Cardiac Imaging Review

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1 Kroft et al. rtifacts in EG- Synchronized MDT oronary ngiography ardiac Imaging Review 09_07_2138_Kroft.fm 7/27/07 L. J. M. Kroft 1. de Roos J. Geleijns Kroft LJM, de Roos, Geleijns J Keywords: artifacts, cardiac imaging, computed tomography, coronary angiography, T, diagnostic imaging DOI: /JR Received December 11, 2006; accepted after revision March 8, ll authors: Department of Radiology, 2S, Leiden University Medical enter, lbinusdreef 2, 2333 Z, Leiden, The Netherlands. ddress correspondence to L. J. M. Kroft (l.j.m.kroft@lumc.nl). ME This article is available for ME credit. See for more information. JR 2007; 189: X/07/ merican Roentgen Ray Society rtifacts in EG-Synchronized MDT oronary ngiography OJETIVE. In MDT coronary angiography, image artifacts are the major cause of false-positive and false-negative interpretations regarding the presence of coronary artery stenoses. Hence, it is important that observers reporting these investigations are aware of the potential presence of image artifacts and that these artifacts are recognized. ONLUSION. The article explores the technical causes for various artifacts in MDT coronary angiography imaging and clinical examples are given. rom the first 4-MDT feasibility F studies to the current clinically applied 64-MDT investigations, MDT coronary angiography has evolved into a reliable 3D imaging technique for detecting and excluding coronary artery stenoses with high accuracy [1 8]; it is now considered an appropriate imaging tool for detecting coronary artery disease in certain clinical contexts [9]. However, 2D conventional invasive X-ray coronary angiography is still considered the standard of reference for evaluating the coronary arteries because of its superior spatial and temporal resolutions compared with MDT. Parts of the coronary arteries cannot be evaluated with MDT because of image artifacts, and image artifacts are the major cause of false-positive and false-negative interpretations. Hence, it is important that observers reporting MDT coronary artery angiography investigations are aware of the potential presence of image artifacts and that these artifacts are recognized on the images. In this article, the impact of artifacts on MDT coronary angiography will be explained. The causes of artifacts will be discussed in detail, with special attention to the effect of coronary artery size and motion. Examples of artifacts will be shown that may help the reader recognize these artifacts when reporting MDT coronary angiography. Impact of rtifacts on MDT oronary ngiography MDT coronary angiography image quality and diagnostic performance have greatly improved after recent technical developments. With 4-MDT, 29% of the coronary arteries could not be evaluated because of artifacts [1]. With 16-MDT, 22 29% of the coronary artery segments could not be evaluated [10, 11]. One study stated that if these segments that could not be evaluated were excluded or considered negative, 25% of patients with a significant stenosis would have been missed [11]. With 64-MDT, 3 11% of coronary artery segments still cannot be evaluated [8, 12 15]. Sensitivities and specificities for detecting significant ( 50%) coronary artery stenoses based on segmental analysis with 64-MDT (conventional X-ray coronary angiography as the standard of reference) have been found to be good to excellent, in the range of 76 99% and 95 97%, respectively [2 8, 16]. However, these study outcomes are difficult to compare because the study methods vary substantially for example, in the selection of patients. Moreover, these study results should be interpreted with care because coronary artery segments that could not be evaluated (3 27%) were excluded from analysis beforehand [2 5, 7, 8]. Interestingly, of the coronary artery segments that were included in 64-MDT studies, the accuracy for detecting stenoses depended highly on image artifacts. Falsepositive and false-negative interpretations were attributed to image artifacts in 91% [6] to 100% [5] of cases, where the major cause was the presence of calcifications. Other authors support these findings [2 4, 6]. Less JR:189, September

2 Kroft et al. TLE 1: rtifacts and auses, Explanations, and Measures to void Them rtifact or Error ause Technical Explanation Solution looming (artifact) Small high-contrast objects appear larger than they are (e.g., caused by dense calcifications, clips, and stents) Not ideal transfer function (point-spread function) Smallest available focal spot, optimal reconstruction filter lurring (artifact) lurring (artifact) Streaks (artifact) Subtle discontinuities and missing data (artifact) Stairstep (artifact) Windmill (artifact) Poor contrast in lumen of coronary arteries (error) frequent causes were motion artifacts [4, 5] and obesity resulting in a poor contrast-tonoise ratio [4]. It can be concluded that artifacts are the cause of coronary arteries that cannot be evaluated and for false-positive and false-negative diagnoses as well. MDT and rtifacts rtifact Definition In T, the term image artifact can be defined as any discrepancy between the reconstructed Hounsfield values in the image and the true attenuation coefficients of the object in such a way that these discrepancies are clinically significant or relevant as judged by the radiologist [17]. In the definition, it is assumed that the radiologist will recognize relevant artifacts when they are present. Technical onsiderations for oronary rtery Imaging and rtifacts oronary artery MDT is technically complex and requires high spatial resolution, high temporal resolution, good low-contrast resolution, intravascular contrast enhancement, and a veraging of real attenuation values in a voxel Smearing out of real attenuation values due to cardiac motion, respiratory motion, or postural motion Reconstruction artifact caused by, e.g., lumen filled with high iodine concentration, metal clips, or metallic implants Respiratory or postural motion, pitch factor too high, or poor-quality EG signal Nonoptimal phase selection during prospective trigger acquisition or during retrospective reconstruction, due to either tachycardia or patient arrhythmia haracteristic of spiral acquisitions; more prominent for multislice acquisitions and at high pitch factors Technical error in contrast administration: extravasation (operator), volume and speed (operator), timing (operator); patient factors, technical limitations: Valsalva maneuver, heart failure, obesity Volume averaging due to voxel size Volume averaging due to motion eam hardening No data for proper image reconstruction Lack of proper EG synchronization Subsequent acquisitions, interpolations, as between different detector rows Poor contrast enhancement and resulting low contrast-tonoise ratio Incomplete coverage (error) Technical error by operator Incorrect planning of scanning range Thinner slices, smaller field of view β-blockers, nitroglycerin agents, sedatives, breath-holding instructions to patients Saline flush to avoid iodine contrast artifacts, use of less dense metal clips and implants Proper instruction of patient, optimal fixing of EG leads, and checking of signal Manual EG editing, cone-beam T (e.g., 256-MDT scanner), β-blockers Optimization of pitch factor might reduce windmill artifact; windmill artifacts are avoided in cases of prospective triggered acquisitions and cone-beam T (e.g., 256-MDT scanner) Instruct operator; update protocol; use high-density contrast agent, automatic (sure) start with accurate scanning delay, higher m and kv settings; instruct patient Instructions to operator short scanning time. The key acquisition parameters in cardiac MDT are section thickness, the rotation time of the X-ray tube, and the pitch factor. The acquisition section thickness is measured along the z-axis and determines the minimal voxel height in a reconstructed image. The rotation time (in milliseconds) is the time needed for a 360 rotation of the X-ray tube. The pitch factor is the ratio of patient displacement along the z-axis direction per tube rotation divided by the total thickness of all simultaneously acquired sections [18]. asically, almost all MDT image artifacts can be explained by limitations relating to spatial resolution, temporal resolution, noise (Table 1), and the reconstruction algorithms used. In the images, artifacts are mainly observed as blurring, blooming, streaks, missing data, discontinuities, and poor contrast enhancement. rtifacts may be grouped in technical- (physics-based, scanner-based, and reconstruction-based), operator-, and patient-related causes [19]. Defining the different artifact categories seems rather arbitrary. For example, poor patient instruction that results in breathing artifacts may be categorized as an operator-dependent artifact, whereas breathing artifacts that occur despite adequate breath-holding instructions may be considered patient-dependent. Spatial Resolution, Temporal Resolution, Noise, and Reconstruction lgorithm Spatial Resolution Spatial resolution, the ability to visualize small structures in the scanned volume, must be considered in three dimensions. voxel is the volume element that is represented by a 2D pixel in the axial xy-plane. The third dimension is the z-axis and the corresponding voxel height. reconstructed field of view of 200 mm and a characteristic pixel matrix result in a pixel size of mm 2 in the axial xy-plane. These values are typical for single-detector helical T acquisitions and for the current MDT scanners. Reconstructions with a smaller field of view can substantially decrease the pixel size in the axial plane. 582 JR:189, September 2007

3 rtifacts in EG-Synchronized MDT oronary ngiography > FWHM < FWHM FWHM The great improvement in spatial resolution with the current MDT scanners is due to the feasibility of acquiring volumes with thinner sections, which is especially important for reduction of the partial volume effect. With 4-MDT, 4 1 mm or 2 mm collimation scans were obtained in 20- to 45- second breath-holds [1, 20 22]. With 16- MDT, mm or 0.75 mm collimation scans were obtained in 16- to 30-second breath-holds [23 25]. With 64-slice and 64- row MDT scanners, mm and mm collimations are achieved within 11- to 15-second breath-holds [2, 5, 6]. Thinner collimation causes smaller voxel height, and shorter breath-holds allow more patients to hold their breath during image acquisition. lso, the scanned volume is obtained in substantially fewer heart beats, thereby decreasing the amount of image artifacts due to variations between heart beats. However, voxel size alone is not sufficient for describing the spatial resolution that can be achieved with T scanners. Voxel size does not take into account the inherent physical limitations of the T scanner such as focal spot size, detector size, and geometry (focus-to-axis of rotation distance and focus-to-detector distance) that result in geometric unsharpness [26]. The unambiguous and clearly defined quantity that physicists use for describing spatial resolution is the full width at half maximum (FWHM) of the response of a very small object. In T, a Fig. 1 Principle of full width at half maximum (FWHM) of response of very small object for describing spatial resolution that can be achieved. and, Visualization of two ideal points separated by distance of less than one FWHM () and separated more than one FWHM (). Response of ideal point is represented by gray area; composite response of two ideal points is represented by curving black line. t separation distance of less than one FWHM, two points cannot be distinguished separately; at distance of more than one FWHM, two points can be observed individually. Note that this criterion assumes static condition, or, in other words, that no motion artifacts are present. tungsten bead with a submillimeter diameter is often used as the small object for which the FWHM is determined. Once the FWHM is established for a scanner, one knows at what separation distance two points can be distinguished separately and at what separation distance two points are perceived as one point. If two ideal points are separated just a distance FWHM apart (or more), there is a fair chance that they will be separated in the image. If the points are separated less than a distance FWHM, then the points will be visualized as one point (Fig. 1). Typical values for the FWHM in the axial plane are mm; the FWHM along the z-axis is slightly worse, with typical values between 0.7 and 1.0 mm [27]. This means that structures with dimensions smaller than the FWHM of the point-spread function will be severely distorted in the reconstructed images, even if a small reconstructed field of view with correspondingly small voxels is being used. In clinical applications of MDT coronary angiography, this aspect should be taken into account for example, when coronary artery plaque (characterization) is considered and for imaging of small coronary artery vessels. Partial Volume Effect Partial volume effect or artifact is caused by limited spatial resolution and is the result of averaging the attenuation coefficient in a voxel that is heterogeneous in composition, FWHM where the average numeric value (in Hounsfield units) is assigned to the corresponding pixel. Partial volume effect is especially present at borders of two tissues or structures with a large difference in Hounsfield units, particularly at the margin of the coronary artery lumen and calcified plaque. The larger the voxels (i.e., pixel size and reconstructed image thickness), the larger the partial volume effect [18]. Partial volume artifacts are most often due to the presence of calcifications and are a major concern in MDT coronary angiography because they cause false-positive and false-negative interpretations in coronary arteries that could otherwise be evaluated [2 6]. Partial volume artifacts, including blurring and blooming, are best avoided by using thin collimation and a small reconstructed field of view [19]. Temporal Resolution Temporal resolution is the ability to resolve fast-moving objects in the displayed T image [18]. Temporal resolution remains the major challenge in MDT coronary angiography. Limitations in temporal resolution are strongly related to coronary artery size and motion. Motion in general causes degradation of contrast and spatial resolution and introduces artifacts [28]. ardiac motion presents as blurring and is the major reason for nondiagnostic coronary artery image quality [11 13]. The three major approaches to limit cardiac motion artifacts were already postu- JR:189, September

4 Kroft et al. lated in the 1970s by Harell et al. [29]: reducing the data collection period by faster rotating X-ray tubes, synchronizing T data acquisition with the cardiac cycle (prospective gating), and reconstructions synchronized to the EG cycle (retrospective reconstructions) after data acquisition. These measures are currently routinely applied in MDT coronary angiography. Irregular heart rates cause stairstep artifacts due to phase misregistration, in which images are not reconstructed at exactly the same phase of the heart cycle [30] (Fig. 2). Irregular heart rates for example, premature atrial contraction can also cause blurring of a coronary artery segment (Fig. 3). EG editing may reduce these artifacts. oronary rtery Size and Motion Mean coronary artery lumen diameters vary substantially from their proximal to their distal parts [31]. More than half the segments are smaller than 2.0 mm and have less image quality than those segments larger than 2.0 mm at 64-MDT [14]. The coronary arteries move substantially during the cardiac cycle with considerable intra- and interpatient variation regarding motion patterns and ranges [32]. The right coronary artery has the greatest velocity and range [33, 34]. Transverse (axial xy-plane) displacement is the major part of motion and ranges between 6 and 42 mm for the right and between 3 and 20 mm for the left coronary artery [32]. Motion velocity increases with heart rate [33, 34], but motion range does not [32]. The right coronary artery is affected most by motion artifacts [12]. Excessive coronary artery motion of an inplane distance greater than its diameter corresponds to visible motion on transverse images [33]. This amount of own diameter motion seems a rather comfortable criterion, particularly when the limitations in temporal resolution affect T of small structures (small-diameter coronary artery segments). With the D Fig year-old man imaged for suspected coronary artery disease. Stairstep artifact due to premature atrial contraction with extra systole, followed by compensatory long R-R interval (between sixth and seventh R-R peaks) D, Note that premature beat is approximately in middle of acquisition (), which is also true in images ( D). Note stairstep in right coronary artery (R) (arrows) at 3D reconstructions and central luminal line projections () and in two long-axis perpendicular curved multiplanar reconstructions (, D). In these perpendicular curved multiplanar reconstructions, coronary artery is usually more affected in one direction than in other. Step had virtually no effect on left anterior descending coronary artery (LD in ). Mean heart rate was 59 beats per minute. R-R interval during acquisition varied between 644 and 1,281 milliseconds. current MDT scanners, coronary artery motion exceeds the velocities needed for motionfree imaging when variations in cardiac motion during the cardiac cycle are not taken into account. However, motion velocity and speed change during the cardiac cycle, and we must use the rest phase for imaging with the fewest motion artifacts (Fig. 4). ut even then, the cardiac rest period, defined as the time with a displacement of the coronary artery of less than 1 mm, has a mean duration of 120 milliseconds but ranges from 66 to 333 milliseconds among patients [32]. onsequently, the temporal resolution of 165 milliseconds that is currently achieved with the fastest 330- millisecond rotation times when using halfscan reconstructions is longer than the mean rest period of 120 milliseconds and is not fast enough to image the general patient free of motion artifacts, notwithstanding optimal phase selection for image reconstruction in the cardiac cycle. 584 JR:189, September 2007

5 rtifacts in EG-Synchronized MDT oronary ngiography Fig year-old woman with suspected coronary artery disease. Image shows blurring due to motion caused by premature atrial contraction. t short R-R interval, rest phase was too short for motion-free imaging of coronary artery segment that presumably had large motion range at this time, causing blurring. This segment of right coronary artery is frequently affected by motion artifacts. Mean heart rate was 66 beats per minute. R-R interval during acquisition varied between 641 and 1,194 milliseconds. Fig year-old woman with suspected coronary artery disease. Image shows motion range for right coronary artery (R) during cardiac cycle. Image reconstructions were performed at 0%, 40%, and 80% of R-R interval and show identical orientation of 3D images in upper row and identical levels of images in middle row. In lower row, level that best displayed origin of R is displayed. Note large amount of motion of R during cardiac cycle. Note that R is displayed sharply at 80% of R-R interval, but not at 0% and 40% time phases. Mean heart rate was 52 beats per minute. R-R interval during acquisition varied between 1,095 and 1,189 milliseconds. With heart rates of less than 65 beats per minute (bpm), the best image quality is predominantly in diastole, whereas in heart rates exceeding 75 bpm, the best image quality shifts to systole in most cases [1, 13, 35]. Increasing heart rates mainly affect and significantly shorten the rest period at diastole [1, 32]. hoosing the optimal time point for reconstruction becomes more crucial for preserving image quality with higher heart rates [34]. With 64-MDT and at heart rates less than 65 bpm, diagnostic image quality can be obtained for all coronary arteries at a single reconstruction interval at mid diastole [15]. When higher heart rates are included, a minority of patients (7%) require additional reconstructions at late systole for optimal visualization of the right coronary artery [12]. To overcome limitations in temporal resolution and motion artifacts, increased gantry rotation speed is favored. However, a rotation time of less than 200 milliseconds to provide a temporal resolution of less than 100 milliseconds, regardless of cardiac frequency (halfscan reconstruction), would already result in an increase in mechanical G-forces that is beyond mechanical engineering limits [36]. Other strategies are as follows: Lowering the cardiac frequency by using β-blockers or dilating the coronary arteries by using nitroglycerin oronary artery image quality is inversely dependent on heart rate [1, 4, 12, 20, 21, 37]. dministering β-blockers for coronary artery MDT is widely used for reducing the heart rate, preferably to a frequency of less than bpm [2 5, 7, 8, 38]. Despite these publications, it has recently been found that the effect of heart rate on image quality is limited and mainly affects the visualization of the left circumflex coronary artery. Instead, the variability in heart rate during acquisition was found to be important. The stabilizing effect of β-blockers was shown to be the major determinant that resulted in superior image quality in patients receiving β-blockers as compared with those who did not [39]. Some authors use vasodilating medication as well for maximizing coronary artery size [40, 41]. In MDT coronary angiography, the use of nitroglycerin has been found to increase proximal coronary artery diameters by 12 21% [41]. However, the added value on diagnostic accuracy is not clear yet. Using segmental reconstruction instead of half-scan reconstruction The single-segment approach (i.e., half-scan reconstruction) and the two or more segment approach (i.e., multisegment reconstruction) are the two image JR:189, September

6 Kroft et al. Fig year-old woman with suspected coronary artery disease. Images show poor contrast enhancement. ontrast timing was good because coronary arteries were already enhancing. D, Note poor enhancement of left ventricle (LV), which should be brightly enhanced () (compare with Fig. 4). lso note stent in circumflex coronary artery ( and ), where artery is moderately enhanced. Patient performed Valsalva maneuver during image acquisition that is recognized by contrast column with convex shape toward superior vena cava (SV on coronal image, D), whereas saline flush should be running through at this time point. High intrathoracic pressure during Valsalva maneuver hampers inflow in right atrium and causes poor contrast enhancement. Mean heart rate was 77 beats per minute. R-R interval during acquisition varied between 776 and 789 milliseconds. reconstruction algorithms used for low and higher heart rates, respectively. With halfscan reconstruction, data obtained from a single 180 gantry rotation are used for image reconstruction [36]. With multisegment reconstruction, data of two or more successive cardiac cycles are combined that cover, as separate segments, 180 acquisition. Multisegment reconstruction algorithms require a stable and predictable heart rate during image acquisition. The temporal resolution is improved by a factor 2n (n = number of cycles and segments) of the rotation time [36]. For 64-MDT with 330 milliseconds rotation time, it was found that image quality achieved with two-segment reconstruction was not significantly improved compared with half-scan reconstruction for heart rates exceeding 65 bmp, although in 65% of patients the best overall image quality was achieved by twosegment reconstruction [13]. Dual or multiple-source T scanners in combination with half-scan reconstruction algorithms new technology, the dualsource MDT technique has recently been introduced with two tubes mounted at an angle of 90, thus improving the temporal resolution by a factor 2. rotation time of 330 milliseconds results in a temporal resolution of 83 milliseconds by half-scan reconstruction; this is achieved independently of heart rate [42]. Development of scanners with more than two source/detector combinations may further improve temporal resolution. Faster scanning with 256-MDT volumetric scanners With these next generation volumetric MDT scanners [43, 44], the entire heart can be covered in only one half rotation. This is expected to eliminate interbeat (stairstep) artifacts, and a short scanning time of 175 milliseconds (half-scan, D prospective triggering) minimizes the breath-holding time, allowing almost all patients to breath-hold during image acquisition. Moreover, multisegment reconstruction for example, two-segment reconstruction may potentially be used to improve the acquisition time. Respiratory Motion reath-holding exercises and instructions to the patients are particularly important in avoiding motion artifacts by breathing and postural motions that cause blurring. With the current 64-MDT scanners, most patients can breath-hold for the 10- to 13-second scanning duration. We instruct the patients to hold their breath after breathing in, at approximately three quarters of their maximum, and to lie still without pressing in order to avoid a Valsalva maneuver that may result not only in breathing artifacts but also in poor contrast enhancement (Fig. 5). Respiratory motion is well recognized at the lung window setting. Noise T noise (quantum mottle) is determined primarily by the number of photons used to make an image. The quantum mottle fraction decreases as the number of photons increases. T noise is generally reduced by increasing the kvp, m, or scanning time. T noise is also reduced by increasing voxel size (i.e., by decreasing the matrix size), increasing reconstructed field of view, by increasing section thickness [45], or by image stacking. ontrast is the difference in Hounsfield values between tissues and tends to increase as kvp decreases but is less affected by m or scanning time. T contrast can be improved by administering an iodinated contrast agent. The displayed image contrast is primarily determined by the T windowwidth and window-level settings [45]. Larger chest sizes (obesity) are associated with a higher level of image noise that negatively affects the quality of MDT coronary angiograms [12]. Inadequate contrast administration (e.g., inadequate volume, injection speed, or timing), inadequate selection of the field of view or region-of-interest placement for bolus tracking, or inadequate breath-holds can result in low contrast-to-noise ratios, resulting in poorly visualized coronary arteries. ontrast media with higher iodine concentrations provide substantially higher attenuation values in the coronary arteries [46], although the added 586 JR:189, September 2007

7 rtifacts in EG-Synchronized MDT oronary ngiography Fig year-old woman with suspected coronary artery disease. Geometric distortion due to spiral acquisition, where black shadow or rod artifact next to contrast-filled right coronary artery is due to miscalculation by reconstruction algorithm. During spiral acquisition, position registered by each view shifts. D, Miscalculation may cause hypodense artifacts (arrows) that rotate around high-density contrast-filled coronary artery. Note change in artifact position from to that is also observed on corresponding levels at coronal reconstruction (D). Mean heart rate was 66 beats per minute. R-R interval during acquisition varied between 916 and 977 milliseconds. value of these higher-iodine-concentration media on diagnostic accuracy in assessing coronary artery disease has not yet been established. rtifacts on the right coronary artery due to high contrast density in the right atrium can be effectively reduced using a saline flush. However, the diagnostic accuracy in detecting coronary artery stenoses has not been found to be substantially different between uni- or biphasic contrast protocols with or without a saline flush [47]. Probably, the contrast dose and injection speed should be balanced for optimal enhancement. Testing the IV access before contrast administration is advisable and is best performed just before coronary MDT by flushing a saline bolus with the patient s arms up in the same position as during coronary artery scanning. D Reconstruction lgorithm Spiral acquisitions may cause artifacts due to suboptimal (large) pitch, which is due to table movement during data acquisition that causes subsequent projections to be acquired from slightly different parts of the object. Resulting inconsistency in the data causes artifacts that increase with pitch. General T artifacts due to spiral scanning in relation to pitch are cone and rod artifacts [48]. With the current generation of MDT scanners, cone artifacts are avoided by various dedicated MDT reconstruction approaches that account for cone-beam geometry [36]. Rod artifacts [48] also referred to as windmill artifacts [36] are caused by the spiralinterpolation process of high-contrast structures that are obliquely oriented along the z- axis scanning plane. During scanning, the position registered by each view shifts; the size of the shift depends on the structure s angulation and the table increment. Rod artifacts occur as hypodense (or hyperdense) structures around structures with high (or low) density. These helical T artifacts are often observed around the ribs, where hypodense artifacts rotate around dense ribs and are seen when scrolling through a stack of images [36, 42]. ecause of the shape, the position, and the high contrast density of the coronary arteries, rod artifacts may occur in coronary MDT angiography as well (Fig. 6), and these artifacts may hamper coronary artery evaluation. These artifacts should be recognized and not be confused with noncalcified coronary plaque. To reduce spiral interpolation artifacts, narrow collimation that improves the z-axis resolution should be used [36]. One may expect these spiral T artifacts to disappear with the introduction of newer generation (256-row) scanners in which acquisition of the entire cardiac volume will be obtained without table movement. eam-hardening artifacts are caused by the polychromatic nature of the X-ray beam. s the lower energy photons are preferentially absorbed, the beam becomes more penetrating, which results in lower computed attenuation coefficient Hounsfield values. eam-hardening artifacts are most prominent at high-contrast interfaces [45]. In cardiac imaging, high-density contrast agent injection can manifest as beam-hardening artifacts by causing dark bands between dense objects in the image [17]. Metal objects can cause complicated artifacts, such as beam hardening and partial volume, that are worsened with object motion, and methods to overcome metal-induced artifacts are particularly difficult to design [17]. In MDT coronary angiography, these artifacts may occur with stents, pacemakers, or surgical clips. If the density of highly attenuating metal objects is beyond the normal range that can be handled by the computer, severe streaking artifacts occur [19] (Fig. 7). Sharp high-resolution kernels may be used for improved stent lumen visualization [49], although sharp kernel filters result in higher image noise and artifacts causing lower in-stent attenuation values [50]. The diagnostic effect of highresolution kernels on accuracy in evaluating stent patency in patients has not been investigated yet. JR:189, September

8 Kroft et al. Fig. 7 Two patients with suspected coronary artery disease. and, 34-year-old man with pacemaker lead in right atrium () that causes subtle artifacts visible at right ventricular surface in 3D view (arrows, ) and through right coronary artery central luminal line reconstruction (arrow, ). Mean heart rate was 78 beats per minute. R-R interval during acquisition varied between 759 and 790 milliseconds. and D, 57-year-old man after bypass surgery with metallic sternal wires (). Severe high-density surgical clip artifacts hamper arterial lumen evaluation at course of left internal mammary artery, which was used for bypassing left anterior descending coronary artery (D). Surgical clips were used for occluding side branches of left internal mammary artery. Mean heart rate was 74 beats per minute. R-R interval during acquisition varied between 760 and 835 milliseconds. Reconstruction and Postprocessing Errors Errors during reconstruction can occur because of inadequate or insufficient phase selection for coronary artery evaluation. Multiple-phase reconstructions may be needed for visualizing all coronary artery segments with diagnostic quality (Fig. 8). t the workstation, automatic tools for segmentation may produce errors (Fig. 9). It is important to always review the nonpostprocessed source images to confirm findings found at the computer-assisted reconstructions. Diagnostic Interpretation Errors For 64-MDT, false-positive and falsenegative coronary artery interpretations have been explicitly explained by obvious technical limitations that is, image artifacts due to calcifications, motion, and obesity [2 6]. This was also the case for 4- MDT [21] and 16-MDT [10, 11] coronary angiography. Therefore, missing coronary artery abnormalities seems to be most commonly related to artifacts and less related to lack of diagnostic perception in the case of well-trained observers. Technical errors that can be avoided are often related to patient handling and postprocessing handling. Suggestions for avoiding MDT coronary angiography artifacts are presented in Table 1. Global measures are the following: Patient preparation Lower the heart rate in patients with heart rates exceeding 65 bpm by using β-blockers (if allowed). Obtain a good EG signal. Prepare contrast injection and prepare the patient with breathholding instructions. cquisition Take care for adequate timing of contrast injection, and use an adequate dose and injection speed. Provide good breath-holding instructions. Postprocessing hoose the best phase for coronary artery reconstruction. Review the source images to confirm findings at advanced workstation reconstructions. Diagnostic interpretation Recognize artifacts and report diagnostic limitations. D 588 JR:189, September 2007

9 rtifacts in EG-Synchronized MDT oronary ngiography Fig year-old man with suspected coronary artery disease., utomatic segmentation reconstruction artifact with interruption and apparent stenosis of right coronary artery (R) at crux where it diverges into R continuation in atrioventricular groove and in posterior descending branch (PD). Point of interruption is where central luminal line (dotted line, ) has lost its way at crux and is not in center of coronary artery (). This is easily repaired by manually replacing erroneous point () correctly in central lumen at curved multiplanar reconstruction. fter replacement, artery is continuous; compare and. Mean heart rate was 52 beats per minute. R-R interval during acquisition varied between 1,124 and 1,172 milliseconds. D Fig year-old woman with suspected coronary artery disease. D, urved multiplanar reconstruction of right coronary artery in two perpendicular longitudinal directions. t time point with least motion at 900 milliseconds (at 76% of R-R interval, and ), right coronary artery is sharply delineated in proximal part but appears interrupted halfway (arrow), whereas further coronary artery segment appears blurred. Severe stenosis cannot be excluded at this time. dditional reconstruction at 500 milliseconds (at 42% of R-R interval, and D) is of moderate but diagnostic quality and shows that suspected right coronary artery segment is actually open. Mean heart rate was 51 beats per minute. R-R interval during acquisition varied between 1,155 and 1,198 milliseconds. JR:189, September

10 Kroft et al. onclusion The main artifacts that hamper MDT coronary angiography image interpretation are motion artifacts that cause blurring and incorrect diagnoses due to coronary artery calcifications. This article has explored artifact causes and provided examples. Recognizing artifacts is important in the diagnostic process. cknowledgment We thank Raoul M.S. Joemai for producing Figure 1. References 1. Giesler T, aum U, Ropers D, et al. Noninvasive visualization of coronary arteries using contrast-enhanced multidetector T: influence of heart rate on image quality and stenosis detection. JR 2002; 179: Mollet NR, ademartiri F, van Mieghem G, et al. High-resolution spiral computed tomography coronary angiography in patients referred for diagnostic conventional coronary angiography. irculation 2005; 112: Leber W, Knez, von Ziegler F, et al. 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