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CARDIAC RADIOLOGY: SYMPOSIUM Table of Contents   
Year : 2007  |  Volume : 17  |  Issue : 1  |  Page : 29-36
Pictorial essay: Artifacts in cardiac CT: A practical system of understanding and correction


Department of Radiology, Jupiter Heart Scan, Mumbai, India

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How to cite this article:
Telkar HB. Pictorial essay: Artifacts in cardiac CT: A practical system of understanding and correction. Indian J Radiol Imaging 2007;17:29-36

How to cite this URL:
Telkar HB. Pictorial essay: Artifacts in cardiac CT: A practical system of understanding and correction. Indian J Radiol Imaging [serial online] 2007 [cited 2020 May 25];17:29-36. Available from: http://www.ijri.org/text.asp?2007/17/1/29/32698
Artifacts are defined as various patterns that appear in images, but are not present in the original object. There are three types of artifacts encountered during CT coronary angiography; patient related, procedure related and reconstruction related.


   I. Patient-Related Top


These may be due to

A. Obesity

B. Breath-hold

C. Movement

D. Dense calcium or metal

A. Obesity

In patients weighing more than 80 kg or with large breasts, there is increased noise resulting in wall irregularity and difficulty in differentiating from plaques [Figure - 1],[Figure - 2].

To eliminate noise, we can:

a. Try to scan the patient at a lower pitch, with overlapping images.

b. Use a smoother kernel.

c. Use a higher mAs [Figure - 2].

d. Use more contrast.

B. Breath-Hold

Holding one's breath for even as short as 8 to 10 seconds can be a problem for some patients [Figure - 3]. To eliminate this problem, the patient must practice breath-holding before the procedure and if required an attendant can be placed in the room next to the patient's head for nose-holding. Slice collimations of 1.2mm can help reduce the scan time, while still maintaining adequate image quality [Figure - 4].

Despite all these steps, in about 1-5% of patients, breathing related artifacts still occur.

C. Patient Movement

Good communication with the patient before the procedure eliminates anxiety and fear and reduces motion related artifacts.

D. Streak Artifacts

The presence of metal, especially after a bypass-surgery or dense calcium can lead to streak artifacts [Figure - 5].


   II. Procedure-Related Top


A. ECG

B. Poor planning

C. Contrast-related.

A. ECG-Related

1. Metallic electrodes on the anterior chest wall.

2. Improper placement of electrodes, resulting in baseline tremors, waveform artifacts and shallow trace.

1. Metallic electrodes on the anterior chest wall

Due to immense improvement in the resolution of the scanners, streak artifacts due to metal in the ECG electrodes are not much of a problem on the 64-slice scanners [Figure - 6].

2. Improper placement of electrodes

Poor contact of the leads or electrodes with the skin or physical contact of the female breasts with the electrode wires may result in an abnormal ECG trace.

a. Abnormal amplitudes [Figure - 7]

b. Baseline tremors [Figure - 8]

c. Wave-form artifact [Figure - 9]

The above changes can be prevented by proper placement of electrodes, with good use of jelly for proper contact and to affix the leads after raising the patients' hands above their heads, so as to prevent creasing of the skin under the lead pads. Occasionally, we can change the position of the leads, such as, by putting them on the back [Figure - 10]. Breasts can be strapped if they come in the way.

B. Poor Planning

Poor planning can generate unwanted artifacts and pseudo-stenosis [Figure - 11]. To prevent this from happening, we should scan from the level of the inferior border of the arch of the aorta downwards. The calcium score axial data can be used to identify the left main (LM). We should then start higher than this level [Figure - 12].

C. Contrast-Related

1. Streak Artifact

Streak artifacts may occur due to contrast in the SVC, at the time of starting the scan. The SVC at this time is still filling with undiluted contrast from the veins and this may give rise to dense streak artifacts over the RCA [Figure - 13]. To avoid this, we can inject from the left arm and use a saline flush.

Contrast in the right ventricle (RV) [Figure - 14]A also causes an artifact over the RCA, which can be eliminated by using a saline chaser [Figure - 14]B. However in patients with a poor cardiac output, the RV opacification may still be dense and cause artifacts.

2. Slab Artifacts

These are caused by the improper use of contrast medium. Often, the concentration of contrast medium is locally too high or the contrast medium injection may have started too early. This gives rise to "contrast" slabs [Figure - 15].


   III) Reconstruction-Related Top


These include artifacts due to

A. High heart rate

B. Variable heart rate

C. Selection of incorrect phase

D. Step artifacts due to misregistration

A. High Heart Rate

The RCA exhibits the highest velocity movement during the cardiac cycle [Figure - 16] and the greatest positional change in the x and y planes, followed by the circumflex branch of the left coronary artery, the left main coronary artery and the left anterior descending artery.

The heart rate should be as low as possible, so as to broaden the mid-diastolic band to get better temporal resolution in the data acquisition band. This is the reason for giving beta-blockers.

B. Variable Heart Rate

The problem of a variable heart rate was more pronounced in 4-slice and 16-slice scanners, where the feeling of warmth after contrast injection, would lead to an increase in the heart rate in the latter part of the acquisition.

Measures to eliminate artifacts in the above conditions

i) Multi-sector reconstructions.

In dual-sector reconstruction, the temporal window can be reduced to a maximum of 1/4 th of the gantry rotation time [Figure - 17]. This increases the flexibility and placement of the temporal window in the shortened R-R interval at high heart rates.

ii) Manipulation of the ECG trace

Various tools are available for manipulation of the ECG trace, which include one or more of the following capabilities.

a. Drag the syncs forward /backward

b. Insert extra syncs

c. Disable and delete syncs

a. Drag the syncs forward /backward

Step artifacts due to improper placement of syncs can be eliminated or reduced by changing the position of these syncs [Figure - 18].

b. Insert extra sync

In cases where the difference between two adjacent heart beats is high, inserting an additional sync helps in averaging the ECG data and resultant uniformity in the trace pattern [Figure - 19].

c. Disable and delete syncs

To overcome ectopic beats, the corresponding syncs can be disabled [Figure - 20]. In this situation, the ectopic beat is not read during image reconstruction and the data from the previous heartbeat is used to fill in the image; this negates any image variation in the ECG pattern thereby removing the step in the final image. The disadvantage is that this leads to interpolation across the R-R interval thereby resulting in loss of information and blurring in the final reconstructed image.

C. Incorrect Phase

The choice of an incorrect phase can give rise to artifacts [Figure - 21]. Choosing the correct phase eliminates these artifacts [Figure - 22].

The optimum phases are as follows

< 60 bpm - 70% and 850/900ms

61- 65 bpm - 70% for the left side and 30% for the RCA

> 65 bpm - 250/300ms (high HR)

However assessing at least two data sets, one using the percentage of the R-R interval and the other using the ms time before or after the preceding R wave, helps in avoiding artifacts and in falsely interpreting stenotic lesions.

D. Step Artifacts due to Mis-registration

Step artifacts due to intrinsic mis-registration of data are known [Figure - 23],[Figure - 24]. These can be eliminated by using one or more of the following measures:

i. Try more than one reconstruction phase

ii. Try to edit the ECG trace

iii. Sometimes a thicker reconstruction can be used, e.g. 1 mm instead of 0.75 mm

iv. A multi-sectorial reconstruction may be used.


   Conclusion Top


Artifacts that result from technical errors in image data acquisition and interpretation may be avoided with appropriate planning and execution of the scanning procedure, including instruction and practice of the patient in breath-holding, as well as the optimal selection of anatomic coverage, scanning delay, pitch and reconstruction window. To improve interpretation errors, it is essential to use multiple (at least two) reconstructions tailored to each case to avoid mis-interpretation and bad image quality. This article is based entirely on the author's experience and highlights practical issues.

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Correspondence Address:
Hemant B Telkar
Jupiter Heart Scan 756 Mancherji Joshi Marg, Parsi Colony Garden View Building, Mumbai
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0971-3026.32698

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    Figures

  [Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6], [Figure - 7], [Figure - 8], [Figure - 9], [Figure - 10], [Figure - 11], [Figure - 12], [Figure - 13], [Figure - 14], [Figure - 15], [Figure - 16], [Figure - 17], [Figure - 18], [Figure - 19], [Figure - 20], [Figure - 21], [Figure - 22], [Figure - 23], [Figure - 24]



 

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