Editing Scanco Administration (section)

==Dose Calculations==

===XtremeCT II radiation dose===

In late 2023, Laura Cerbone visited the lab and did a series of X-ray dose calculations with equipment provided by Errol Stewart at AHS. She wrote a full report titled REPORT Dose assessment on the XtremeCT II (9Jan2024).pdf

For the knee protocol we have 936 mAs (for the 6 stacks), meaning 156 mAs for a single 168 slices stack.
For the tibia protocol we have 71 mAs.
Rescaling the DLP or the CTDI we measured for the tibia we would have:
 
 DLP(knee) = (156/71)*1.04 = 2.28 mGy
 CTDI(knee) = (156/71)*10.2 = 22.40 mGy

As for the k factors, there is no k-factor for the knee in that table, but Laura found a paper (“Estimates of Effective Dose for CT Scans of the Lower Extremities”, DOI: 10.1148/radiol.14132903) 
where they computed the k-factor for lower extremities for a spiral CT. The kV and the current used for this study are slightly different (e.g., we have a cone beam CT, not a spiral).
Nevertheless, using their k-factor to convert the DLP(tibia) to the effective dose we got 2µSv (lower but of the same magnitude as the one declared by Scanco). In principle, one could 
use the factor they found in this paper (0.0004 mSv/(mGy*cm)) for the knee, which would yield an effective dose of around 10 uSv (for a scan length of 10.2 mm, otherwise 60 uSv for the whole knee exam). 

The effective dose is computed by multiplying the DLP by the k-factor or, equivalently, multiplying the CTDI_vol (equivalent to weighted for us because we have a cone beam CT) by the scan length and the k-factor.
In our case we have
 
TIBIA:
 ED = DLP * k-factor = CTDI_vol * scan length * k-factor = 10.16 mGy * 1.02 cm * 0.0002 mSv mGy^-1 cm^-1 = 2.07 x 10^-3 mSv = 2.07 microSv
 
KNEE:
 ED = DLP * k-factor = CTDI_vol * scan length * k-factor = 22.4 mGy * 6.12 cm * 0.0004 mSv mGy^-1 cm^-1 = 0.0548 mSv = 54.8 microSv
 
Now, this calculation is not precise and it relies on factors that were computed for a different type of exam. In addition, we don’t know how they compute the effective dose at Scanco.

Stefan Hämmerle wrote on January 16, 2024:

 We go with the [https://www.icrp.org/publication.asp?id=ICRP%20Publication%20103 ICRP Publication 103] where to my knowledge the definition of effective dose comes. There you have different weighting factors for different organs and you have to take into account if an organ is only partly irradiated. For e.g. total skin surface and bone surface and so on we use some estimation from other ICRP Publications and can estimate with that the fraction of these organs we irradiate with the primary beam. Additionally we add something for the stray radiation and for the scout view. With that we end up to something below 3µSv, which we rounded up to below 5µSv to be on the safe side. 


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===XtremeCT radiation dose===
Some notes about radiation dose to help decide acceptable scanning protocols.  These comments are based on scanning an adult (not children having significant red marrow at the scan region).

:*The annual dose limit for the hand for an occupational user is 500 mSv (as per the ICRP), and with the x-ray energy of our XtremeCT that translates to 500 mGy local dose per year. For non-occupational subjects, the limit is generally 10x lower (i.e., our volunteers should not have more than 50 mGy local dose per year).
:*Our scanner delivers a local dose of about 5.3 mGy per scan.  However, if we have adjacent scans, there is some overlap due to the penumbra of the x-ray beam which results in about double the local dose (a conservative guess would be 10 mGy) for a ~1mm overlap region. Averaging over the whole scanned regions (overlap and non-overlap regions) we can say that the average delivered local dose is about 7-8 mGy. The local dose is not additive.  If we're very conservative, and we take 10 mGy as the local dose, then we could theoretically scan a person 5 times before exceeding the 50 mGy limit based on a tenth of the occupational limit (500 mGy).


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===vivaCT CTDI dose===
====Dose calculation====

Assuming the following settings on the vivaCT for "high mode", we will calculate the CTDI dose.

 2000 projections (360 degrees)
 0.109 mA
 200 ms IT

I calculate the time current value as:

 0.109 mA * (0.200 + 0.012) S * 2000 proj= 46.22 mAs

The 0.012 is the integration time between projections.

Taking the proportion of the 68 mAs at 55 kV resulting in 720 mGy (see the document on example
dose calculation on this Wiki site), we have:

 46.22 / 68 * 720 = 490 mGy

Since CTDI in air is approximately two times the weighted CTDI, and since our setup is more like in air, we get:

 2 * 490 = 980 mGy per scan.

It's always a good idea to look at the time-current product in the control file to get an approximate estimate.


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====Dose chart and example====
*Refer to [[Media:VivaCT dose information-fyi.pdf|'''VivaCT CTDI Dose Calculation Example''']].

We have measured the following CT dose indices (CTDI). 
:*CTDIw:   Weighted CTDI (chart below)
:*CTDIair: CTDI in air (2 times CTDIw - calculate from chart below).

The detector was placed in the center of a 35 mm PMMA cylinder which was placed in the center of rotation.

<pre>
 70kV  300ms 85uA  Field of view 38.9mm 1000 Proj./180°:   CTDI = 260 mGy
 55kV  300ms 109uA Field of view 38.9mm 1000 Proj./180°:   CTDI = 220 mGy
 45kV  300ms 133uA Field of view 38.9mm 1000 Proj./180°:   CTDI = 190 mGy

 70kV  300ms 85uA  Field of view 30.7mm 1000 Proj./180°:   CTDI = 420 mGy
 55kV  300ms 109uA Field of view 30.7mm 1000 Proj./180°:   CTDI = 350 mGy
 45kV  300ms 133uA Field of view 30.7mm 1000 Proj./180°:   CTDI = 305 mGy

 70kV  300ms 85uA  Field of view 21.5mm 1000 Proj./180°:   CTDI = 850 mGy
 55kV  300ms 109uA Field of view 21.5mm 1000 Proj./180°:   CTDI = 720 mGy
 45kV  300ms 133uA Field of view 21.5mm 1000 Proj./180°:   CTDI = 620 mGy
</pre>

 high res = 1000 Proj./180°
 medium  = 500 Proj./180°
 standard  = 250 Proj./180°

The CTDI scales linear with the integration time (s), with the current (uA) and number of projections. If you have different settings there, it is easy to calculate the CTDI.

CTDI Definition:
<pre>
Integral from +50mm to -50mm of D(z)/NxT
</pre>
:*where D(z) is the dose profile along a line z which is perpendicular to the tomographic plane
:*N is the number of tomographic sections produced in a single rotation
:*T is the nominal tomographic section thickness

Example: How to estimate how much radiation would be incurred by a mouse scanned at 25
um resolution (medium resolution -- MR) at 55 kVp, 109 uA, 150 ms IT, for about 30 minutes?

If you take your settings you can calculate the CTDI.  First thing you have to be aware is that the CTDI is something like the local dose but it takes the cone beam geometry and the not perfect collimation of the x-ray beam into account (The CTDI value is always higher than a local dose measurement with for example TLDs). It does not matter how many stacks you are scanning. The dose you have at one point which is within the volume you are scanning gets the same local dose if you are performing a 1 stack or a 4 stack measurement.

With the 55kVp/109uA HR with 300ms we have a value at the 30.7 mm position of 350 mGy. Because you have MR we can divide the values by a factor of 2; and because you have 150ms instead of 300ms there is an other factor of 2. You would get 87.5 mGy for the position 30.7 mm. Because of the cone beam the dose has a quadratic dependency to the diameter of the field of view. (30.7 mm/25 mm)2 = 1.5. The CTDI value for your settings is in the order of 87.5 mGy * 1.5 = 132 mGy.

Note that for in vivo measurements, we should multiply the result by a factor of two since we are doing the measurement in AIR, not WATER.  The charts above are assuming the CTDI done in water.


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