Opportunistic CT

Radiation Dose

X-rays are a form of energy – like light and radio waves. X-rays are also called radiation. Unlike light waves, x-rays have enough energy to pass through your body. As the radiation moves through your body, it passes through bones, tissues, and organs differently. This allows a radiologist to create images of them. The radiologist is a specially trained doctor who can examine these images on a computer display. X-rays allow the radiologist to see the structures in your body in very fine detail.

Dose Length Product

Dose length product (DLP) is a measure of CT tube radiation output/exposure (measured in mGy.cm). It is related to volume CT dose index (CTDIvol), but CTDIvol represents the dose through a slice of an appropriate phantom. The dose length product accounts for the length of radiation output along the z-axis (the long axis of the patient). It may be described by the following equation:

DLP = (CTDIvol in mGy) . (length of scan in cm)

The dose length product does not take the size of the patient into account and is not a measure of absorbed dose. If the AP and lateral dimensions of the patient are available, then the size specific dose estimate (SSDE) can be used to estimate the absorbed dose.

Effective Dose

Some estimates of effective dose are summarized in the table below, taken from the cited sources.

Estimates of Effective Dose^1,2,3

Scan	Effective Dose (mSv)
Head	2.2
Full Spine	20.6
Cervical	3.6
Thoracic	12.0
Lumbar	5.0
Chest	6.0
Chest/Abdo/Pelvis	19.0
Abdo/Pelvis	15.6
Knees	0.16
Ankles	0.14

¹Conference of Radiation Control Program Directors (CRCPD) Nationwide Evaluation of X-ray Trends (NEXT) 2005-2006 report (https://crcpd.org/document/nationwide-evaluation-of-x-ray-trends-next-tabulation-and-graphical-summary-of-the-2005-2006-survey-of-computed-tomography/)

²CRCPD, Publication E-07-2, Tabulation and Graphical Summary of 2000 Survey of Computed Tomograph, (NEXT), August, 2007

³Biswas et al. Radiation Exposure from Musculoskeletal Computerized Tomographic Scans. The Journal of Bone and Joint Surgery, 2009.

Notes:

• Full Spine: From the base of the skull (C1) to the sacrum (through L5), effective dose of 20.6 mSv (adding up dose of all spine scans)

• Extended Chest/Abdo/Pelvis (or Full Spine + Abdo/Pelvis): From the base of the skull (C1) through the proximal femur. Effective dose of 25.2 mSv (cervical spine + chest + abdo/pelvis). Likely this is an overestimation.

Dose Length Product (DLP) of ARTININ scans is usually between 270 – 400 mGy-cm.

Converting between mGy-cm to effective dose (as described by the American Association of Physicists in Medicine (AAPM) Report No. 96):

E (mSv) = k × DLP

where k is the normalized effective dose per dose-length product for adults.

Estimated effective dose for ARTININ (as according to DLP):

E (mSv) = 0.015 × 400 
E (mSv) = 6 mSv

Note that in the ARTININ consent we list 10 mSv, which is on the high end (which would be for a larger person).

Full spine (estimates of DLP taken from a few participants in the spinal cord injury group): 705 - 958. 55m mGy-cm

Note that DLP is over twice what is observed in ARTININ

E (mSv) = 0.015 × 958.55 
E (mSv) = 14.5 mSv

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Calibration

Do some more stuff about calibration. All of these methods are available in our GitHub repository called Ogo.

Phantom calibration

This uses a phantom.

Asynchronous calibration

This also uses a phantom, but not at the same time as the scan of the patient. Sometimes it can be months or even years away.

Internal calibration

This doesn't use a phantom, but instead uses the internal tissues as referents.

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