spect/ct: how much radiation dose ct constitute
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Hybrid Imaging
Shahid YounasM.Sc. Medical Physics (Surrey) UK
M.Sc. Applied Physics (UET) Lahore
Sr. Medical Physicist & RPA
Picture courtesy of:
Shahnawaz Shahid,
Shazanan Shahid,
Shazeen Shahid
Department of Nuclear Medicine SKMCH&RC, LAHORE
SPECT CT: HOW MUCH RADIATION DOSE
Thanks to:
SPECT CT HYBRID
Material and Methodology
Reference 1: The Mnaual of SYMBIA SPECT/CT:SIEMENS, GERMANY.
We analyzed 151 patients from a dual-headed SIEMENS’ SYMBIA
T-16 unit with an integrated 16-slice CT scanner.
This CT portion of this system has,
variable tube current (20– 345 mA),
slice thickness of 0.6–10 mm,
rotational speed of 0.6–1.5 secs.
tube voltage up to 130 kVp.
Effective Dose
Reference 2: E. J. Hall and A. J. Giaccia, Radiolobiology for the Radiologist, Lippincott Williams &
Wilkins, Philadelphia, Pa, USA, 6th edition, 2005.
The quantity Effective Dose can have practical value for comparing
the relative doses related to the stochastic effects and comparing
different technologies [2]
Radiation Dose Conversation Mechanism
Reference 3: The Essential Physics of Medical Imaging, Third Edition Third, North American Edition Edition by Jerrold T. Bushberg (Author),
J. Anthony Seibert , Edwin M. Leidholdt Jr. , John M. Boone
Game of 10-10 sec [3]
Risk Factor
Reference 2: E. J. Hall and A. J. Giaccia, Radiolobiology for the Radiologist, Lippincott Williams &
Wilkins, Philadelphia, Pa, USA, 6th edition, 2005.
According to ICRP, the risk of developing a lethal cancer is
approximately 4% per Sievert. The lifetime risk for cancer has been
shown to be greater in the younger patients.
Dose Conversion
Reference 4: (AAPM, 2008), Report, Shope et al, 1981
CT dose index Volume (CTDIv) and Dose Length Product (DLP) are
two basic quantities used to estimate the average absorbed dose in
the organs and tissues based for the helical (volumetric) CT
scanners.[4]
Dose Length Product & CT Dose Indexvolume
CT Dose Calculations
Reference 5: International Commission on Radiological Protection (ICRP) Publication 53 and 80.
𝐷𝑜𝑠𝑒 𝐿𝑒𝑛𝑔𝑡ℎ 𝑃𝑟𝑜𝑑𝑢𝑐𝑡 = 𝐶𝑇𝐷𝐼𝑣 ∗ 𝑃𝑎𝑡𝑖𝑒𝑛𝑡 𝐿𝑒𝑛𝑔𝑡ℎ
𝐷𝐿𝑃 = 𝐶𝑇𝐷𝐼𝑣 ∗ 𝐿
DLP is measured in mGy*cm
CT Dose Calculations
Reference 5: International Commission on Radiological Protection (ICRP) Publication 53 and 80.
𝐸𝑓𝑓𝑒𝑐𝑡𝑖𝑣𝑒 𝐷𝑜𝑠𝑒 = 𝐷𝑜𝑠𝑒 𝐿𝑒𝑛𝑔𝑡ℎ 𝑃𝑟𝑜𝑑𝑢𝑐𝑡 . 𝑐𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 𝐹𝑎𝑐𝑡𝑜𝑟
Effective dose is measured in mSv
𝐸𝑒𝑓𝑓 = 𝐷𝐿𝑃 . 𝑘
CT Dose Conversion Factors
Reference 5: International Commission on Radiological Protection (ICRP) Publication 53 and 80.
Reference 6: Doses to Computed Tomography Examinations in the UK – 2003 Review (Table 3)
Region of Body Effective Dose per DLP for adult
(mSv per mGy.cm)
Head & Neck 0.0031
Head 0.0021
Neck 0.0059
Chest 0.014
Abdomen & Pelvis 0.015
Trunk 0.015
Nuclear Dose Conversion
Reference 7 : “Radiation dose to patients from radiopharmaceuticals ICRP Publication 53,” Annals of the ICRP, vol. 18, no. 1–4, 1987.
J. Valentin, “Radiation dose to patients from radiopharmaceuticals (Addendum 2 to ICRP publication 53) ICRP publication
80 approved by the commission in September 1997,” Annals of the ICRP, vol. 28, no. 3, pp. 1–126, 1998.
Effective dose per unit administered activity” conversion factors
listed in the International Commission on Radiological Protection
(ICRP) Publication 53 and 80.
Results
Reference 8: F. A. Mettler, W. Huda, T. T. Yoshizumi, and M. Mahesh, “Effective doses in radiology and diagnostic nuclear medicine: a
catalog,” Radiology, vol. 248, no. 1, pp. 254–263, 2008.
Primary Scan99mTc-MDP Bone Mean: 868 MBqAverage DLP: 46.98
% increase in patient dose due to CT Component
CT Dose
from
SPECT
8Diagnostic
CT Dose
NM Dose NM + CT Dose % increase
w.r.t. RP Dose
Chest 0.65 7
4.95
5.6 13.13
Abd & Pelvis 0.87 8-14 5.75 16.17
Trunk 10.45 8-12 15.4 211.11
Neck 0.29 3 5.24 5.85
All doses are in milli-Sievert (mSv)
Results
Primary Scan99mTc-MIBI ParathyroidMean: 842.5 MBqAverage DLP: 135
% increase in patient dose due to CT Component
CT Dose
from
SPECT
8Diagnostic CT
Dose
NM Dose NM + CT
Dose
% increase
w.r.t. RP Dose
Neck 0.8 3 7.58 8.37 10.55
All doses are in milli-Sievert (mSv)
Results
Primary Scan99mTc-ThyroidMean: 165.5 MBqAverage DLP: 63.5
% increase in patient dose due to CT Component
CT Dose
from
SPECT
8Diagnostic
CT Dose
NM Dose SPECT + CT
Dose
% increase w.r.t.
NM Dose
CT Region: Neck 0.01 3 2.15 2.16 0.46
All doses are in milli-Sievert (mSv)
Results
Primary Scan131I-Post-therapyMean: 2790 MBqAverage DLP: 63.5
% increase in patient dose due to CT Component
CT Dose
from
SPECT
8Diagnostic CT
Dose
NM Dose NM + CT
Dose
% increase
w.r.t. RP Dose
Neck 0.43 3 2790 2790.43 0.015
All doses are in milli-Sievert (mSv)
Secret to low dose
As images are not meant to be diagnostic in the SPECT mounted
CT consequently Effective Dose would be expected to be lower
than typical diagnostic values – This is the secret (lower mA).
Cancer Risk from SPECT
In our calculations the increased risk of cancer by additional
exposure of low does CT ranges from 0.01% to 0.04 %.
Cancer Risk from SPECT
Although imparted CT dose is quite lower however this lower dose
can be balanced by decreasing the injected radiotracer dose at the
cost of increased image time.
Thanks
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