Patient Dosimetry

 

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Three main aspects are covered

  1. ICRP dosimetry data

  2. Dosimetry data based on our model

  3. fetal dosimetry for pregnant patients

The main issue is to calculate the cumulative activity for Kr81m and for Tc99m-MAA in the lungs ie the area under the activity-time curves.

ICRP

The ICRP model for Kr81m assumes that all the Kr81m activity stays in the lungs ie only decay is assumed.  They calculate effective dose values in units of mSv/MBq.s and give the following information.

example

Assume a 300 MBq generator with an efficiency of 70% (effective Kr81m activity of 210MBq)

4 ventilation views are taken with an average time per view of 80s

each view delivers a Kr81m activity of the Kr81m spike value and the flow for 80s

Kr81m activity per view   =   210 + ( 0.053 x 210 x 80)    MBq

                                           =    1100 MBq

With the ICRP assumption that only decay removes Kr81m from the lungs, then the area under the activity time-curve for one view is

1100 x T1/2 / ln(2)   =   1100 x 13 / 0.693      =      20635   MBq.s

for four views, the total Kr81m activity-time area is therefore  82540 MBq.s

 

Our Model

We have developed a very simple model of the Kr81m administration process.

Unlike the ICRP model this takes account of the fact that the patient also breathes out, loosing Kr81m in the process.

example

with the same assumptions as above (effective Kr81m activity of 210MBq, 4 views of 80s), the area under the time activity curve for normal breathing is -

area   =   xxxxx MBq.s

The effective dose to a standard adult is =                0.04 mSv

 

Patient Dosimetry (mSv) for Kr81m and Tc99m MAA
Age Kr81m Doses for a typical 4-view study Tc99m-MAA
ICRP Our Model Activity MBq ICRP
Newborn 2.13 1.43 15 3.8
1yr 0.77 0.52 15 1.5
5yr 0.39 0.26 27 1.4
10yr 0.25 0.17 46 1.6
15yr 0.18 0.12 81 2.0
adult (female) 0.15 0.1 100 2.1
adult (male) 0.12 0.08 100 1.7

 

Fetal Dosimetry

There is essentially no transfer of Kr81m from the lungs of the patient to the bloodstream or any other organ. Therefore the radiation dose to the fetus is from the Kr81m content of the patient's lungs during the study.

The MIRD scheme has a model for pregnancy and this can be used together with the values of cumulative activity derived above.

We have presented calculations based on our model of a typical four view study

 

Fetal Dosimetry (mSv)
fetal age Kr81m (4400MBq) Tc99m (100MBq)
3 mnth 0.0005 0.02
6 mnth 0.0007 0.04
9 mnth 0.0008 0.05
 

 

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Last modified: November 29, 2002