Three dimensional imaging using 64 detector row multi-slice CT should be used more widely for the diagnosis and management of congenital heart disease

Although 64 detector row multi-slice CT (MSCT) is the latest advance in multi-slice technology, it remains less frequently utilized for the diagnosis and management of congenital heart disease (CHD) compared to diagnostic cardiac catheterization (DCC) with biplane angiography, two dimensional echocardiography (2DE), and magnetic resonance imaging (MRI). When used, the interpreting physician is usually a radiologist who visualizes 2D slices in the axial plane and rarely in three dimensions (3D). Many pediatric cardiologists complain that the radiation exposure (RE) from MSCT is too high to ethically justify its use in children, but ignore the effective dose to which their patients are exposed during DCC and interventional CC (ICC) using biplane fluoroscopy and cineangiography. Gating image acquisition to the ECG signal is rarely done because it is thought to dramatically increase RE. And the literature is not very helpful. Methods to calculate RE are complex especially in infants and children, arcane, and very poorly explained. Radiation units of measure and their meaning vary from report to report, and explanatory language is abstruse and often incomprehensible to those who would order such tests. The natural history of many complex forms of congenital heart disease is incompletely understood in individuals who might benefit from a detailed anatomic 3D analysis especially for those lesions that are obligatorily palliated and thus have a different natural history and risk of developing radiation sequelae. Amid this confusion, the default imaging methods for CHD are 2DE, DCC, and MRI, because 2DE and MRI entail no radiation, and DCC has been used for more than half a century with RE that is universally accepted as safe. Appropriately, 2DE is the primary imaging modality for congenital heart disease (CHD) at all ages. Pediatric cardiologists stress anatomy while their adult counterparts emphasize physiology. When physiologic measurements are sought in infants and children, they are often acquired with the patient crying or struggling to escape, thus obviating their value. This is especially true with sub-costal imaging which most unsedated infants and children resist. A common 2DE acquisition format consists of sweeps, moving the transducer from location x to location y during image acquisition in an attempt to show the relationship of structure a to structure b. After the fact, these phrenetic images are incomprehensible except to the person who acquired them. Unfortunately, infants and children are rarely routinely sedated, and when they are, a variety of drugs is used in dosages that often are too small to allow a comprehensive examination. And a full understanding of how to sequentially acquire a set of 2D black white, color flow, and Doppler images that represent a 3D whole is rarely taught or understood. These self-imposed pediatric 2DE limitations cry out for improvements in techniques that allow better measurement of pressures, flows and resistances. Ironically adult echocardiographers have been using 2DE for this purpose for years, but in pediatrics this information is still sought primarily through DCC. Everyone accepts that patients need anesthesia for DCC and ICC in order to eliminate pain and prevent movement. Many forget that the primary need is to obtain a representative physiologic steady state during which pressures and oxygen saturations can be accurately obtained and flows and resistances calculated. The corresponding need for infants and children to be sedated during 2DE seems intuitive, but is rarely effectively practiced. And ignored are the depressant effects of general anesthesia on cardiac function (Filner and Karliner, 1976), the most common sedation modality for both DCC and ICC. Cardiac output drops, and the data often bear little resemblance to the awake steady state. Witness the child with an aortic systolic pressure of 80 mmHg under general anesthesia whose pressure when awake or under light sedation is 110! Nevertheless, the numbers obtained are accepted as accurate making the term “DCC” highly problematic. Unfortunately today pediatric and adult cardiac trainees are more likely to be exposed to and taught how to do interventional procedures rather than to understand and record accurate steady state physiology. And it is now rare to calculate LV or RV volumes, ejection fractions, and flows angiographically. These values are simply “eyeballed” from whatever imaging format is used. Even though DCC is well established as the standard for imaging CHD, its radiographic application varies from center to center, the technology is old, anatomy is poorly or incompletely visualized, and it requires use of large volumes of contrast agents with their attendant risk of allergic reactions and renal functional impairment. Each angiogram requires a separate bolus. And angiography “sees” only that pathology within the one or two 2D orthogonal planes visualized and only to the degree there is adequate contrast. As the number of 2DE increases, the number of DCC is falling, but the need to see anatomy accurately and record physiology faithfully remains unchanged. To meet this need new imaging modalities are emerging including, MSCT, 3D echo (3DE), and MRI. Cardiologists commonly hold that MRI represents the gold standard for ventricular volumetric analysis and that these calculations are automatically rendered by the application of MRI cardiac software. For example, operated patients with tetralogy of Fallot with chronic moderate or severe pulmonary regurgitation by 2DE often undergo MRI in order to obtain RV end-diastolic volume as an index to help determine when to replace the pulmonary valve (Lindsey et al., 2010). And while these data have long been available using standard biplane angiography or more recently by 3DE (Hubka et al., 2002), few laboratories retain the expertise or have the necessary equipment. What is not so well known is that, like 2DE and angiography, ventricular shapes by MRI must be accurately drawn by hand for true volumes to be calculated. Failure to understand the differences between LV and RV morphology with different CHD and physiologic conditions or failure to accurately identify end-systole and end-diastole will produce significant errors. Few physicians realize that volumes and outputs can be calculated for both ventricles and atria using MSCT. Although MRI offers the potential for 3D imaging with no radiation risk, albeit with a lower spatial resolution compared to MSCT (Friedrich, 2010), few radiologists bother to reconstruct MRI in 3D. MRI requires longer anesthetic times averaging 30 min to an hour or longer and is not as easily scheduled, and fewer centers have access to expensive modern MRI cardiac software. Compared to MSCT, MRI is no less subject to errors and is simply not as practicable.