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CCTA Z axis shift with 64 slice Siemens
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 Posted: Wed Apr 4th, 2007 09:00 pm
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bratboy
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I am currently using a Siemens Somatom Sensation 64-slice CT scanner to do Cardiac and other Angiographic procedures. I have been in a quandary about how to eliminate what appears to be a Z axis shift during the acquisition for CCTA’s. I have been over the dissertation by Siemens that always leads the error back to technologist error or patient motion. I noticed a "banding" effect on every CCTA done with this machine. This was explained as the contrast attenuation during the acquisition. I noticed on the ctisus.com site the same artifact. When I put the entire VRT in the lateral position as an MIP, I noticed the Z axis shift through the ribs corresponds to the banding effect. Do you have any suggestions how to remedy this or have you seen this on other scanner acquisitions?

Attachment: banding effect.jpg (Downloaded 59 times)


 Posted: Thu Apr 5th, 2007 04:50 pm
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efishman
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excellent question and its your lucky day. Tony Cook from Siemens is here and has the explanation for this issue. Tony is responsible for CT Cardiac Education for Siemens and here is his response.

The "banding" artifact you see on MPR cardiac images is completely normal and caused by the fact that when gating a study, you are essentially removing "time" data from the image as contrast is being injected. This makes the the transition between the remaining ED data used to create the images much more noticeable to the eye, especially in the Right atrium where the contrast density is still changing during the scan because of inflow from the SVC. You notice this effect much less in the Left ventricle where the blood/contrast mixing is more uniform. As long as the slabs of data are properly positioned on top of each other without misregistration, you have a good scan.

You also mention a Z axis shift and this sometimes WILL cause one or more slabs to not line up properly although the ECG trace during the scan looks fine with obvious problems or PVCs. This is almost always caused by the pt swallowing or tightening the diaphram during the scan. Coronary CTAs require a negative pitch so the same anatomy is scanned over many rotations. Tighenting the abdomen even a little will sometimes cause just enough intra abdominal pressure to slightly move the heart up and down in the Z direction essentially causing a "double exposure." This will cause the data to appear to jump when you scroll through it and if you put the data into a lung window you may even see the blurring of the lung anatomy or two heart borders. There is nothing that can be done to correct this after the scan as it is pt motion and not ECG in nature. The best defense is to explain to all cardiac patients the importance of proper breathold, no swallowing and not to valsalva or move the abdominal muscles during the scan in any way.

Hope this helps!

Tony Cook R.T. (CT)



 Posted: Thu Apr 5th, 2007 08:52 pm
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bratboy
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Thank you Tony,

I understand the banding effect as you explain it. I thoroughly explain the breathing instructions to the patients to include swallowing and Valsalva movement. I added a third breath to the hyperinflation as well as leaving an additional 3 second delay from the time the API stops until the exposure begins to allow for pt "settling". I have done over 450 CCTA's on this machine alone and can't understand why every case has the same artifact. When I went back and reviewed the cases that Seimen's App's people did during the training, the artifact is there as well. The App's acquisitions were from the proprietary protocol. The acquisition was done with 3mm X 0.64 as opposed to the .75 X 0.64. With the smaller acquisition the problem is more apparent. The artifact seems to be worse at the initiation of the scan which led me to think there was a problem with the table motion during takeoff. My F.E. did a thorough check of the system and found it to be in order. The ECG leads show the same artifact on all studies as well. Perhaps it is all inclusive to the resampling and gating. Is there a possibility that the two different targets on the anode utilized by Z sharp technology and the detector configuration may be causing a minor FFD or SID anomaly? I'll take any advice and help to improve this problem.


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