Image Reconstruction for ECG-Triggeredand ECG-Gated Multislice CT 4

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45

T HOMAS F LOHR AND T INSU P AN

INTRODUCTION

Computed tomography (CT) imaging of the heart and the coronary anatomy requires high temporal resolution to avoid motion artifacts and to achieve sufficient spatial resolution—

at best submillimeter—to adequately visualize small anatomi- cal structures such as the coronary arteries. Furthermore, the complete heart volume has to be examined within the time of one breath-hold. First attempts to use single-slice spiral CT systems for cardiac scanning were not convincing because of poor temporal resolution and insufficient volume coverage with thin slices (1,2). Since 1999, 4-slice CT systems with higher volume coverage speed and improved temporal resolution thanks to faster gantry rotation (rotation time down to 0.5 s) have been clinically used for electrocardiogram (ECG)-trig- gered or ECG-gated multislice CT (MSCT) examinations of the cardiac anatomy (3–9). Coverage of the entire heart volume with thin slices (4 × 1-mm/4 × 1.25-mm collimation) within one breath-hold period became feasible, allowing for new applica- tions such as high-resolution CT angiographies of the coronary arteries (6–9). First clinical studies have demonstrated the abil- ity of MSCT to characterize lipid, fibrous, and calcified coro- nary plaques (10). Despite all promising advances, challenges and limitations remain for cardiac MSCT with 4-slice detec- tors. Spatial resolution is still not sufficient to clearly depict stents or severely calcified coronary arteries (6,7). Temporal resolution is not yet adequate for patients with higher heart rates, and a diagnostic outcome cannot be guaranteed in these cases despite careful selection of the reconstruction interval

(11,12). The scan time of about 40 s required to cover the entire

heart volume (approx 12 cm) with 4 × 1-mm, 4 × 1.25-mm collimation is at the limit for a single breath-hold scan. In 2000, a shorter scan time was realized with 8 × 1.25-mm collimation, cardiac MSCT, which enables scan times of about 20 s. In 2001, a new generation of MSCT systems was introduced. With simultaneous acquisition of up to 16 submillimeter slices and gantry rotation times down to 0.4 s, spatial resolution in the transverse direction and temporal resolution are further improved, while examination times are considerably reduced

(13,14). Sixteen-slice systems have the potential to overcome

the limitations of established 4-slice/8-slice CT scanners, and first clinical studies have already demonstrated enhanced clini- cal performance (15,16). While system properties such as gan- try rotation time and detector slice width determine the intrinsic temporal and spatial resolution of the data, dedicated scan and image reconstruction techniques are needed to optimize the outcome of cardiac CT examinations. In this chapter, we present the basics of ECG-triggered and ECG-gated MSCT scanning.

We give an overview on image reconstruction techniques, start- ing with single-segment partial scan reconstruction and ending with multisegment approaches. We discuss the pros and cons of single- and multisegment reconstruction. We demonstrate the properties of reconstruction algorithms with patient scans, and end with a short summary and discussion.

SCAN TECHNIQUES FOR ECG-CONTROLLED MULTISLICE CT: ECG-TRIGGERED AXIAL AND ECG-GATED SPIRAL SCANNING

For ECG-synchronized examinations of the cardio-thoracic anatomy, either ECG-triggered axial scanning or ECG-gated spiral scanning can be used. The most basic approach is pro- spectively ECG-triggered axial scanning, which was previously introduced with electron beam CT. The patient’s ECG signal is monitored during examination, and axial scans are started with a predefined temporal offset relative to the R waves, which can be either relative (given as a certain percentage of the RR inter- val time) or absolute (given in ms), and either forward or reverse

(5). Data acquisition is therefore “triggered” by the R waves of

the patient’s ECG signal. A schematic illustration of absolute and relative phase setting is given in Fig. 1. The principle of ECG-triggered multislice axial scanning is illustrated in Fig. 2.

The coordinate system in Fig. 2 shows the patient’s ECG-signal as a function of time on the horizontal axis and the position of the detector slices relative to the patient on the vertical axis (in this example, four detector slices are indicated). Usually, par- tial scan data intervals (180° + detector fan angle) are acquired.

Thanks to optimized half-scan reconstruction algorithms with adequate data weighting, a temporal resolution up to half the gantry rotation time per image (250 ms for 0.5 s gantry rotation) can be achieved in a sufficiently centered region of interest.

The number of images acquired with every scan corresponds to the number of active detector slices. In between the individual axial scans, the table moves to the next z position; the heart

From: Contemporary Cardiology: CT of the Heart:

Principles and Applications

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

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46 FLOHR AND PAN

Fig. 1. Schematic illustration of absolute and relative phase setting for electrocardiogram (ECG)-controlled CT examinations of the cardio- thoracic anatomy.

Fig. 2. Principle of electrocardiogram (ECG)-triggered multislice axial scanning. The patient’s ECG signal is indicated as a function of time on the horizontal axis. The position of the detector slices relative to the patient (four slices in this example) is shown on the vertical axis. Usually, partial scan data intervals (180° + detector fan angle) are acquired, which are marked as boxes.

volume is therefore covered in a “step and shoot” technique. As a result of the time necessary for table motion, only every sec- ond heart beat can be used for data acquisition, which limits the minimum slice width to 2.5 mm with 4-slice or 1.25 mm with 8-slice CT systems if the whole heart volume has to be covered within one breath-hold period.

With retrospective ECG gating, the heart volume is covered continuously by a spiral scan. The patient’s ECG signal is recorded simultaneously to allow for a retrospective selection of the data segments used for image reconstruction. The prin- ciple of retrospectively ECG-gated spiral scanning is illustrated in Fig. 3. Similar to Fig. 2, the patient’s ECG signal is indicated as a function of time on the horizontal axis, and the position of the detector slices relative to the patient is shown on the vertical axis. The table moves continuously, and continuous spiral scan data of the heart volume are acquired. Only scan data acquired in a predefined cardiac phase, usually the diastolic phase, are used for image reconstruction (5,17). These scan data segments are indicated as boxes in Fig. 3. A variety of dedicated recon- struction approaches for ECG-gated spiral MSCT have been introduced with 4-slice CT scanners (3–5,17), resulting in a temporal resolution of half the gantry rotation time (250 ms for 0.5 s gantry rotation time) or better, thanks to multisegment

Fig. 3. Principle of retrospectively electrocardiogram (ECG)-gated spiral scanning. The patient’s ECG signal is indicated as a function of time on the horizontal axis, and the position of the detector slices relative to the patient is shown on the vertical axis. The table moves continuously, and continuous spiral scan data of the heart volume are acquired. Only scan data acquired in a pre-defined cardiac phase, usually the diastolic phase, are used for image reconstruction, which are marked as boxes.

reconstruction. Different from ECG-triggered axial scanning, scan data from every heart cycle can be used for image recon- struction; therefore, the entire heart volume can be covered with a 4-slice system within one breath-hold period using 1-mm or 1.25-mm slices. Image reconstruction during different heart phases is feasible by shifting the start points of the data seg- ments used for image reconstruction relative to the R waves.

For a given start position, a stack of images at different z posi- tions covering a small subvolume of the heart can be recon- structed thanks to the multislice data acquisition (4,5,17).

Prospective ECG triggering combined with “step and shoot”

acquisition of axial slices has the benefit of smaller patient dose than ECG-gated spiral scanning, since scan data are acquired in the previously selected heart phases only. It does not, however, provide continuous volume coverage with overlapping slices, and misregistration of anatomical details cannot be avoided.

Furthermore, reconstruction of images in different phases of the cardiac cycle for functional evaluation is not possible.

Because ECG-triggered axial scanning depends on a reliable prediction of the patient’s next RR interval by using the mean of the preceding RR intervals, the method encounters its limi- tations for patients with severe arrhythmia. To maintain the benefits of ECG-gated spiral CT but reduce patient dose, ECG- controlled dose-modulation has been developed (18). During the spiral scan, the output of the X-ray tube is modulated according to the patient’s ECG. It is kept at its nominal value during a user-defined phase of the cardiac cycle—in general, the mid- to end-diastolic phase. During the rest of the cardiac cycle, the tube output is reduced. Clinical studies with 4-slice CT systems have demonstrated dose reduction by 30–50%, depending on the patient’s heart rate, using ECG-controlled dose modulation (18).

IMAGE RECONSTRUCTION FOR ECG-TRIGGERED MULTISLICE AXIAL SCANNING

Using prospective ECG-triggering, axial scan data are

acquired. Since the table is stationary for each individual scan

and moves only between scans, no multislice spiral interpola-

tion is necessary. In general, partial scans are performed, with

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a scan data segment covering 180° plus the detector fan angle (about 50–60°, depending on system geometry). Including some additional data for smooth transition weighting of complementary rays to avoid streaking artifacts resulting from data inconsistencies at the beginning and at the end of the scan interval, the total scan interval of a partial scan is ∆α

p

= 240–

260°. This is the minimum necessary for image reconstruction throughout the entire scan field of view (SFOV) of usually 50–

cm diameter. The temporal resolution at a certain point in the SFOV is determined by the acquisition time window of the data contributing to the reconstruction of that particular image point.

Similar to slice sensitivity profiles (SSP), temporal resolution may be characterized by time sensitivity profiles (TSP). The temporal resolution ∆T

ima

assigned to an image is the full width at half maximum (FWHM) of the TSP. In a conventional ap- proach, the entire partial scan data segment is used for image reconstruction in any point of the SFOV. Redundant data are weighted using algorithms such as the one described by Parker

(19). The temporal resolution ∆Tima

in this case is ∆α

p

/360°

times the rotation time of the scanner; for 0.5 s rotation, ∆T

ima

= 0.33–0.36 s. To improve temporal resolution, modified reconstruction approaches for partial scan data have been pro- posed (5,17), which are best explained in parallel geometry. A modern CT scanner acquires data in fan beam geometry, char- acterized by the projection angle α and by the fan angle β within a projection. Another set of variables serving the same purpose is θ and b. θ is the azimuthal angle and b denotes the distance of a ray from the iso-center (see Fig. 4). θ and b are used to label rays when projection data are in the form of parallel projec- tions. A simple coordinate transformation relates the two sets of variables:

θ = α + β, b = R

_F

sinβ. (1)

Using this equation, the measured fan beam data can be transformed to parallel data, a procedure called “rebinning.” In parallel geometry, 180° of scan data are necessary for image reconstruction. For data acquisition in fan beam geometry, a partial scan interval larger than 180°, namely 180° plus the detector fan angle, is necessary to provide 180° of parallel data for any image point within the SFOV. In the center of rotation, for β = 0, 180° of the acquired fan beam data are sufficient to provide 180° of parallel data; see Eq. (1). If all redundant data are neglected, temporal resolution in the center of rotation can be as good as 180°/360° times the rotation time of the scanner;

for 0.5 s rotation, ∆T

ima

= 0.25 s. As a consequence of the rebinning Eq. (1), data with different fan beam projection angles α and hence different acquisition times contribute to a parallel projection at projection angle θ. Fig. 5 is a sinogram and illus- trates the rebinning procedure. Obviously, the temporal resolu- tion is not constant, but depends on the position of the image point in the SFOV. This is illustrated in Fig. 5 for the example of three different image points a, b, and c. Measurement values contributing to point a in the iso-center are located on a straight vertical line in the sinogram (β = 0). Hence, the temporal reso- lution for this particular point is half the rotation time of the scanner. Data contributing to points b and c are located on sinosoidal curves in the sinogram. A measure for temporal reso- lution is given by the path lengths of the sinosoidal curves within the gray shaded parallel sinogram. Obviously, the total acquisition time of the data for point b is longer than for the central point a, whereas it is even shorter for point c. For clini- cal applications, the heart should be sufficiently centered within the SFOV to maintain a stable temporal resolution of half the gantry rotation time. It is not possible to make use of the areas

Fig. 4. Geometry of CT data acquisition. A modern CT scanner

acquires data in “fan beam geometry,” characterized by the projection angle α and by the fan angle β within a projection. Another set of variables serving the same purpose is θ and b. b denotes the distance of a ray from the iso-center. θ and b are the coordinates of a ray in

“parallel geometry.”

Fig. 5. Sinogram illustrating parallel rebinning according to Eq. 1 for the partial scan data segment. The sinogram curves for three image points a, b, and c at different positions within the scan field of view are indicated.

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with better temporal resolution, as start and end position of the X-ray source during ECG-triggered acquisition depend on the patient’s heart rate and cannot be fixed.

IMAGE RECONSTRUCTION FOR ECG-GATED MULTISLICE SPIRAL SCANNING

Using ECG-gated multislice spiral scanning, the heart vol- ume is covered continuously by a spiral scan. Image recon- struction for ECG-gated multislice spiral scanning therefore consists of two parts: multislice spiral interpolation to compen- sate for the continuous table movement and to obtain scan data at the desired image z position, followed by a partial scan reconstruction of the axial data segments as described above. A

“single-slice” partial scan data segment is generated for each image using a partial rotation of the multislice spiral scan that covers the given z position. For each projection angle within the multislice data segment, a linear interpolation is performed between the data of those two detector slices that are in closest proximity to the desired image plane z

_ima

. The temporal reso- lution, which is limited to half the gantry rotation time for pro- spective ECG triggering, can be improved up to 1/(2N) times the rotation time by using scan data of N subsequent heart cycles for image reconstruction (17). With increased N, better tempo- ral resolution is achieved, but at the expense of reduced volume coverage within one breath-hold time or loss of transverse reso- lution. To maintain good transverse resolution and thin-slice

images, every z position of the heart has to be seen by a detector slice at every time during the N heart cycles. As a consequence, the larger the N and the lower the patient’s heart rate, the more the spiral pitch has to be reduced. If the pitch is too high, there will be z positions which are not covered by a detector slice in the desired phase of the cardiac cycle. To obtain images at these z positions, far-reaching spiral interpolations have to be per- formed, which degrade SSPs and reduce transverse resolution.

SINGLE-SEGMENT RECONSTRUCTION

At low heart rates, a single-segment reconstruction (N = 1)

yields the best compromise between sufficient temporal reso-

lution on the one hand and adequate volume coverage with thin

slices on the other. For N = 1, consecutive multislice spiral data

from the same heart period are used to generate the single-slice

partial scan data segment for an image. The spiral interpolation

scheme for a 4-slice scanner using one segment of multislice

data is illustrated in Fig. 6, together with the calculation of the

spiral interpolation weights for some representative projection

angles. In general, no interpolation between projections mea-

sured at different projection angles, i.e., different acquisition

times, is performed, even in cases where this would be possible

(180°-type interpolation). The temporal resolution is constant

and equals half the gantry rotation time of the scanner using

optimized partial scan reconstruction techniques as described

above. For 0.5-s gantry rotation time, temporal resolution is

Fig. 6. Spiral interpolation scheme for a four-slice scanner using one segment of multislice data for image reconstruction. The z position of the four detector slices changes linearly relative to the patient due to the constant spiral feed. The spiral interpolation is indicated for some representative projection angles α. Please note that in general no interpolation between projections measured at different projection angles, i.e., different acquisition times, is performed, even in cases where this would be possible (180°-type interpolation). On the bottom, the electrocardiogram signal is shown schematically.

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∆T

ima

= 0.25 s. Some of the recently introduced 16-slice CT systems offer gantry rotation times even shorter than 0.5 s, such as 0.42 s or 0.4 s. In this case, temporal resolution can be as good as ∆T

ima

= 0.21 s or 0.2 s.

MULTISEGMENT RECONSTRUCTION

At higher heart rates, temporal resolution can be improved by dividing the partial scan data segment used for image recon- struction into N = 2–4 subsegments acquired in subsequent heart cycles. Each subsegment is generated by using data from one heart period only, and there are temporal gaps between the multislice data segments used for image reconstruction. Simi- lar to the case N = 1, for each projection angle α within subsegment j, a linear interpolation is performed between the data of those two detector slices that are in closest proximity to the desired image plane. The result are N single-slice partial scan subsegments located at the given image z position z

ima

(see Fig. 7 for the example N = 2).

With this technique, the patient’s heart rate and the gantry rotation time of the scanner have to be properly desynchronized to allow for an improved temporal resolution. Two requirements have to be met: firstly, start- and end-projection angles of the subsegments have to fit together to build up a full partial scan interval (see Fig. 8). As a consequence, the start projections of subsequent subsegments have to be shifted relative to each other. Secondly, all subsegments have to be acquired in the

same relative phase of the patient’s heart cycle to reduce the

total time interval contributing to an image. If the patient’s

heart cycle and the rotation of the scanner are completely syn-

chronous, the two requirements are contradictory. For instance,

for a heart rate of 60 beats per minute (bpm) and a 360° rotation

time of 0.5 s, the same heart phase always corresponds to the

same projection angle segment, and a partial scan interval can-

not be divided into smaller subsegments acquired in successive

heart periods. Then no better temporal resolution than half

the gantry rotation time is achieved. In the best case, when the

patient’s heart cycle and the rotation of the scanner are opti-

mally desynchronized, the entire partial scan interval may be

divided into N subsegments of equal length, and each

subsegment is restricted to a data time interval of 1/(2N) times

the rotation time within the same relative heart phase. Gener-

ally, depending on the relation of rotation time and patient heart

rate, temporal resolution is not constant but varies between one

half and 1/(2N) times the gantry rotation time. There are “sweet

spots,” heart rates with optimum temporal resolution, and heart

rates where temporal resolution cannot be improved beyond

half the gantry rotation time. Multisegment approaches rely on

a complete periodicity of the heart motion, and they encounter

their limitations for patients with arrhythmia or patients with

changing heart rates during examination. In general, clinical

practice suggests the use of N = 1 segment at lower heart rates

and N ≥ 2 segments at higher heart rates. In some CT scanners,

Fig. 7. Spiral interpolation scheme for a four-slice scanner using N = 2 subsegments of multislice data from consecutive heart periods for image reconstruction. Both subsegments have to fit together to build up a partial scan data interval. The spiral interpolation is indicated for some representative projection angles α.

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the partial scan data segment is automatically divided into one or two subsegments depending on the patient’s heart rate dur- ing examination (Adaptive Cardio Volume ACV algorithm

[17]). At heart rates below a certain threshold, one subsegment

of consecutive multislice spiral data from the same heart period is used. At higher heart rates, two subsegments from adjacent heart cycles contribute to the partial scan data segment. In some other CT scanners, the single-segment partial scan images are reconstructed prospectively as baseline images, followed by a two-segment reconstruction retrospectively for a potential gain of temporal resolution for higher heart rates. Alternatively, a two-segment prospective reconstruction can be prescribed with a slower table feed for scanning the same cardiac anatomy in two heart cycles, followed by a retrospective single-segment reconstruction for comparison with the two-segment recon- struction. A single-segment reconstruction is usually preferred for its reliability, even though a two-segment reconstruction may yield a better image quality.

Another approach is to prospectively adjust the rotation time of the scanner to the heart rate of the patient to obtain best possible temporal resolution for a multisegment reconstruc- tion. The range of heart rates is preferred to cover 15 bpm for each selected gantry cycle, to account for the patient’s heart rate variation during a scan. Figure 9 shows an example of using 0.5-s and 0.6-s gantry rotation time for this implementa- tion. The 0.5-s two-segment reconstruction covers low 60s to 75 bpm, followed by 0.6-s three- to four-segment reconstruc- tion for 75–90 bpm, and 0.5-s three- to four-segment recon- struction for the heart rates of over 90 bpm. Figure 10 shows another example of using 0.42-s and 0.5-s gantry rotation time in combination with an automatic selection of one- and two-

segment reconstruction (ACV algorithm). The 0.5-s two-seg- ment reconstruction covers 64–73 bpm, whereas the 0.42-s two- segment reconstruction is preferable for 73–90 bpm. Figures 11 and 12 show examples of patient scans at higher heart rates, to illustrate the potential of multisegment reconstruction to improve image quality in selected cases. Again, prospectively adapting the rotation time of the scanner and exploiting multisegment reconstruction requires a stable and predictable heart rate during examination and complete periodicity of the heart motion.

DISCUSSION AND SUMMARY

We have reviewed the scanning and reconstruction tech- niques for ECG-controlled MSCT. Two basic scanning tech- niques are discussed: ECG-triggered axial scanning and ECG-gated spiral scanning. For ECG-triggered axial scanning, the X-ray will be turned on only for the duration of the data for image reconstruction; therefore, the dose to the patient can be kept to a minimum. For ECG-gated spiral scanning, the dose is higher, since the X-ray is turned on for the whole duration of the scanning. Ways of reducing the X-ray dose have been pro- posed, such as ECG-controlled dose modulation (18). There are two major reconstruction techniques associated with spiral scanning: one is single-segment reconstruction, where each image is reconstructed with the data from a single cardiac cycle;

the other is N-segment reconstruction, where each image is reconstructed with the data from N contiguous cardiac cycles.

The two-segment reconstruction has the advantage of extend-

ing from single-segment reconstruction without changing the

gantry rotation cycle. The three- and four-segment reconstruc-

tions can be used when the heart rate is becoming too high (>75

Fig. 8. Schematic illustration of the two-segment cardio reconstruction approach. The circular graph represents the full partial scan interval in parallel geometry (180° of parallel data), which is composed of two subsegments acquired in subsequent heart cycles. These two subsegments have to be measured in the same phase of the cardiac cycle, and start- and end-projections have to fit together. To fulfill both requirements, the patient’s heart cycle and the rotation of the scanner have to be asynchronous.

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Fig. 9. A design of N-segment reconstruction with gantry cycles of 0.5 and 0.6 s to improve temporal resolution for higher heart rates. The 0.5-s two-segment reconstruction covers from the low 60s to 75 bpm, followed by 0.6-s three- to four-segment reconstruction for 75–90 bpm, and 0.5-s three- to four-segment reconstruction for heart rates of over 90 bpm.

Fig. 10. A design of automatic selection of single- and two-segment reconstruction (ACV algorithm) with gantry cycles of 0.42 and 0.5 s.

0.42-s gantry rotation is preferable for low heart rates (single-segment reconstruction) and for 73–90 bpm (two-segment reconstruction).

The 0.5-s two-segment reconstruction yields better temporal resolution for 64–73 bpm.

bpm), and a slower gantry cycle of 0.6 s can be used to optimize the temporal resolution. Single-segment reconstruction is the clinically most robust reconstruction technique. The reliability of obtaining good quality images with N-segment reconstruc- tion generally goes down when N becomes larger from 2, to 3, to 4. Multisegment reconstruction requires a stable heart rate during examination and complete periodicity of the heart motion.

The cardiac MSCT has also benefited significantly from the advancements of 4 × 1-mm/4 × 1.25-mm to 16 × 0.75-mm/

16 × 0.625-mm collimation and faster gantry speeds of 0.5 s to 0.42 s or 0.4 s. The wider coverage with thinner slices (16 × 0.75 mm/16 × 0.625 mm) has improved the spatial resolution

in the cranio-caudal direction (or Z coordinate) to submillime-

ter, with already submillimeter resolution in the in-plane direc-

tion (or X and Y coordinates); this is critical for coronary artery

imaging, and has shortened the scan time of 40 s (4-slice) to

20 s (8- or 16-slice) or even 10 s (16-slice), which is well under

a single breath-hold. The gantry rotation cycle of 0.5 s to 0.42

or 0.4 s has improved the temporal resolution of 0.25 s to 0.21

and 0.2 s, which will help the MSCT become a scanner for most

patients. The image quality that can be obtained in clinical

routine is demonstrated in Fig. 13. Meanwhile, first clinical

experience has demonstrated the potential of 16-slice technol-

ogy for cardiac imaging (15,16,20).

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Fig. 11. A patient study with heart rates of 97–101 bpm with three- to four-segment reconstruction. Shown here are the volume-rendered image, and the curved reformatted images of the left anterior descending (LAD), left circumflex (LCX), and right coronary artery (RCA). It demon- strates the potential for three- to four-segment reconstruction to improve the temporal resolution for higher heart rates.

Fig. 12. A patient study of heart rates of around 92 bpm with two-segment reconstruction. Shown here are the volume-rendered image, and a “spider view” maximum intensity projection showing the origins of left anterior descending, left circumflex, and right coronary artery (courtesy of Dr. Ropers, Erlangen University, Germany).

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Fig. 13. Case of a 68-yr-old male patient with a history of stroke secondary to a bilateral carotid obstruction. The CT scan was obtained on a 16-slice CT system. The images show severe calcification in the left anterior descending (LAD) and right coronary artery (RCA), as well as large pericardial calcifications. The calcifications are displayed in endoscopic and maximum intensity projection viewing techniques. Corre- sponding to the coronary angiogram, a 40% stenosis of the left main (LM) and 80% stenosis of the LAD and RCA were found (courtesy of Prof.

Oudkerk, Groningen University, Groningen, the Netherlands).

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REFERENCES

1. Bahner M, Boese J, Lutz A, et al. Retrospectively ECG-gated spiral CT of the heart and lung. Eur Radiol 1999;9:106–109.

2. Kachelriess M, Kalender W. Electrocardiogram-correlated image reconstruction from subsecond spiral computed tomography scans of the heart. Med Phys 1998;25:2417–2431.

3. Kachelriess M, Ulzheimer S, Kalender W. ECG-correlated image reconstruction from subsecond multi-slice spiral CT scans of the heart. Med Phys 2000;27:1881–1902.

4. Taguchi K, Anno H. High temporal resolution for multi-slice helical computed tomography. Med Phys 2000,27(5):861–872.

5. Ohnesorge B, Flohr T, Becker C, et al. Cardiac imaging by means of electro-cardiographically gated multisection spiral CT—initial experience. Radiology 2000;217:564–571.

6. Achenbach S, Ulzheimer S, Baum U, et al. Noninvasive coronary angiography by retrospectively ECG-gated multi-slice spiral CT.

Circulation 2000;102:2823–2828.

7. Becker C, Knez A, Ohnesorge B, Schöpf U, Reiser M. Imaging of non calcified coronary plaques using helical CT with retrospective EKG gating. AJR 2000;175:423–424.

8. Knez A, Becker C, Leber A, Ohnesorge B, Reiser M, Haberl R. Non- invasive assessment of coronary artery stenoses with multidetector helical computed tomography. Circulation 2000;101:e221–e222.

9. Nieman K, Oudkerk M, Rensing B, et al. Coronary angiography with multi-slice computed tomography. Lancet 2001;357:599–603 10. Schroeder S, Kopp A, Baumbach A, et al. Noninvasive detection and

evaluation of atherosclerotic coronary plaques with multi-slice computed tomography. JACC 2001;37(5):1430–1435.

11. Hong C, Becker C, Huber A, et al. ECG-gated reconstructed multidetector row CT coronary angiography: effect of varying trigger delay on image quality. Radiology 2001;220:712–717.

12. Kopp A, Schröder S, Küttner A, et al. Coronary arteries: retrospectively ECG-gated multi-detector row CT angiography with selective optimization of the image reconstruction window. Radiology 2001;221:683–688.

13. Flohr T, Stierstorfer K, Bruder H, Simon J, Schaller S. New technical developments in multislice CT, part 1: approaching isotropic resolution with sub-mm 16-slice scanning. Fortschr Röntgenstr 2002;174:839–845.

14. Flohr T, Bruder H, Stierstorfer K, Simon J, Schaller S, Ohnesorge B.

New technical developments in multislice CT, part 2: sub-millime- ter 16-slice scanning and increased gantry rotation speed for cardiac imaging. Fortschr Röntgenstr 2002;174:1022–1027.

15. Kopp AF, Küttner A, Heuschmid M, Schröder S, Ohnesorge B, Claussen CD. Multidetector-row CT cardiac imaging with 4 and 16 slices for coronary CTA and imaging of atherosclerotic plaques. Eu Radiol 2002;12(Suppl 2):S17–S24.

16. Nieman K, Cademartiri F, Lemos PA, Raaijmakers R, Pattynama PMT, de Feyter PJ. Reliable noninvasive coronary angiography with fast submillimeter multislice spiral computed tomography. Circula- tion 2002;106:2051–2054.

17. Flohr T, Ohnesorge B. Heart rate adaptive optimization of spatial and temporal resolution for ECG-gated multi-slice spiral CT of the heart. JCAT 2001;25(6):907–923.

18. Jakobs TF, Becker CR, Ohnesorge B, et al. Multislice helical CT of the heart with retrospective ECG gating: reduction of radiation exposure by ECG-controlled tube current modulation. Eur Radiol 2002;12:1081–1086.

19. Parker D. Optimal short scan convolution reconstruction for fanbeam CT. Med Phys 1982;9(2):254–257.

20. Ropers D, Baum U, Pohle K, et al. Detection of coronary artery stenosis with thin-slice multi-detector row spiral computed tomography and multiplanar reconstruction. Circulation 2003;107:664–666.