ).
PRF ( % ) = Retrograde flow ( mL ) / Antegrade flow ( mL )
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
