Research - Cardiac CT

 

ARTIFACTS AND PITFALLS IN CORONARY ARTERY CALCIUM SCORING

Kelly D. Ludema, D.O., MSU Radiology

 

INTRODUCTION

Numerous articles in the recent scientific literature have discussed the clinical application of coronary artery calcium scoring (CACS). Our institutional experience with CACS indicates there are many artifacts and interpretation pitfalls related to this relatively new technology. These limitations have been less widely disseminated in the literature and indicate falsely a shallower learning curve for CACS. A host of factors contribute to complexities interpreting these procedures: CT acquisition parameters, post processing software settings, user error, patient movement, respiratory and cardiac motion, and irrelevant calcifications. A physician interpreting CACS studies must be cognizant of these error sources to consistently obtain reproducible results. We will demonstrate representative artifacts and pitfalls as an aid to practicing physicians regularly interpreting CACS examinations.

 

Abbreviations used:

RCA: Right coronary artery.
LM: Left main coronary artery.
LCX: Left circumflex coronary artery.
LAD: Left anterior descending coronary artery.
GCV: Great cardiac vein. MCV: Middle cardiac vein.
CAC: Coronary artery calcium.
CA: Coronary angiography.
ICUS: Intra-coronary ultrasound.

 

TECHNIQUE

All the CACS studies referenced here were obtained on a GE LightSpeed Plus system (4 detectors) utilizing prospective cardiac gating. Scoring parameters include craniocaudal scan direction, 4x2.5mm collimation and 500ms gantry rotation speed. The kV is set at 120 and the mA was varied dependant on patient weight (220-350). A matrix of 512x512 and 25cm FOV is standard. The threshold is set 90 and 130HU with a 2 pixel minimum area to be scored. However we only report scores at the 130HU threshold. The scores reported here are modified Agatston scores (Agatston*0.83) to correct for the larger 3mm slice thickness on electron beam CT (EBCT). The examinations were scored on a stand alone workstation utilizing SmartScore software (GE Medical Milwaukee, Wisconsin).

 

ARTIFACTS

The most problematic aspect of cardiac scoring is the artifacts caused by cardiac and respiratory motion. This motion can cause false positive or false negative scoring results due to areas of hyper-attenuation being scored as calcium or the calcification being out of the plane of imaging. This also decreases reproducibility and makes follow-up of standard Agatston scores in the short term problematic. Respiratory motion artifact is less common due to more rapid helical scanning allowing reduced breath hold times in the 20-25 second range.

Since most of the high attenuation and poor visualization artifacts are caused by cardiac motion, imaging of the heart should be completed if at all possible during the point after the “T” wave in the cardiac cycle (diastole). This is the point of cardiac muscle re-polarization and offers the most optimal window to image the coronary arteries with CT. In EKG triggered CAC scoring, imaging at the correct R-R interval then becomes essential. The default R-R interval included on vendor software (70% on GE) has been shown to be inadequate to achieve optimal results. Optimal EKG triggering should be set at the end of systole (T wave) as opposed to later in diastole as the other longer R-R interval will achieve. This will give a longer time of relative decreased coronary artery motion. The reproducibility of CAC measurements improve if the optimal EKG triggering time varies according to the baseline heart rate. Lu, et al. used R-R triggering percentages of 31-46 dependant on the heart rate to reduce coronary artery motion artifacts from 80% of the subjects to 26%.¹ Mahnken, et al. utilized a standard 50% R-R interval in retrospectively gated MSCT to decrease the variance in CAC score between studies.²

Motion artifacts: The most motion prone coronary artery has been shown to be the RCA. Even under the most ideal circumstances some right coronary artery motion will likely be seen, since the temporal resolution to completely stop cardiac motion has not been achieved by either EBCT or helical CT. This motion can either cause artifacts of hyper-attenuation (Fig. 1A), or displaced areas of real coronary artery calcification (Fig. 1B). Both of these artifacts can cause false elevation of the CACS, especially with the lower 90HU threshold. Lopressor is utilized in our practice in patients with heart rates above 85-90 (<5% of patients) in order to achieve decreased cardiac motion (Fig 2). Reduction of cardiac motion artifacts by use of Beta-blockade increases reader confidence and study reproducibility.

High-attenuation artifacts: Any anatomical structure adjacent to the coronary arteries can cause areas of hyper-attenuation during respiratory or cardiac motion (Fig 3). These areas can mimic linear coronary artery calcification and cause false score elevation and decreased reproducibility.

Energy starvation (x-ray photon starvation): This can cause areas of artifact, especially at the base of the heart, due to increased tissue in this region. This increased noise artifact can be especially prominent in EBCT and may cause artificially elevated scores in both the 130HU and the 90HU thresholds. This is particularly seen in patients with increased body mass index and may preclude completely accurate scoring of the distal coronary arteries even with higher mA settings. The SmartScore auto score function can be used to assist in determination of scorable areas. However this auto-score function (highlight scorable regions button) will falsely highlight this artifact causing a “speckled” appearance of the distal coronary arteries and veins (Fig 4). The inexperienced or inattentive reader could then falsely score these regions. We have found that these speckled areas are isolated to the distal RCA, PDA, LCX, and distal cardiac veins. This artifact is almost never seen in the more proximal coronary arteries and represents noise that should not be scored. Punctate areas of calcification can be seen but are distinct from the “speckle” artifacts (Fig. 5). In these heavier patients it helps to turn off this auto score function on the work station. An experienced reader can then easily distinguish the speckle artifact from real areas of punctate calcification.

 

INTERPRETATION PITFALLS

Missing extra-cardiac findings: The standard field of view (25cm) for CACS is necessary in order to achieve an accurate score by maintaining the correct pixel size. However this standard field of view inhibits visualization of the surrounding anatomy. All examinations should be also looked at after the images have been retrospectively reconstructed. This allows visualization of the anatomy in the normal non-scoring field of view. In our experience extra-cardiac findings that may be clinically significant are found in upwards of 10% of patients (un-published data). These include liver and adrenal masses, chest wall masses, lung lesions, vascular abnormalities and mediastinal adenopathy (Fig. 6- Fig. 7).

Missing soft plaque: A recent article has shown excellent correlation with CT coronary angiographic characterization of plaque with ICUS.³ In this article the attenuation characteristics of the soft lipid plaques were low enough (6, and -5 HU) that theoretically these may be distinguished from the attenuation of blood without the administration of IV contrast if the spacial resolution is sufficient. This brings out the possibility that with the advancing spacial and temporal resolution of MSCT these lipid core soft plaques may be able to be visualized in the proximal coronary arteries even without the added benefit of IV contrast enhancement. We have found that these plaques can be occasionally seen on CACS studies with standard technique (Fig.8 - Fig.9). The intermediate plaques however are likely to still be missed in non CTA images due to their HU near that of blood (83, and 51HU in #3). The possibility of visualization of these lipid core soft plaques should at least be kept in mind when reading CAC scoring examinations. More research needs to be done to determine the significance of these findings.

Being unaware of standard anatomy: A detailed survey of coronary artery anatomy is beyond the scope of this poster (an excellent anatomy reference can be found in #4). A general understanding is necessary to label the areas of calcification and for presentation of results to clinicians and patients. The interpreting physician should primarily be able to distinguish the cardiac veins from the coronary arteries. Since the cardiac veins do not accumulate calcium, any hyper-attenuating focus originating in them can safely be discounted.⁴ The GCV can be quite prominent at the level of the LM/LAD bifurcation and runs very closely with the LCX in the atrioventricular groove (Fig. 10).

The Coronary sinus is also closely opposed to the distal LCX as well as the distal RCA. The Middle cardiac Vein can be seen directly adjacent to the PDA at the base of the heart draining into the coronary sinus (Fig. 11). Readers should also be aware of variations of the coronary arteries. These include minor anomalies such as LCX arising from the RCA or the right coronary sinus (most common at 0.5%). Major anomalies include the LM from the right sinus, the RCA from the left sinus (Fig 12), or single coronary ostia. These major anomalies are important because complications include myocardial infarction, CHF and sudden death.⁵

Extraneous calcifications: These include aortic and mitral valve/annular, lymph nodes, and pericardial calcifications that can be confused with CAC by the inexperienced reader. The mitral valve is located between the left atrium and left ventricle. The prevalence of mitral annular calcification is approximately 1-3% in patients less than 70 years old (Transamerica reinsurance Data 2002). The LCX can be seen in slices near the mitral valve. Care should be taken that the mitral valve or annular calcification is not included with the LCX score (Fig 13). The LCX will usually be surrounded by a perceptible area of pericardial fat. Mitral annular calcification is also within the cardiac muscle unlike the LCX calcification. The Mitral valve can be distinguished from the LCX by its lack of linear progression through the atrioventricular groove and its large areas of calcification that is atypical for the LCX in this region (Fig. 14).

 

CONCLUSIONS

CACS is rapidly becoming a widely utilized screening procedure. In order for results to be reproducible and accepted by the clinical community, every effort to maintain strict quality control should be undertaken. One of these quality control measures is for the reading radiologist to recognize the artifacts and pitfalls in interpretation of this modality. Although many of the potential problems listed here become intuitive after reading 25-30 studies, inexperienced readers may have many questions concerning the specifics of interpretation. The artifacts and pitfalls we have discussed will allow the beginning CACS reader to recognize these problems.

 

REFERENCES 

1. Lu B, Zhuang N, Mao SS, et al. EKG-triggered CT data acquisition to reduce variability in coronary arterial calcium score. Radiology 2002 224: 838-844.
2. Mahnken AH, Sinha AM, Wildberger JE, et al. The influence of motion artifact conditioned by reconstruction, on the coronary artery calcium score in multislice spiral CT. Rofo Fortschr Geb Rontgenstr Neuen Bildgeb Verfahr 2001 173(10): 882-92.
3. Kopp AF, Schroeder S, Baumbach A, et al. Non-invasive characterization of coronary lesion morphology and composition by multislice CT: first results in comparison with intracoronary ultrasound. Eur. Radiol November 2001: 1607-1611.
4. Sevrukov A, Jelnin V, Kondos G. Electron Beam CT of the Coronary Arteries: Cross-Sectional Anatomy for Calcium Scoring. AJR 2001 177: 1437-1445.
5. Click RL, Holmes DR Jr, Vliestra RE, et al. Anomalous coronary arteries: Location, degree of atherosclerosis and effects on survival--a report from the Coronary Artery Surgery Study. J Am Coll Cardiol 1989 13(3):531–537.

 

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