Multidetector-Row CTfor Assessment of Cardiac Valves 15

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Multidetector-Row CT

for Assessment of Cardiac Valves

J ÜRGEN K. W ILLMANN , MD AND D OMINIK W EISHAUPT , MD

INTRODUCTION

Imaging of the heart with computed tomography (CT) is challenging because the heart is continuously moving during data acquisition. As a result of the limited temporal resolution, the use of single-detector helical CT for noninvasive cardiac imaging was limited and resulted often in images with a high content of artifacts. The introduction of multidetector-row CT (MDCT) scanners by the end of 1998 laid the foundation for increased clinical use of CT for cardiac imaging. Partial view acquisition and retrospectively electrocardiogram (ECG)-gated helical reconstruction offered by 4-channel MDCT scanners, allow for a temporal resolution of up to 125 ms, combined with both a high spatial resolution and a high signal-to-noise ratio

ning of the whole heart with an even higher temporal resolution of up to 105 ms within a convenient short breath-hold of about 18 s (2). Apart from visualization of coronary artery lumen and stenosis as well as detection and quantification of coronary calcification (3,4), these technical developments in CT tech- nology also improved visualization of morphological details of the heart including the cardiac valves (5,6).

In this chapter, we describe the potential clinical and research applications of MDCT, with special focus on the assessment of the aortic and mitral valves.

CONVENTIONAL IMAGING TECHNIQUES

Conventional angiography allows assessment of functional data, including measuring the transvalvular pressure gradient as well as calculating the valve area using the Gorlin formula

tional angiography has been demonstrated to be inaccurate. In addition, conventional angiography does not allow precise quantification of valvular calcification.

TRANSTHORACIC AND TRANSESOPHAGEAL ECHOCARDIOGRAPHY

Transthoracic echocardiography is a widely available, non- invasive, and safe imaging modality which allows an expedi- tious assessment of the anatomic and functional status of the

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cardiac valves. In patients with stenotic aortic and mitral dis- ease, precise quantification of mean and maximum trans- valvular pressure gradients as well as determination of valve area can be performed. Alternatively, transesophageal echo- cardiography can be performed, in particular in patients with poor transthoracic acoustic windows, such as patients with large and thick chest walls, small hearts, chest deformities, or in elderly patients. Transthoracic as well as transesophageal echocardiography allow assessment of the entire spectrum of valvular disease, in particular visualization of infective endo- carditis and its sequelae, including perivalvular abscesses.

However, both transthoracic and transesophageal echocardio- graphy permit only a rough quantification of valvular calcifica- tion, as only indirect signs of calcification, including increased echogenicity and thickening of the valves, may be used for quantification (8).

MDCT

Aortic valve stenosis is the most common cardiac valve lesion in the developed countries, with a prevalence of 2% to 7% in the population above 65 yr of age (9). Degeneration and calcification of the aortic valve cusps and the aortic annulus are the most common cause of aortic valve stenosis (10). Several studies have identified the presence and extent of aortic valve calcification as a strong predictor both for the progression as well as for the outcome of aortic valve stenosis (11,12). The positive therapeutic effect of a lipid-lowering pharmacologi- cal therapy with HMG-CoA reductase inhibitors on the natural history of calcific aortic valvular disease has been demon- strated recently (13). Therefore, an imaging modality for quan- tification of aortic valve calcium in patients with aortic valve stenosis is desirable. From a surgical point of view, preopera- tive knowledge about anatomic details of the aortic valve is of particular interest, since the procedure of aortic valve sur- gery itself and the choice of the type of valve prosthesis to be implanted depend on various parameters, including the aortic valve morphology (tricuspid vs bicuspid), its diameter, as well as the presence and extent of aortic valve and annulus calcifi- cation (14–16). Moreover, the presence of aortic valve calcifi- cation extending to the interventricular septum as a surgical finding at aortic valve replacement predicts the need for per- manent pacing postoperatively (17). Similarly, two studies

From: Contemporary Cardiology: CT of the Heart:

Principles and Applications

Edited by: U. Joseph Schoepf © Humana Press, Inc., Totowa, NJ

MDCT for assessment of the cardiac valves may be per- formed either with or without administration of intravenous contrast agents. Usually, for assessment of valvular morphol- ogy, a contrast-enhanced CT scan is performed. For mere determination of valvular calcification, a nonenhanced CT scan is sufficient.

The scan is planned on a low-dose anteroposterior scout view (120 kV, 50 mAs) from the ascending aorta to the apex of the heart. Four-channel MDCT acquisition is performed with 4 × 1-mm collimation (1.25-mm slice width for reconstruc- tion), 500-ms rotation time, table feed of 2.5–4.5 mm per rotation, 300 mA at 120 kV, and a 0.6- to 0.8-mm image recon- struction increment. The acquisition parameters for a 16-chan- nel MDCT acquisition include a 12 × 0.75-mm collimation (0.75-mm slice width for reconstruction), a table feed of 6.7 mm/s, a gantry rotation time of 420 ms, and a reconstruction increment of 0.4 mm. For retrospective reconstruction of the MDCT data set, a digital ECG file from the patient is simulta- neously recorded during MDCT scanning. Image reconstruc- tion is performed with a medium-sharp body convolution kernel in the mid- to end-diastolic phase of the cardiac cycle using between 50% and 70% relative delay to the R waves of the ECG signal. With 4-channel MDCT scanning, this results in a voxel size of 0.35 × 0.35 × 1.25 mm

(field of view [FOV], 18 cm;

matrix size, 512 × 512 pixels). For 16-channel MDCT, the resulting voxel size is 0.35 × 0.35 × 0.75 mm

. For data analy- sis, the reconstructed MDCT data are best transferred to an independent workstation, which allows multiplanar reconstruc- tions in sagittal, coronal, and oblique planes. For assessment of the valvular morphology and quantification of valvular cal- cium, routine reconstruction of volume renderings or multiplanar reformations is not necessary.

If the MDCT scanning is performed with intravenous administration of an iodinated contrast agent, we first deter- mine the optimal scan delay using the test bolus method. For this purpose, a test bolus of 20 mL of iodinated contrast agent followed by a 50-mL chaser of saline is injected by a power injector. Delay times are determined by visually evaluating the contrast material at the level of the aortic valve by using ten consecutive transverse images obtained every 2 s without table feed. The MDCT scan is then performed after administering 120 mL of nonionic contrast material through a 20-gage needle placed in an antecubital vein at 3–5 mL/s followed by a 50-mL saline chaser bolus. Alternatively, the delay time can be obtained by using a bolus-tracking technique (e.g., CARE- Bolus, Volume Zoom Navigator, Siemens). For this purpose, a single nonenhanced low-dose scan (10 mAs) at the level of the ascending aorta is performed. Based on this transverse image,

MDCT allows reliable differentiation between bicuspid and tricuspid aortic valves. In a prospective study of 25 patients with aortic valve stenosis, Willmann et al. (5) have shown that contrast-enhanced MDCT is highly accurate for prediction of valve morphology when compared to surgery and echo- cardiography. Using contrast-enhanced MDCT there was a 100% agreement between MDCT and surgical or echocardio- graphic findings with regard to the morphology of the aortic valve (Fig. 1) (5). On nonenhanced MDCT data sets, the aortic valve could be correctly classified in 87% of the cases (5).

MDCT also allows for a precise measurement of the diameter of the aortic valve annulus (Fig. 2). In the same study by Willmann et al. (5), the diameter of the aortic valve annulus as measured on contrast-enhanced 4-channel MDCT images correlated highly with the intraoperative measurement with only an overestimation by 0.7 mm on contrast-enhanced MDCT data sets (5).

The morphology of the mitral valve and its apparatus, including the mitral valve annulus, mitral valve leaflets, tendi- nous cords, and papillary muscles can also be visualized on MDCT (Fig. 3). Recently, our group reported on the prelimi- nary experience with mitral valve imaging in 20 patients (6).

Good-to-excellent image quality of the mitral valve annulus and its leaflets were obtained in 15 of 20 consecutive patients (75%). In 19 of 20 patients (95%) papillary muscles could also be visualized to good or excellent advantage. However, visibil- ity of tendinous cords was inferior. In 14 of 20 patients (70%) tendinous cords were not or only moderately visible (6).

VALVULAR CALCIFICATION

MDCT is in particular helpful for the assessment of aortic valve calcification. According to the echocardiographic grad- ing of aortic valve calcifications (11), we use a four-point grading scale for assessment of the degree of the calcification of the aortic valve. This grading scale is as follows: grade 1, no calcification; grade 2, mild calcification (small, isolated spots of calcification); grade 3, moderate calcification (mul- tiple larger spots of calcification); and grade 4, heavy calci- fication (extensive calcification of all aortic valve leaflets) (Figs. 4 and 5). By using this grading scale there was an 84%

agreement between contrast-enhanced MDCT findings and the true calcification status of the aortic valves as assessed during surgery (5).

As with the aortic valve, the calcifications of the mitral valve

may be located either on the mitral valve annulus or on the

mitral valve leaflets. MDCT yielded a 95–100% agreement

compared to echocardiography and intraoperative findings with

regard to assessment of calcifications of the mitral valve annu-

lus and mitral valve leaflets (6).

Fig. 1. (A) 57-yr-old male with severe aortic valve stenosis who underwent contrast-enhanced retrospectively electrocardiogram- gated multidetector-row CT (MDCT) before surgery. Double oblique reconstruction shows heavily calcified bicuspid aortic valve with large calcific deposit (arrows). (B) Image obtained during surgery shows excellent agreement between both MDCT and intraoperative status.

Note presence of bicuspid aortic valve with large deposits of calcium at the free edges of the aortic valve (arrows).

Fig. 2. Precise assessment of aortic valve diameter is possible with contrast-enhanced, retrospectively electrocardiogram-gated multidetector-row CT (MDCT). Preoperative assessment was per- formed on a sagittal oblique reconstruction of the heart in this 63-yr- old female with aortic valve stenosis. There was only a minimal difference between the diameter as assessed by MDCT and the diam- eter as measured during surgery.

Fig. 3. 73-yr-old female with mitral regurgitation. Long-axis view reconstruction contrast-enhanced, retrospectively electrocardiogram- gated multidetector-row CT data set demonstrates anterior mitral valve annulus (large arrow), mitral valve leaflets (small arrows), ten- dinous cord (arrowhead), and the anterior papillary muscle (P). LV, left ventricle; LA, left atrium; AA, ascending aorta.

All these findings imply the potential of MDCT for a longi-

tudinal monitoring of patients with aortic and mitral valve cal-

cifications by measuring the change of valvular calcification

over time. Apart from a visual semi-quantitative assessment of

valvular calcification, future studies will demonstrate whether

MDCT imaging may also allow an absolute quantification of

valvular calcium. In vitro studies with absolute determination

of aortic valve calcium by ashing or enzymatic digesting of noncalcified areas of the aortic valve and subsequent measure- ment of the mass of the remaining aortic valve calcium are warranted to correlate the real amount of calcium and the cal- cium obtained on MDCT. Furthermore, the reliability of differ- ent calcium scores used for quantification of coronary artery calcium need to be evaluated when quantifying aortic valve calcium, including the Agatson score (20), the volume score

eral mass score (22).

LIMITATIONS OF MDCT IMAGING

So far, MDCT imaging has demonstrated its capability only for evaluation of the morphology of mitral and aortic valves, including the semi-quantitative assessment of calcifications.

As a result of the limited temporal resolution of 4-channel MDCT scanners, functional assessment of cardiac valves as well as quantification of valvular area, in particular of the aortic valve, has not been possible yet. With the introduction of 16-channel scanners, as well as further reduction of the rotation time of the CT tube, additional improvement of spatial and temporal resolution may also allow accurate assessment of valvular area.

A second limitation includes the radiation dose, which is inherent to CT scanning. Since the data are acquired with an overlapping helical pitch and continuous radiation exposure, there is a considerable applied radiation dose. Mean effective dose values of up to 13 mSv have been calculated for imaging the entire heart using a 4-channel MDCT scanner (23). Com- pared to the mean effective dose values calculated for conven- tional coronary angiography (usually between 2.1 and 2.5 mSv), the mean effective dose caused by MDCT angiography is higher by a factor of about 5 (23). However, by reducing the tube output during heart phases that are not likely to be targeted by the ECG-gated reconstruction (i.e., reconstruction intervals excepting 50–70% of the cardiac cycle), a dose reduction of up to 48% is possible (24). Furthermore, improvements of dose utilizations of recent generation 16-channel MDCT scanners may also help in reducing radiation dose (25).

CONCLUSION AND FUTURE DIRECTIONS

MDCT is an emerging tool for noninvasive imaging of the

heart. Apart from evaluation of the coronary arteries, a compre-

hensive work-up of the heart also includes assessment of mor-

phological details of the heart chambers, including cardiac

valves. Preliminary studies have shown that the aortic and

mitral valves can be visualized with high quality. At this point, MDCT of the aortic valve seems to be of special interest for the assessment of aortic valvular calcification. Future studies are warranted to exploit the potential of MDCT for evaluation of the pulmonary and tricuspidal valves as well as to establish tools for quantitative analysis of valvular function and quanti- fication of the calcium deposits.

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