The Role of CT & MRI in Congenital Heart Disease Treatment
Before Congenital Heart Disease treatment can begin in a child, those suspected with it undergo a number of tests to confirm the diagnosis. For example, an Echocardiogram can provide ultrasound imaging of the heart. During an echocardiogram study, the cardiologist studies the structure of the heart using two-dimensional (2D) imaging as well as three-dimensional (3D) imaging techniques. They also obtain functional information about the heart using additional techniques such as M-mode (motion mode), traditional Doppler techniques (Pulse-wave Doppler, continuous wave Doppler and color Doppler), tissue Doppler imaging, speckle tracking and blood stream imaging. These advanced techniques enable the cardiologist to understand the changes that happen in the chambers of the heart during systole and diastole and enable them to identify abnormalities in heart function before they become apparent on 2D imaging. 2D echocardiogram in children has excellent spatial and temporal resolution. However, limitations in current technology means that 3D echocardiography does not have the same spatial or temporal resolution. Echocardiogram (similar to all other medical applications of ultrasound) is also operator dependent and requires a high level of skill and training to identify complex Congenital Heart Diseases (CHD). While it is almost always possible to obtain excellent images of the heart structure in neonates, infants and toddlers by echocardiogram, it becomes increasingly difficult to obtain good images of the heart as well as the blood vessels connected to the heart in older children and adolescents. Cardiologists hence resort to other modes of imaging before subjecting the child to any type of Congenital Heart Disease treatment like a cardiac surgery or intervention. What is CT & MRI? For advanced imaging modality, doctors consider both a computerized tomography (CT) and magnetic resonance (MR) imaging well. Cardiac CT angiography is often the first advanced imaging modality which cardiologists request in children with CHD detected during echocardiogram. CT scans produce images of the body using x-rays. Unlike a traditional x-ray, the X-ray generator as well as the detector can move all around the body obtaining images of the body in multiple angles. These images are then reconstructed by a computer to produce the images. Modern versions of the CT, the so-called “128-slice” or “256-slice” CT can differentiate parts of the body which are less than a millimeter apart (excellent spatial resolution). These are also “ultra-fast” and the entire study is completed in a fraction of a second. Children older than 4 or 5 years can cooperate for a CT scan without requiring anesthesia. CT can also be obtained in neonates and young infants by a “feed and wrap” technique. Infants are put to sleep after a breast feed and wrapped tightly to keep them warm. As the entire study only takes a few seconds, this can be obtained without any sedation. The excellent spatial resolution of CT angiography enables congenital cardiologists and radiologists to reconstruct the anatomy of the heart as a 3D model and permits them to understand the relationship of the structures within the heart. A 3D model generated from the CT can be printed using a 3D printer. This allows the congenital cardiac surgeon to plan his surgery in real time and also perform sham surgeries in order to decide on the best strategy. This saves precious time during the surgical procedure and ensures a better outcome for the child. CT scans are widely available and are cheaper than MR imaging.
CT scan unfortunately images the heart during only one phase of the cardiac cycle. The imaging is performed at the end of systole or end of diastole depending on the heart rate of the child. Hence CT scan does not provide any temporal resolution at all. CT angiography hence provides excellent anatomical information but almost no functional information. CT also involves ionizing radiation which carries a small but definite risk for cancers (A disputed study from 2007 suggested that as much as 2% of all future cancers can be attributed to a CT scan). It is for this reason that cardiac CTs are requested cautiously by pediatric cardiologists.
Cardiac MR employs non-ionizing electromagnetic waves and strong magnets to obtain information about the tissue in the body. This information is then reconstructed into anatomical images. Unlike a CT where images are only obtained in the transverse (axial) plane, MR images can be obtained in transverse, longitudinal (coronal) and sagittal planes providing a truly 3D view of the heart (unlike a CT where the information from 1 dimension is reconstructed into a 3D image). MR allows for the heart to be studied in various phases and permits detection of the amount of blood flow through various blood vessels. This allows cardiologists to obtain information about the functioning of the heart which is not available through any other imaging modality. MR angiography images can also be converted into 3D models which can be printed similar to CT angiography. The use of non-ionizing radiation means that there is no additional risk of cancer due to repeated MR imaging. These advantages have made cardiac MR a popular imaging modality for pediatric and congenital cardiologists in the recent years. However cardiac MR carries some disadvantages. The entire study takes 45 to 60 minutes to be completed. This means that the child will have to remain inside the scanner for a prolonged period. Children often become claustrophobic and frightened. The MR imager is also very noisy, and this frightens children more. It is often challenging to obtain cardiac MR in children less than 8-9 years without some form of anesthesia. The spatial resolution of MR is also inferior to that of CT (MR can differentiate structures which are 1-2mm apart while CT can differentiate structures a fraction of a millimeter apart). Cardiac MR imaging is more expensive and is not as widely available as CT. It needs specialized training of radiology technicians, radiologists and cardiologists. It must now be apparent to the readers that non imaging modality is perfect to obtain all the anatomic and functional information about the heart. The Echocardiogram is sufficient to obtain information in most cases. Pediatric cardiologists use a judicious combination of these 3 modalities to obtain all the information required. In a small minority of cases, additional modalities such as conventional angiography and radio-isotope studies are also necessary.
