Editing Scanco Administration (section)

===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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