Chapter 6. Computed Tomography of the Adult Cardiac Surgery Patient: Principles and Applications

Over the last decade, advances in computed tomography (CT) technology have revolutionized the diagnosis of cardiovascular disease. As illustrated in this chapter, CT has dramatically reduced, and for some clinical indications eliminated, the need for diagnostic arterial catheterization. In the process, CT has become invaluable in cardiac diagnosis and surgical planning.

CT is based on a source of x-rays and a system to detect those x-rays that pass though the patient. The x-ray source and detector are mounted on the “CT gantry” that rotates around the patient. As detailed below, two major technology advances have enabled CT to image the beating heart. The first is the speed at which the CT gantry rotates. The second is the incorporation of an increasing number of detectors that can resolve small anatomic detail. One of the great promises of modern CT is the ability to noninvasively exclude coronary artery disease in as little as one heartbeat (Fig. 6-1). However, the role of CT extends far beyond the coronary arteries alone. Using the same CT acquisition, native coronary imaging can be extended to coronary bypass grafts, the beating myocardium, valve motion, ventricular outflow tracks, and cardiac lesions.

In order to understand the clinical contribution of CT and to avoid pitfalls in image interpretation, it is essential for the surgeon to appreciate the basic principles of CT used in cardiac imaging. This chapter is divided into two sections. The first section describes the technical considerations for cardiac CT. By understanding each component, the surgeon will be better able to distinguish image artifacts from pathology. The second section reviews those CT examinations that are most frequently performed in the noninvasive cardiovascular imaging program at Brigham and Women's Hospital (BWH), detailing the strengths and limitations of each exam.

Cardiac CT Protocols

Introduction

Most advances in cardiac CT, for example in coronary CT angiography (CTA), have focused on the development of protocols consistent with the rapid incremental improvements in the technology. One of the major technological advances has been the incorporation of multiple elements into the CT detector system, called Multi-Detector CT (MDCT). MDCT is synonymous with multi-slice CT. Data from each of the detectors is used to reconstruct an axial slice perpendicular to the long axis, or z-axis, of the patient. The width of the detectors determines the slice thickness and thus the ability to resolve small anatomic detail (spatial resolution) of the scanner. Thinner slices yield superior spatial resolution; however, comparing two scanners that produce the same number of slices, the scanner with thinner slices will have less z-axis (ie, craniocaudal) coverage per gantry rotation and thus will have a longer scan time. To date, the minimum detector width and largest number of detector rows is 0.5 mm and 320, respectively.1 This yields 16 cm (0.5 mm - 320) z-axis coverage per gantry rotation, and thus the entire heart can be imaged with data acquired over a single R-R interval.

Temporal Resolution

Successful cardiac imaging by any modality relies on the ability of the hardware to produce motion-free images or, in other words, to image faster than the heart beats. Because it requires that the gantry be rotated around the patient, CT is inherently slower than digital subtraction angiography (DSA) where each frame corresponds to a single projection image. As described below, CT has recently become much faster and thus cardiac CT can now be routinely performed.

Temporal resolution is the metric that measures imaging speed. For a CT scanner with a single photon source, the temporal resolution is one half of the CT gantry rotation time. This is because image reconstruction requires CT data acquired from one half (180 degrees) of a complete gantry rotation. All manufacturers have gantry rotation times less than or equal to 350 milliseconds. Using this gantry rotation time as an example, an ECG-gated cardiac image can be reconstructed (using single-segment reconstruction, described below) with CT data acquired over 175 milliseconds of the cardiac cycle. Thus, the reconstructed images inherently display the average of the cardiac motion over the 175 milliseconds during which the data was acquired. This is how ECG gating enables cardiac CT. Without gating, cardiac images are nondiagnostic because the reconstruction “averages” the motion over the entire R-R interval, for example over 1000 milliseconds for a patient with a heart rate of 60 beats per minute.

There are additional strategies to improve temporal resolution. The first uses two independent x-ray CT sources and two independent (64-slice) detector systems built into the CT gantry. The second x-ray source is positioned 90 degrees from the first x-ray source, and the second detection system is positioned 90 degrees from the first detection system. With respect to temporal resolution, the practical consequence of this CT configuration is that 180 degrees of gantry rotation can be achieved in half the time (eg, 82.5 ms as opposed to 165 ms). This halves the temporal resolution (to 82.5 ms), and thus for this “dual-source” CT configuration, motion is averaged over only 82.5 ms. Another strategy uses both x-ray CT source-detector systems to sample the entire heart within a single R-R interval by rapidly moving the patient through the scanner.2 Both implementations of such systems have technical advantages and disadvantages that are beyond the scope of this chapter.

For single-source scanners, temporal resolution can be improved by adopting a so-called “multisegment” image reconstruction. The difference between single-segment and multisegment reconstruction is that in the former, 180 degrees of data are acquired from a single heartbeat, whereas multisegment reconstruction uses several heartbeats to obtain the one half gantry CT data. For example, in a two-segment reconstruction, two heartbeats are used to generate a single axial slice, and thus the temporal resolution is halved. Similarly, if four heat beats are used (four segment reconstruction), only 45 degrees of data are used from each heartbeat. This yields a fourfold reduction in the effective temporal resolution, making it theoretically possible to perform high spatial resolution cardiac CT in patients with a rapid (eg, >70 beats per minute) heart rate. However, because multiple heartbeats are used to fill the 180 degrees of gantry rotation necessary for the reconstruction, stable periodicity of the heart is essential. When beat-to-beat variations in heart rate occur, image quality is degraded significantly. In our experience, multisegment reconstruction works well in patients with high heart rates who are being studied for clinical indications where the highest image quality may not be required (studies of graft patency, pericardial calcification, etc.). For more demanding applications (eg, native coronary CT angiography) we still routinely employ beta-blockade for heart rates greater than 60 beats per minute.

Beta-Blockade for Heart Rate Control

As suggested above, beta blockade is an important component of most cardiac CT examinations. As the temporal resolution of cardiac CT improves, the dependence on lowering the heart rate will naturally be mitigated. However, the speed of all CT scanners is inferior to coronary catheterization, and thus beta-blockade is recommended for the large majority of patients in whom it is safe. In our experience, many surgical patients have standing orders for beta-blockers as part of their medical therapy, and image quality is excellent. When this is not the case, either oral and/or IV metoprolol is routinely administered.

ECG Gating

ECG gating refers to the simultaneous acquisition of both the patient's electrocardiogram (ECG) tracing and CT data. By acquiring both pieces of information, CT images can be reconstructed using only a short temporal segment of the R-R interval. Each segment is named by its “phase” in the cardiac cycle; the common nomenclature is to name the percentage of a specific phase with respect to its position in the R-R interval. For example, reconstruction of 20 (equally spaced) phases would be named as 0%, 5%, 10%…95%. The period in which the heart has the least motion is usually (but not always) in mid diastole, near 75%. Thus, the CT exposure (and subsequent patient radiation level) can be lowered by limiting the exposure to a small part of the R-R interval where coronary motion is expected to be a minimum. This is termed “prospective” ECG gating because the reconstruction phase and width is determined prospectively. The disadvantage of this approach is that cine loops of the entire R-R interval cannot be reconstruction because there is no complete data set throughout the R-R interval. If this is desired, so-called “retrospective” ECG gating can be used, at the expense of high radiation levels.

For patients who require imaging of bypass grafts, it is important to note that periodic displacement of both saphenous vein grafts (SVG), radial grafts, and internal mammary artery (IMA) grafts is far less than the motion of the native coronary arteries. Thus, for these vessels, a single reconstruction at mid-diastole is usually sufficient (Fig. 6-2), and prospective ECG gating is routinely used. However, the benefits of lower radiation in this population are relatively moot, because the latent period for a radiation-induced malignancy is roughly a decade for blood tumors and significantly longer for solid tumors. Patients under consideration for repeat bypass surgery typically have a shorter life expectancy based on cardiac status. Thus, it is essential that the surgeon not only recognizes that motion has degraded image quality, but also realizes that additional reconstructions, and even repeat imaging, can and should be performed. If the entire course of the graft is not clear to the surgeon at a single cardiac phase, it is almost always the case that another phase will yield motion-free depiction of the graft segment that was poorly seen. At BWH, open communication between the radiologist and surgeon for every case has eliminated this pitfall and ensures that the maximum amount of imaging data is incorporated into presurgical planning.

In cine CT, such as imaging the aortic valve over the entire R-R interval, images are acquired with retrospective ECG gating and subsequently reconstructed throughout the cardiac cycle and then played, in cine mode, to demonstrate function. Each individual image (Fig. 6-3) offers an outstanding assessment of the aortic valve and root structure. Cine CT can also be used to assess ventricular-wall motion. In comparison with magnetic resonance imaging (MRI), the gold standard for global- and regional-wall motion abnormalities, CT often has inferior temporal resolution. However, it is important to emphasize that cine CT does not require a separate image acquisition. The entire CT data set (coronary, valve, myocardium, pericardium) is acquired in a single breath hold; cine CT is simply part of the image postprocessing.

For surgical patients, CT has the distinct advantage over MRI in that it is by far the best imaging modality to identify and quantify calcification. Also, the most common contraindications for cardiac CT (eg, impaired renal function as measured by glomerular filtration rate or alternatively by serum creatinine) differ from those for MRI (pacemaker), and thus CT can often be used for patients who cannot have MRI. Single heartbeat cardiac CT is now a clinical reality, with an entire cardiac acquisition in approximately one second.3 In addition to the fact that patient radiation is decreased, multiple scans can be performed with the same injection of iodinated contrast material, creating the opportunity for a host of additional studies (eg, myocardial perfusion)4,5 that are, at present, largely in the domain of cardiac MRI and nuclear cardiology.

Patient Irradiation

Because ECG gating is required, ascending aorta and cardiac CT delivers more patient irradiation than CT of any other body part. Although details regarding cardiac CT dosimetry are beyond the scope of this chapter, discussions regarding CT dose must be based on sound principles. The radiation risk most commonly quoted relates to the probability that the CT scan will result in the development of a fatal radiation-induced neoplasm. Human data for radiation at this low level (the level delivered in ECG gated cardiac CT) is very sparse; all anecdotal reports support a long latency period as described above. For this reason, patients should be separated into two groups: those with a life expectancy of roughly 10 to 15 years or less, and those with a longer life expectancy. In the former group, the only dose consideration is whether the radiation could cause a skin burn (the only short-term complication of any consequence). X-ray skin burns are extremely uncommon, particularly in CT (even for ECG-gated studies), and typically result from multiple exams repeated at short-term intervals. Thus, for this subset of patients, radiation dose should not be a consideration in determining a modality for coronary imaging. For those patients for whom radiation is an important consideration, prospective ECG gating should be used. For cine evaluations in these younger patients, x-ray current modulation is a strategy to lower the radiation dose. The tube current (expressed as the mA) is modulated over the course of the cardiac cycle so that the desired (high) diagnostic current is delivered only in diastole. The patient dose is decreased because the tube current is reduced for the remainder of the cardiac cycle. Although current modulation is helpful in many cases (eg, pediatric patients), the decision to use it should be made after consultation between surgeon and radiologist because the potential drawbacks are significant. Most importantly, when current modulation is used, images reconstructed during phases with low tube current are relatively noisy because less tube current is used to generate them.

Scanning Parameters

The scan time refers to the time required to complete the CT acquisition along the z-axis of the patient. As described above, better temporal resolution decreases the scan time, not only decreasing cardiac motion, but also enabling breath hold CT. This is important in cardiac CT because in comparison with nongated CT, ECG gating not only increases patient radiation but also increases the scan time.

In practical terms, a 64-slice ECG-gated cardiac CT scan (craniocaudal, or z-axis imaging over ~15 cm) can be performed in roughly 10 seconds, versus 20 to 25 seconds with a 16-slice scanner. One great benefit of wide area detector CT is faster (single heartbeat) scans. If this option is not available, increasing the thickness of the detectors increases the z-axis coverage per rotation and thus decreases the scan time. For example, in a patient who cannot perform the breath hold, using thicker detector widths (eg, 1mm thickness as opposed to 0.5mm thickness) will decrease the scan time by providing more z-axis coverage per rotation. However, routinely increasing the width of the detectors for cardiac applications is undesirable because it degrades the spatial resolution of the examination. In general, spatial resolution refers to the ability to differentiate small detail in an image. This is an essential component to coronary imaging because the diameter of the proximal coronary arteries are on the order or 3 to 4 mm. Substitution of 0.5mm reconstructed images with 1mm images thus impacts the ability to see detail that may be required for accurate diagnoses. Routine consultation between the surgeon and radiologist is essential to best understand and optimize the tradeoff between scan time and slice thickness. For example, imaging of the myocardium and ascending aorta almost never requires submillimeter slices, because the pathology is larger. Thus, for dyspneic patients who require only imaging of the ascending aorta, thicker slices are used to cut the scan time.

The scanning parameters that primarily determine the number of photons used to create a CT image are termed “mAs,” or milliamperes-seconds and “kV,” or kilovolts. The former represents the x-ray tube current; the latter refers to the voltage applied within the tube. For the surgeon, choosing the best numbers (typical values are 550 to 700 effective mAs, 120 kV) is far less important than understanding the fact that modern cardiac CT pushes the limits of technology, and thus creates tradeoffs with respect to the x-ray CT source. The source generates photons that are either attenuated by the patient or reach the detectors. When more photons reach the detectors, the image quality is higher because there is less noise. The decision to image with thinner slices (eg, 0.5mm as opposed to 1mm) means that fewer photons reach the detector; thus, thinner slices have more noise. This is especially important in obese patients because their increased body mass absorbs more photons than thin patients. For the same effective mAs and kV, images of obese patients can be dramatically degraded by greater image noise.

If there were no limit to the number of photons that an x-ray CT source could produce, the solution would be to simply increase the number of photons (and the radiation dose) until image noise was satisfactory. Unfortunately, because the x-ray CT tube heats excessively when pushed to its maximum, there is a limit to the number of photons that can be produced. This is why image noise becomes problematic with thin slice imaging of obese patients. When this is the case, consultation between surgeon and radiologist is important because diagnostic images can often be obtained by increasing the image thickness, scanning a smaller z-axis field of view (FOV), or both. The latter can be particularly useful if the exam can be tailored to the most important structure. Scanning a smaller z-axis means that more photons can be generated and used before the x-ray CT tube reaches its heat limit.

On the other hand, whenever possible, the z-axis FOV should be generous, as unexpected pathology can extend both cranially and caudally. For example, an ECG-gated cardiac and ascending aorta exam to evaluate extension of the intimal flap into the coronary arteries can reveal extension into the great vessels. Also, scanning must allow for variations in the FOV induced by breath holding. As a general rule, for scanning the native coronaries alone, the superior border of the FOV is set at the axial slice corresponding to the top of the carina. This is typically 2 to 3 cm superior to the origin of the left main. The inferior border should scan through the entire inferior wall of the heart and should include several slices of the liver to account for cardiac displacement during breath holding. For bypass graft imaging, the superior border of the field of view must include the subclavian arteries and the origin of both IMAs.

Contrast Material

With the exception of scans performed solely for the assessment of cardiac and aortic calcification, CT examinations are performed with iodinated contrast material. Effective communication between surgeon and radiologist is important in optimizing the use of contrast material, particularly in the decision of whether to use a single or a dual injection system. A single system injects only contrast, whereas a dual system has two reservoirs to inject contrast followed by saline. Dual injection is essential for many applications, and it is routine in coronary imaging. The contrast and saline delivery are timed so that the left heart, aorta, and coronary arteries are enhanced with contrast while the right heart is filled with saline. The use and the timing of the saline are essential parts of the exam because artifacts that limit interpretation of the right coronary artery (RCA) will be induced if the right heart and central veins are densely enhanced with contrast (as opposed to saline). However, for patients in whom imaging requires enhancement of both the left and right heart (eg, assessment of both the mitral and tricuspid valves), saline cannot be used and less dense contrast material may be chosen to lessen potential artifacts.

Summary

Advances in technology such as submillimeter spatial resolution, up to 320 detector rows, and rapid gantry rotation times with dual x-ray CT source and detector systems have enabled ECG-gated CT to make a positive contribution to the care of cardiac surgery patients. The surgeon must recognize that cardiac protocols push the limit of technology, and thus CT of the heart is more complicated than a scan of any other body part. Consequently, routine and effective communication between the surgeon and the radiologist will result in the best possible patient outcomes.

Applications in Cardiac Surgical Patients

Native Coronary CTA

One of the most common clinical indications for cardiac CT is to evaluate the native coronary arteries for stenosis (Figs. 6-4, 6-5, 6-6, and 6-7). Numerous validation studies have evaluated cardiac CT for this purpose. 6–19 In these studies, data are typically reported on a per coronary artery segment basis, comparing CTA and DSA. A significant stenosis is generally defined as greater than 50%, determined by quantitative coronary angiography. Data are also analyzed on a per patient basis regarding the value of CTA in ruling-in or excluding CAD. Literature to date reports on patient populations with a relatively high prevalence of CAD (ie, patients already scheduled for DSA.

1. Nonevaluable coronary segments were included as being positive for a significant coronary stenosis.

2. Sensitivity and specificity were not calculated separately for 16-slice and 64-slice scanner.

3. Percentage of nonevaluable “patients”

4. Sensitivity and specificity were calculated on “vessel”-based analysis.

5. CT images were interpreted independently by two radiologists with unequal experience in reading coronary CTA.

Table 6-1 summarizes published data. Among the most consistent findings is a very high negative predictive value (NPV) of coronary CTA when performed with 64- or greater detector row scanners. The data and our experience suggest a very high NPV, effectively arguing that cardiac CTA can effectively exclude CAD in patients with low to intermediate pre-test probability of disease. Consequently, CTA has become increasingly useful for the cardiac surgeon in managing patients scheduled for noncoronary cardiac surgery. If the clinical suspicion is low, but not insignificant, CTA affords the surgeon a method of assessing coronary disease without subjecting the patient to femoral arterial puncture with its known complications. For example, patients undergoing isolated mitral valve surgery for degenerative, myxomatous disease have a low prevalence of CAD, making CTA an ideal alternative to conventional angiography to exclude significant CAD. When the CT protocol described in Section A is followed, high-quality imaging is routine, and when CTA excludes CAD, surgeons have increased confidence in using CTA alone. Follow-up with DSA can be reserved for those patients who might benefit from catheter-based intervention.

Coronary Artery Bypass Graft CTA

Cardiac CT provides the cardiac surgeon with a non-invasive method to assess graft patency after CABG. Studies using early 16-detector row CT scanners suggest 100% sensitivity and specificity for identifying occluded versus patent grafts20–22 with benefits from advanced image postprocessing tools.23 In clinical practice, this application is of value in the evaluation of the symptomatic patient in the early postoperative period in whom graft failure is being considered (Fig. 6-8). It is particularly useful for the demonstration of patent grafts in patients with a remote surgical history and unknown graft anatomy prior to DSA, and in patients in whom conventional angiography fails to demonstrate a known graft (Fig. 6-9). In the reoperative setting, such data has virtually revolutionized surgical decision-making and planning prior to surgery in patients who have already undergone CABG.

Patients with recurrent angina after bypass surgery may have developed stenosis or occlusion in bypass grafts, or may have progression of native coronary disease. In these patients, CTA can be more limited. For example, exclusion of significant stenosis in a graft may be problematic because of metallic surgical clips that cause artifact (Fig. 6-10). Moreover, native CAD in these patients is often advanced and heavily calcified. A large volume of calcium may result in an uninterpretable study for many segments of the native coronaries (Fig. 6-11). CTA for plaque assessment in such patients may not be definitive; consequently conventional angiography may be preferred in this population. 24 Obviously, the less calcification and metal artifact present, the more useful CTA is.

In the research context, graft patency is an important outcome in the evaluation of different surgical techniques. Randomized controlled trials utilizing conventional angiography for assessment of graft patency typically demonstrate a 10 to 20% rate of noncompliance. This noncompliance is at least partly attributable to the invasive nature of the test. 25–27 Cardiac CT is an attractive, noninvasive, and very accurate method to assess graft patency for clinical trials, again obviating a traditional arterial puncture with its risks and known complications. Cardiac CT may also be used for routine postoperative control of grafts following implementation of a new surgical technique in a local center practice (Fig. 6-12).

The Aorta: The Root, the Ascending Aorta, the Arch, and the Descending Aorta

Surgery of the aortic root, the ascending aorta, the arch, and the descending aorta is becoming increasingly commonplace as the population ages. CT has been used for many years to assess the thoracic aorta. Non–ECG-gated CT is highly accurate in the assessment of the aortic arch and descending thoracic aorta because they are not subject to significant cardiac motion. However, ECG-gated cardiac CT adds motion-free imaging of the aortic root and ascending aorta. Without question, any aorta surgery involving the aorta (whether the root, the arch, the ascending, or the descending portion) requires preoperative CTA for surgical decision-making. The superior imaging of ECG-gated CTA better defines the pathology and hence facilitates the preoperative planning. For example, CTA is by far the best imaging modality to clearly define aortic calcification.

If portions of the aorta are calcified on CTA, then aortic cross clamping and cannulation for cardiopulmonary bypass at those sites are contraindicated in order to avoid embolic phenomenon and stroke. Thus, preoperative cardiopulmonary bypass strategy and myocardial protection are often critically altered by preoperative CTA.

As with calcification, CT is the most accurate modality to evaluate the aortic root, with both 2D and 3D visualization (Fig. 6-13). Not only can the aortic root be sized from multiple imaging planes, but also the exact location of the aneurismal pathology with respect to the valve and sinotubular junction can often be defined.28 This assessment is critical for preoperative decision making and surgical planning. In patients with ascending aortic aneurysm, if the aortic root is determined to be aneurysmal near the coronary ostia, surgical decision making changes from a simple tube graft repair for the aneurysm to a much more complex composite root repair with coronary reimplantation. In addition, motion-free images obtained with ECG gating allow for definitive exclusion of type A dissection (Fig. 6-14). Three-dimensional volume rendering optimally depicts other aortic root pathologies such as coronary anomalies or a sinus of valsalva aneurysm (Fig. 6-15). In patients with a sinus of valsalva aneurysm, as opposed to a root aneurysm, CT alters surgical strategy for repair.

For known ascending aortic aneurysms that do not meet size criteria for surgery, CTA is excellent to periodically assess size or change. For those patients who require surgery, CTA with 3D volume-rendered images provides the surgeon with preoperative visualization of aneurysm size and extent. The extension of an ascending aortic aneurysm into the arch can be demonstrated and, as mentioned above, the expected location of aortic cross-clamping can be determined preoperatively (Fig. 6-16). The location of normal aorta distally and the extent of arch involvement will preoperatively determine the arterial cardiopulmonary bypass cannulation site as well as the need for concomitant arch repair, circulatory arrest, or selective antegrade perfusion. Selective antegrade cerebral perfusion itself is dependent on intact right axillary and innominate arteries, and CTA is optimal for defining this anatomy. Because the success of a procedure can be compromised by unexpected intraoperative findings, CT has contributed enormously to surgical planning by defining anatomy that would not be preoperatively visualized with any other imaging modality.

In patients with aortic dissection, CTA allows the surgeon to understand the extent of the intimal flap. In particular, ECG-gated CT offers information regarding dissection of the ascending aorta (the proximal extent of the dissection flap and its relationship to the coronary arteries and the aortic valve) that was not available before gating was routinely performed. The location of the true and false lumen is critical in the preoperative planning, the operative sequence, and the extent of repair. For example, in dissection of the descending aorta, end-organ perfusion is assessed by demonstrating contrast enhancement of individual organs and related compromise of the celiac, superior mesenteric, inferior mesenteric or renal arteries. As is the case for a nonsurgical aneurysm, if a descending thoracic aortic dissection is stable and to be followed expectantly, CTA remains the gold standard for periodic assessment.

Cardiac Masses

Cardiac MRI is the modality most commonly used for high spatial resolution cross-sectional imaging to evaluate cardiac and pericardial masses. However, CT may be preferred if the mass is known to extend into the mediastinum, chest wall, or lung, or if the patient has a contraindication to MRI. CT also evaluates extracardiac thoracic structures with high spatial resolution; thus, it can define the full extent of disease (Fig. 6-17). CT is also useful as a single follow-up examination in patients with metastases to the heart and lungs because it avoids the need for periodic assessment with both conventional chest CT and cardiac MRI. Fat-containing lesions are very amenable to evaluation by CT, because lesions have a characteristic appearance, appearing black relative to water (Fig. 6-18).

Imaging of the Pericardium

In patients with clinically suspected constrictive pericarditis, cardiac MRI or CT can be used to confirm and measure pericardial thickening. In comparison with MRI, CT far better demonstrates the presence and extent of calcification. This may be of value in confirming chronic calcific pericardial thickening, supporting the diagnosis of constrictive pericarditis (Fig. 6-19). In these cases, 3D volume rendering to illustrate regional localization of pericardial abnormality functions as an outstanding preoperative planning tool prior to pericardial stripping (Fig. 6-20).

Valve Imaging

In patients with suspected valve dysfunction based on echocardiography, cine CT from retrospective ECG gating as described in Section A provides valuable additional data. As noted earlier, reconstruction is performed for all phases of the cardiac cycle, and the reformatted data can be played in a cine loop. This has relevance in assessment in native, bioprosthetic, and mechanical aortic valves. Recent experience suggests an excellent correlation between planimetric valves areas obtained by CT, MRI, and TEE (transesophageal echocardiography).29 Consequently, CT can be used as an alternate modality for evaluation of aortic valve area. This is relevant if transthoracic echocardiography is of poor technical quality or discrepant with a clinically expected result. Because patients may be evaluated with CT for concomitant aortic aneurysm or coronary artery disease prior to aortic valve surgery, valve area may be obtained from CT with no additional scanning and only a small amount of image postprocessing. CT can be used in correlation with valve area as determined with echocardiography (Fig. 6-21).

Postoperative evaluation of bioprosthetic valves may also be conducted using CT scanning. Although echocardiographically determined transvalvular gradients are the reference standard for determination of “effective” orifice area, CT provides a useful correlative modality—particularly when the echo is technically challenging or discordant with clinical findings. In patients with unexpectedly high gradients post valve implantation, increasing use of CT will provide more information to guide management (Fig. 6-22). In patients with suspected bioprosthetic valve endocarditis, CT can be an invaluable modality to delineate paravalvular and valvular sequela of the infection (Fig. 6-23).

Cardiac CT also permits high resolution functional evaluation of mechanical aortic valve prostheses without artifact (Fig. 6-24). CT can be readily incorporated to evaluate valve dysfunction, measure opening angles, and to elucidate the underlying cause of valve failure. Fig. 6-25 illustrates correlation between CT and surgical specimen in a mechanical aortic valve patient with restricted opening angle and elevated gradients. CT readily made the preoperative diagnosis of pannus ingrowth.

Preoperative Planning and Surgical Guidance

Reoperative Surgery

Reoperative cardiac surgery with live coronary grafts after previous CABG represents one of the most difficult problems in cardiac surgery. Reoperative sternotomy is challenging secondary to adhesions, loss of tissue planes, and the potential for injury to patent grafts, the aorta, and the right ventricle. Injury to a patent left internal thoracic artery graft to the LAD is associated with a mortality of 50%.30,31 Cardiac CT has been revolutionary in precisely defining the relationship of important structures (e.g, the aorta, right ventricle, or live grafts) to the midline and sternum for reentry planning (Fig. 6-26). At BWH, every reoperative surgery includes a preoperative CTA with z-axis coverage to include all grafts and the entire course of the IMAs. Preoperative identification of all structures at risk is mandatory, and different, specific operative approaches always remain in consideration.32 Experience suggests that preoperative cardiac CT will lead to a modification in surgical strategy for 1 in 5 patients undergoing redo cardiac surgery.23 For example, if CT demonstrates a patent left internal mammary artery (LIMA) is close to the midline or a right ventricle directly adherent to the posterior table of the sternum, cardiopulmonary bypass is instituted prior to reentry. Definition of live grafts with respect to their proximal placement of the aorta is instrumental in determining, before the operation, the precise manner in which those grafts will be handled. For example, in reoperative surgery for aortic valve replacement (AVR) in the setting of live grafts, CTA allows the surgeon to plan preoperatively whether or not those grafts will have to be divided in carrying out the aortotomy for the AVR. As described above, CT also allows the cardiac surgeon to preoperatively plan the precise location of the aortotomy itself.

Minimally Invasive Surgery Coronary Artery Bypass Grafting

Minimally invasive coronary artery bypass surgery is becoming an alternative to open surgery. With limited intraoperative access for direct visualization, aspects of coronary artery anatomy such as vessel diameter, extent of calcification, and the presence of intramyocardial segments become even more important to define preoperatively (Fig. 6-27). In addition, 3D models that combine visualization of a partially transparent thoracic cage over mediastinal structures allow the surgeon to obtain detailed preoperative understanding of the patient's cardiothoracic anatomy (Fig. 6-28). Preoperative CT has demonstrated usefulness for MIDCAB33 and totally endoscopic coronary artery bypass surgery.34 CT is also expected to become invaluable for procedures such as multivessel small thoracotomy coronary revascularization.

CT versus MRI of the Heart

Throughout this chapter, comparisons between CT and MRI have illustrated the strengths and limitations of each. Both modalities give the surgeon valuable pre- and postoperative information. Although the modality best suited for specific clinical indications evolves with the technology, at present certain generalizations can be made. Cardiac CT is invaluable in reoperative cardiac surgery because it offers higher quality noninvasive angiography of native coronary arteries and bypass grafts. Because 3D volume rendering with CT has higher quality and better spatial resolution, it is preferred for preoperative planning for reoperative CABG or minimally invasive cardiac surgery. All calcification is poorly seen on MRI and superbly seen with CT, and thus CT is far superior in demonstrating coronary, myocardial, pericardial, and valvular calcification. The same is true for mechanical valve prostheses; because of large areas of artifact on MR images, functional evaluation is only possible with CT. Finally, all patients with a surgical problem of the aorta require evaluation with CT.

The strengths of MRI include high temporal resolution, greater blood-myocardial image contrast, multiparametric functional evaluation, and techniques for myocardial tissue characterization. In addition, cardiac MRI is invaluable in assessing myocardial function, contractility, and actual tissue perfusion and viability. For these reasons, MRI remains the gold standard to assess biventricular volumes, function, and myocardial mass. A variety of MRI pulse sequences can accurately delineate areas of chronic myocardial infarction, identify certain specific cardiomyopathies, and confirm the presence of neoplasm. With the use of parallel imaging techniques to increase the speed of the MRI acquisition, very high temporal resolution (20 to 30 milliseconds) can be obtained to study bioprosthetic and native valve function. In addition, measurements of flow parameters through a vessel cross-section can be used to, for example, accurately quantify valvular regurgitant lesions.

One distinct advantage of MRI over CT is that MRI delivers no patient radiation. As detailed in Section A, the radiation exposure in ECG-gated CT is higher than that for CT of any other body part. In younger patients, who typically have less comorbid disease and may require multiple follow-up examinations over years or decades, MRI should be used when possible.

Summary

Dramatic progress in CT technology has enabled advanced cardiac imaging for cardiac surgery patients. As the newest cardiac imaging modality to enter clinical practice, cardiac CT has rapidly demonstrated broad ranging applicability, and in particular CT has revolutionized the preoperative assessment of patients for reoperative surgery. Understanding the technical considerations will allow the surgeon to appreciate the inherent strengths and weaknesses of cardiac CT and to optimally communicate with the radiologist. This, in turn, will result in the best quality diagnostic examination in the vast majority of cardiac surgery patients.