[Intl_DxMedPhys] rotation time artifact

Mon Jan 5 22:32:17 EST 2026

Thank you, Dr. Szczykutowicz, for chiming in and pointing out my misunderstanding as well. Yes, with the issue of less angular sampling, adjusting mAs and pitch is nonsensical.

Weirdly enough, though, at such low rotation time, Philips’ dynamic focal spot deflection technique should kick in and provide enough angular sampling coverage, and it seems as if it is not applied in Idris’ images. I forgot where I could check for the views per rotation information, but I will pay attention to my next survey.

Best,

Sang Hoon

From: Szczykutowicz, Timothy P <TSzczykutowicz at uwhealth.org>
Date: Monday, January 5, 2026 at 11:50 AM
To: Cuddy-Walsh, Sarah (HC/SC) <sarah.cuddy-walsh at hc-sc.gc.ca>, Idris Elbakri <idris at canqap.co>, intl_dxmedphys_wd_osu_list at lists.osu.edu <intl_dxmedphys_wd_osu_list at lists.osu.edu>, Chong, Sang hoon (LHMC - Radiology) <SangHoon.Chong at lahey.org>
Subject: [External] Re: [Intl_DxMedPhys] rotation time artifact

I totally agree with Sang Hoon.

Looks like classic view angle aliasing (view angle aliasing always has streak like line artifacts that go in straight lines, whereas detector aliasing has curved lines). Some vendors use a variable sampling rate to keep the total number of views per rotation fixed, other dont. It looks like at faster rotation times,  the sampling rate is not high enough for this phantom. That looks like a soft tissue kernel, it probably is even worse in a lung/bone kernel? I think the only thing you could do is increase rotation time as changing mA or pitch probably wont change the angular range of each sample.

Detector aliasing you can usually see in a bone/lung kernel coming off a drainage tube, it will have the curved lines. I pasted some examples from my book on this below.

-stick

[cid:867a9072-9804-465b-9f02-df781223b966]
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[cid:f6527ec0-02e6-4ad1-a7ff-387b4d11f286]

Timothy P. Szczykutowicz, Ph.D., DABR
Professor
Departments of Radiology, Medical Physics and BME
University of Wisconsin Madison
Cell# 1-716-560-7751<tel:(716)%20560-7751>
Office# 1-608-263-5729
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From: Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list-bounces+tszczykutowicz=uwhealth.org at lists.osu.edu> on behalf of Chong, Sang hoon (LHMC - Radiology) via Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list at lists.osu.edu>
Sent: Friday, January 2, 2026 3:44 PM
To: Cuddy-Walsh, Sarah (HC/SC) <sarah.cuddy-walsh at hc-sc.gc.ca>; Idris Elbakri <idris at canqap.co>; intl_dxmedphys_wd_osu_list at lists.osu.edu <intl_dxmedphys_wd_osu_list at lists.osu.edu>
Subject: Re: [Intl_DxMedPhys] rotation time artifact

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Dear Idris,

0.33s is pretty close to the minimum rotation time of the CT scanner (0.27 s), and your artifact might have been caused by not enough sampling. Is this the first time seeing such an artifact from the particular protocol?

I’d suggest increasing mAs, decreasing pitch, or using a different reconstruction algorithm, and see if that improves the artifact. Or to test, maybe try using a smaller phantom and see if the problem persists. I hope someone else can also chime in to confirm.

Best,

Sang Hoon

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Sang Hoon (Bryan) Chong, Ph.D.

Diagnostic/Nuclear Medicine Medical Physicist

Department of Radiology

41 Mall Road P.O. Box 541 Burlington, MA 01805

T: 781-744-8177 | C: 267-218-5887

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From: Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list-bounces+sanghoon.chong=lahey.org at lists.osu.edu> on behalf of Cuddy-Walsh, Sarah (HC/SC) via Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list at lists.osu.edu>
Date: Friday, January 2, 2026 at 10:11 AM
To: Idris Elbakri <idris at canqap.co>, intl_dxmedphys_wd_osu_list at lists.osu.edu <intl_dxmedphys_wd_osu_list at lists.osu.edu>
Subject: [External] Re: [Intl_DxMedPhys] rotation time artifact

Unclassified / Non classifié

I haven’t seen this before but am intrigued. Is the DAP the same for all protocols (i.e. does the dose rate increase as the rotation time decreases?). Have you tried checking the position of the detector/x-ray source in the header for each projection? What is the extent of the rotation (ex. 210 degrees)?

Happy New Year!

Sarah Cuddy-Walsh, Ph.D.

From: Idris Elbakri <idris at canqap.co>
Sent: 2025-12-31 7:27 AM
To: intl_dxmedphys_wd_osu_list at lists.osu.edu
Subject: [Intl_DxMedPhys] rotation time artifact

Dear colleagues, I am curious to see if anyone has encountered a CT artifact that appears when certain rotation times are used. This artifact started appearing a few weeks ago on our head protocol on a Philips Spectral 7500. I investigated a

Dear colleagues,

I am curious to see if anyone has encountered a CT artifact that appears when certain rotation times are used. This artifact started appearing a few weeks ago on our head protocol on a Philips Spectral 7500. I investigated a bit and found that the artifact appears when the rotation times are short (0.33 and 0.4 seconds) and disappears at 0.5 sec or higher. Is the fast rotation inducing some sort of vibration? Nothing in the protocol has changed from a few weeks ago when there was no artifact.

Upon further ingestigation I also think the artifact appears in other protocols, but is not as obvious, and its dependence on rotation time is not evident. I still suspect some sort of mechanical effect.

I suspect the vendor will push back and say that there is nothing wrong with the scanner as they have just completed a PM. I appreciate any insight you can provide if you have seen something like this and how you might have resolved it.

Cheers,

Idris Elbakri, PhD, MCCPM

PS. Figure from left to right: 0.33, 0.4, 0.5 and 0.75 sec rotation times.

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