nary calcium by EBCT whereas its clinical appli- cation will be discussed in Chapter 7.
1. The Scanner
The EBCT scanner, unlike conventional CT scan- ners, uses electromagnetically focused electrons to sweep stationary tungsten target rings to produce X-rays. There have been three iterations of EBCT scanners from the original C-100 in mid-1980s to C-150, which was introduced in 1993, and then C-300 in 2000.6The latest in the series is e-Speed (General Electric) introduced in 2003 and is capable of up to 33-ms true tem- poral resolution. In the evolution of EBCT scan- ners, the basic technical principle has remained unchanged, but improvements have occurred in spatial resolution, data manipulation and man- agement, data display, and storage. Most of the literature for use of EBCT for coronary calcium scoring has been produced using C-100 and C- 150 scanners with temporal resolution of 100 ms and currently no validation studies for the e- Speed scanner at high temporal resolution mode are available.
In a current-generation EBCT scanner, the electrons generated by an electron gun are accel- erated up to nearly light velocity; the electron beam is then steered by an electromagnetic deflection system to sweep around the anodes (four tungsten 1/2circular target rings, forming a 210-degree arc below the patient) where X-ray
1. The Scanner . . . 93
2. Protocol for Calcium Scoring Scan . . . 94
3. Scoring of Coronary Calcification . . . 94
4. Reproducibility . . . 97
5. Coronary Angiography with EBCT . . . 97
6. Conclusion . . . 98
The term EBCT or electron beam computed tomography is linked to coronary artery calcification and thus diagnosis of coronary artery disease. It was in 1979 that the design of the EBCT scanner was first proposed with the aim of rapidly scanning the beating heart.1This gantry design did not use any moving parts and thus had much shorter scan times compared with conventional computed tomography (CT) scanners. In the early years, the EBCT scanners (also called “cine CT” or “ultrafast CT”) were used to evaluate myocardial thickening, wall motion, and regional blood flow.2 It was not, however, until 1989 that the first use of an EBCT scanner in detection of coronary artery calcification was described.3
Calcification in the coronary arteries as seen on fluoroscopy had been long known to predict the degree of coronary artery disease and hence the prognosis.4,5Because ordinary CT was highly sensitive in detecting calcium in the body, use of EBCT with its much faster temporal resolution made it ideally suited to identify coronary artery calcification.
This chapter describes the techniques and principles underlying the assessment of coro-
6
Computed Tomography Techniques and Principles.
Part a. Electron Beam Computed Tomography
Tarun K. Mittal and Michael B. Rubens
93
beams are produced and directed through the patient (Figure 6a.1; see color section). The fan- shaped X-ray beams are then intercepted by sta- tionary detector rings, forming a 216-degree arc, located above the patient, and converted into digital information by the data acquisition system. These data are then reconstructed into diagnostic images. The image acquisition is prospectively triggered with patient electrocar- diogram (ECG), and temporal resolution of up to 30 frames per second can be obtained. Coronary artery calcium scoring with EBCT yields effec- tive radiation doses of 1.0 and 1.3 mSv for male and female patients, respectively.7
2. Protocol for Calcium Scoring Scan
Standardized methods for imaging, iden- tification, and quantification of coronary artery calcium using EBCT have been pub- lished.8,9 The scanner is operated in the high- resolution, single-slice mode with continuous, non-overlapping slices of 3-mm thickness and an acquisition time of 100 ms per tomogram.
Patients are positioned supine, and after localization of the main pulmonary artery, a sufficient number of tomographic slices are obtained to cover the complete heart through the left ventricular apex (usually 36–40 slices). Elec-
trocardiographic triggering is done at end dias- tole at a time determined from the continuous ECG tracing recorded during scanning. Current clinical protocols of EBCT perform triggering during the cardiac cycle as varied depending on the patient’s resting heart rate to minimize coro- nary artery motion artifacts. Coronary arteries are identified as soft tissue structures, usually surrounded by fat, in the cardiac grooves (Figure 6a.2a–d). Calcified coronary deposits are seen as bright white areas along the course of the coro- nary artery (Figure 6a.3a–d).
3. Scoring of Coronary Calcification
For detection and scoring of calcification in the coronary arteries, an arbitrary value of +130 Hounsfield units (HU) [HU is the attenuation value of various tissues related to water which is calibrated to zero (HU = 0)] and an area greater than 1.0 mm2are often used. The +130 HU level was selected because it lies well above the +30 to 50 HU of unenhanced myocardium and soft tissue. A region of interest is placed manually around the pixels thought to be in the line of the coronary arteries to separate them from the calcification in the aortic root, mitral annulus, pericardium, etc. Because individual pixels above +130 HU are frequently seen throughout
Figure 6a.1. Configuration of a typical EBCT scanner (e-Speed, General Electric).
in mm2and calculates the Agatston score, volume score, and now also the mass measurement.
The Agatston score10(see also Chapter 7) is the oldest and most frequently used calcium scoring system. It takes into account the peak attenua- tion of the calcified lesion as well its area. The area of the lesion with the peak pixel between 131 HU and 200 HU is multiplied by a factor of the heart because of noise, a minimum area
threshold of 2–4 contiguous pixels (generally 3 pixels, which represents an area of 1.03 mm2with a 30-cm field of view and a 512 ¥ 512 matrix) is chosen in the evaluation of coronary calcification. Once the regions of interest have been placed, the scanner software displays the peak calcification attenuation in HU and the area
c d
a b
Figure 6a.2. Normal origin and course of the left main stem (arrowhead) coro- nary artery with continuation as left anterior descending (LAD, arrow) and diag- onal branch (a), origin of the right coronary artery (RCA, arrowhead) anteriorly and circumflex artery (CXA,arrow) in the posterior atrioventricular groove (b),RCA
(arrowhead), LAD (arrow), and CXA (double arrowheads) in their respective grooves along their mid course (c), and posterior descending branch in the infe- rior interventricular groove (d).
one, 201 HU–300 HU lesions by a factor of two, 301 HU–400 HU lesions by a factor of three, and lesions greater than 401 HU by a factor of four.
The sum of the individual lesion scores is the score for that artery, and the sum of all lesion scores is the total calcification score. The volume score first proposed by Callister et al.11is the total volume of the voxels (i.e., total number of pixels
¥ slice thickness) above the 130 HU threshold.
Calcium mass measurement is the representation of the total mineral content of the lesion(s) as the integral (sum) of all HU in a lesion multiplied by the voxel volume in mm3. Because a calcified lesion is a complex of different bone ash equiv- alent, a scaling factor specific to a scanner is used to convert the Hounsfield values to the bone ash
c d
a b
Figure 6a.3. Marked calcification along the course of proximal left anterior descending (LAD, arrowhead) and origin of the circumflex artery (CXA, arrow) (a), further in the right coronary artery (RCA, arrow), mid LAD (arrowhead), and
CXA (double arrowheads) artery (b), and in the distal part of RCA (c). Corre- sponding angiogram showing significant obstructive disease in the proximal LAD and CXA artery (d).
4. Reproducibility
The ability to assess progression of atheroscle- rotic burden over time accurately depends on the reproducibility of the technique. The median interscan variability has been shown to be 7%–12% for calcium score and 5%–11% for volume in recent studies using standard trigger- ing technique.14,15 Lower interscan variability has been shown with increasing calcium scores, patients with stable heart rates (heart rate changing less than 10 beats per minute during scanning), patients with no visible coronary motion, and those with an optimal ECG trigger- ing method (P < 0.05 for all).15It has been shown that ECG trigger of 40% rather than 80% of the R-R interval significantly reduces the interscan variability.16Volume score diminishes the vari- ability because it does not take into account the peak attenuation value, which is affected by image noise.14Similarly, measurement of mass besides reducing the interscan variability as dis- cussed above, also has a low interobserver vari- ability (1%–3%) in the recent studies with EBCT.15
5. Coronary Angiography with EBCT
Several studies have shown feasibility of per- forming coronary angiography with EBCT.17–19 These studies have reported sensitivities and specificities in the range of 78%–93% and 88%–98% for the detection of significant coro- nary stenoses. Up to 25% of the coronary seg- ments may, however, not be assessable using the equivalent and thus provide the calcium mass
measurement.12Unlike Agatston score, no arbi- trary value for peak attenuation is required for such measurement, thus making it least affected by the attenuation values and also by partial volume effects caused by differences in slice thickness and slice spacing. Calcium mass esti- mation is thus more reproducible compared with Agatston and volume scores, and hence has more applicability between different scanner types, particularly with the increasing use of the multislice scanners.
The prevalence of coronary calcium normally increases with age, with women lagging by about 10 years with respect to men. Thus, although the total calcium score may represent the total plaque burden, the age and sex of the individual has to be taken into account to understand the significance of a particular score. Also, it has been shown by several studies that instead of absolute calcium scores, it is the percentile scores for a given age and sex that are more helpful in assessing the risk in asymptomatic individuals and in detection of obstructive disease in symptomatics, with calcium scores above the 75th percentile being more significant than scores below the 25th percentile. This has led to development of sets of nomograms for calcium scores based on age and sex and grouped by percentile ranking. The largest of these is the Kondos database (Table 6a.1), which gives the Agatston calcium scores on 35 246 asymptomatic individuals.13 Rumberger and Kaufman12 have provided the first normal database for volume and the mass measure- ments in 11 490 individuals besides the Agatston scores.
Table 6a.1. Electron beam tomographic coronary artery score percentiles for men and women within each age strata Age (yr)
Percentile <40 40–44 45–49 50–54 55–59 60–64 65–69 70–74 >74
Men (25 251) 3504 4238 4940 4825 3472 2288 1209 540 235
25th 0 0 0 1 4 13 32 64 166
50th 1 1 3 15 48 113 180 310 473
75th 3 9 36 103 215 410 566 892 1071
90th 14 59 154 332 554 994 1299 1774 1982
Women (9995) 641 1024 1634 2184 1835 1334 731 438 174
25th 0 0 0 0 0 0 1 3 9
50th 0 0 0 0 1 3 24 52 75
75th 1 1 2 5 23 57 145 210 241
90th 3 4 22 55 121 193 410 631 709
Source: Reproduced with permission from Hoff et al.13
current technology because of low contrast to noise ratio and motion artifacts caused by respi- ratory and cardiac movement. Also, coronary calcification can be a hindrance in evaluation of degree of stenosis, although the presence of a high calcium score by itself would indicate the presence of atheromatous disease.
6. Conclusion
EBCT is a highly sensitive technique for detec- tion and measurement of coronary artery calcification and thus noninvasive estimation of atherosclerotic plaque burden. It offers a high spatial resolution with low acquisition time and has good reproducibility and interobserver agreement with optimized ECG triggering.
EBCT is a mature technique for calcium detec- tion with a continuously expanding literature.
With growing technological advances, cardiac CT has a promising role in clinical practice as well as research.
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