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Achieva 1.5T devoted to congenital heart disease at CHB

Best Practice
Geva, Tal Boston, Children's Hospital Boston USA
Powell, Andrew, M.D. Boston, Children's Hospital Boston USA

Achieva 1.5T devoted to congenital heart disease at Children's Hospital Boston

The Cardiovascular MR Program at Children's Hospital Boston (CHB) began in 1995, with the goal of bringing the diagnostic power of MRI to patients with congenital heart disease. As demand for cardiac studies increased, cardiologists knew that a world-class cardiovascular MR program would require an optimized scanner. In 2005, CHB acquired the Achieva 1.5T I/T Cardiovascular system, which became the hospital's dedicated cardiac MR scanner. Today, CHB's Cardiovascular Program thrives, equipped with all proven cardiac diagnostic modalities and featuring the unrivaled capabilities of MR for evaluating many congenital heart defects.

Achieva 1.5T is integral to CardioVascular Program

Renowned worldwide for its longstanding expertise in congenital heart defects, Children's Hospital Boston cares for a prolific yearly volume of pediatric and adult patients with these problems. It was logical, then, that CHB would establish the nation's first dedicated pediatric cardiovascular MR program. When the construction of a new cardiovascular imaging center at CHB was announced, selection of the right cardiac MR system became paramount to the program's continuing success.

 

"Philips' clinical CMR applications put it at the forefront," says Andrew Powell, M.D., Director of Cardiovascular MR. "And, Philips clearly was committed to improving cardiovascular MR - particularly for congenital heart disease - and helping us with clinical science support and access to innovative pulse sequences."

 

"The Achieva 1.5T I/T Cardiovascular platform was attractive to us," adds Tal Geva, M.D., Director of Noninvasive Imaging. "But we were impressed with Philips' interest and willingness to work with us to further optimize the system, sequences and coils for imaging children."

 

The Achieva system became an important cornerstone in designing CHB's new imaging and diagnostics floor, which also houses four state-of-the-art cardiac catheterization laboratories, advanced echocardiography and electrophysiology labs and a 10-bed recovery room. All services are close to each other to facilitate collaboration between pediatric cardiology sub-disciplines and minimize the need to transport patients.

 

"Our cardiovascular program has evolved to where MR is regarded as a routine diagnostic test," Dr. Powell says. "Patients visiting their cardiologists often have their MRI studies the same day - in some cases instead of echocardiography. MR is very much integrated into our patients' routine care."

 Tal Geva, M.D. (center) and Andrew Powell, M.D. (right). Achieva 1.5T
Tal Geva, M.D. (center) and Andrew Powell, M.D. (right).
Achieva 1.5T

Cardiac challenges still exist

MR imaging of the human heart still presents challenges - the organ and its components are constantly moving at varying speeds, and important structures are often best visualized in oblique imaging planes. Scanning a child's heart is further complicated by its faster rate, smaller size and the need for a shorter study time to accommodate the young patient's limited ability to lie still.

 

"The anatomy and physiology in congenital heart disease also can vary tremendously," Dr. Geva observes. "The examination requires constant adjustments. Cardiac MR doesn't lend itself easily to set protocols."

 

Still, MR developers have made major progress in optimizing cardiac protocols to account for motion and small anatomy and to generate more quantitative and qualitative information about congenital heart defects. With their Achieva system, CHB cardiologists evaluate cardiovascular anatomy and quantify ventricular and myocardial function.

Most CMR patients in follow-up mode

Seventy-eight percent of CHB's CMR patients are at least 10-years-old, with 42 percent of this group 10- to 20-years-old. Most have had transcatheter or surgical procedures to treat their defects and are in long-term follow-up. "MR's diagnostic value increases in these groups, especially for those who have had heart surgery, which limits their echocardiographic window," Dr. Geva says. "Transducer-to-heart distance, scar tissue, air interfaces and chest wall deformities decrease echo image quality, and serial follow-up catheterizations are simply impractical."

 

"Common CMR indications are repaired tetralogy of Fallot, aortic coarctations or other abnormalities of the aorta, transposition of the great arteries following an atrial or arterial switch operation, and functional single ventricles following a Fontan operation," Dr. Powell adds. "A second patient category has MR scans instead of catheterization presurgically and a third group benefits from a unique MR capability, such as in myocardial assessment or cardiac iron quantification."

CMR enlisted for common birth defects

Over 35,000 infants annually are born with heart defects in the United States, making them among the most common birth defects and the leading cause of birth defectrelated deaths.1 However, advances over the last 40 years have dramatically increased survival rates, making long-term diagnostic follow-up particularly critical.

 

Children's Hospital Boston cardiologists see the aforementioned broad range of congenital heart defects. This article will focus on CMR's use for tetralogy of Fallot, Kawasaki disease, single ventricle heart disease and thalassemia syndromes.

Tetralogy of Fallot

In tetralogy of Fallot, clinicians encounter four anomalies at once: narrowing of the right ventricular outflow tract (RVOT), a ventricular septal defect (VSD), overriding aorta and right ventricular hypertrophy. This combination of anomalies is thought to result from a single developmental anomaly. Treatment consists of surgical closure of the VSD and enlargement of the RVOT, which eliminates cyanotic symptoms, but may render the pulmonary valve incompetent.

 

This leads to chronic pulmonary regurgitation and right ventricular dilation and dysfunction, ultimately increasing the chances of heart failure, arrhythmia and death. "Typically, complications increase in the patient's 30s and 40s," Dr. Geva notes. "Imaging goals are mostly functional and are aimed at assessing regurgitation, the right ventricle's size and function and the extent of scar tissue."

 

On the Achieva system, cardiologists employ a retrospective VectorCardiogram (VCG)- gated, breath hold Balanced-TFE cine imaging to evaluate anatomy and function. Cine phase contrast is used to measure flow and quantify valve regurgitation. Clinicians also study the pulmonary arteries and other extracardiac vasculature.

 

"In these patients, we do off-line quantitative assessment of the size and function of both ventricles and analyze pulmonary regurgitation and cardiac output," Dr. Geva explains. "We also look for residual shunts and quantify any significant tricuspid valve regurgitation. Additionally, we survey for known complications, such as residual RVOT obstruction or aneurysms."

Kawasaki disease

The most common acquired heart disease in developed countries, Kawasaki disease is a generalized vasculitis of unknown etiology affecting small- and medium-sized arteries and developing in children typically less than five-years-old. CHB is an international global referral center for Kawasaki disease. "Echocardiography works well for most Kawasaki disease patients but a small fraction develop giant coronary aneurysms, which are better imaged with MR. Aneurysms put patients at elevated risk for myocardial ischemia and infarction," Dr. Geva observes. "Recently, we are receiving more MR referrals to image the coronary arteries and assess myocardial function and health."

 

Imaging objectives are to look for coronary artery aneurysms and thromboses. "We also conduct myocardial imaging and dobutamine stress MR to assess for inducible ischemia," he says. "Philips, early on, devoted substantial resources to optimize coronary artery imaging and we're benefiting from that," Dr. Powell adds. "For instance, we're using the Philips coronary artery protocol that enables free breathing during scanning."

Thalassemia

Thalassemia is a hereditary anemia often severe enough to require life-long repeated blood transfusions. Unfortunately, because the body cannot adequately eliminate the iron load that accompanies the transfused blood, it accumulates in organs, including the liver and heart. Iron deposition can cause cellular damage and lead to cirrhosis, arrhythmia and death. Drugs called chelators are available to help remove iron from the body, but are associated with side effects, and thus their dosage must be carefully controlled.

 

"We need an accurate way to serially monitor a patient's iron status and risk for cardiac complications," Dr. Powell explains. "While biopsy's invasiveness precludes its routine use, MR enables non-invasive quantification of iron levels in the heart and liver by measuring tissue relaxation times."

 

The MR protocol, developed for Achieva by Philips clinical scientist, Marc Kouwenhoven, utilizes a VCG-gated gradient echo sequence that produces images of the heart at different TE's, enabling calculation of T2* relaxation times. The more iron within a tissue, the shorter the T2* relaxation time.

 

Once a patient's iron status is measured, their risk for organ dysfunction can be gauged, and their chelator regimen individualized. Because it is noninvasive, the T2* MR technique is well-suited to monitor the effectiveness of these modifications or the introduction of new chelators.2

Single ventricle heart disease

Increasingly, CMR is used to image infants born with single ventricle heart defects before one of their major corrective surgeries, a bidirectional superior cavopulmonary anastomosis. Single ventricle heart defects include a heterogeneous group of anomalies in which there is only one rather than two functional pumping heart chambers, causing dysfunctional mixing of pulmonary and systemic blood flows.

 

"Sequential surgeries ultimately separate pulmonary and systemic circulations," Dr. Geva says. "Preoperative catheterization is the current standard before a bidirectional superior cavo-pulmonary anastomosis."

 

CHB investigators recently completed a prospective, randomized clinical trial comparing MR versus catheterization for preoperative evaluation in this patient group3, and presented results at the 2006 American Heart Association meeting.

 

"In this study, we found that the outcomes using the non-invasive MR strategy are as good as catheterization, and complications and costs were reduced," Dr. Geva notes. "Clearly, MR offers an alternative to catheterization for many of these patients."

 

CHB cardiologists use cine, black blood, flow quantification and MRA sequences in these infants, often modifying the standard parameters to better suit the small size of their patients and their fast heart rates. "These methods quantify ventricular size, function and flow dynamics to determine a patient's suitability for the surgery and also help plan the operation," Dr. Powell says. "In addition, CMR may help clinicians detect problems not diagnosed by echocardiography, such as a coarctation, that may require immediate transcatheter intervention."

CMR continues mainstream integration at CHB

Children's Hospital Boston is a pioneer in dedicated CMR for congenital heart disease and pediatric cardiology, and continues to probe its potential for diagnosis and treatment, a trend that will be reflected nationally.

 

"MR will continue to be better integrated into routine clinical practice in pediatric cardiology," Dr. Geva predicts. "Further in the future are MR-guided cardiac interventions and MR-based molecular imaging. In the latter, for example, MR contrast agents could be bound to fibrin antibodies for the detection of blood clots and prevention of thromboembolic complications."

 

"CMR improvements will include faster image acquisition, and coils and hardware designed specifically for pediatric-sized patients," Dr. Powell adds. "Refinements in the user interface also will significantly enhance CMR workflow."

References

  1. American Heart Association.
    Congenital Heart Defects.
    Accessed 2/1/07.

  2. Anderson LJ, Holden S, Davis B et al.
    Cardiovascular T2-star (T2*) magnetic resonance for the early diagnosis of myocardial iron overload.
    Eur Heart J 2001 Dec;22(23):2171-9.

  3. Brown DW, Lang P, Colan SD, del Nido PJ, Odegard KC,  Gauvreau K, Powell AJ, Geva T.
    Randomized clinical trial comparing cardiac MRI with routine cardiac catheterization prior to superior cavopulmonary anastomosis.
    Circulation 2006;114:II-450.


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Best Practice
Achieva 1.5T
Release 1, Release 2
Nova, Nova Dual, Pulsar
Cardiac, Pediatric
 

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