High resolution (4 mm) tof PET-CT (positron emission tomography-computed tomography) from Philips of model Ingenuity TF is newly installed at Institute of Nuclear Medical Physics (INMP). 128 slice CT component incorporated with PET provides comparatively lower dose than the 511 keV annihilation photons associated with positron decay from PET scan. So, for designing shielding in our PET-CT facility, only 511 keV annihilation photons energy has been considered. The main objective of this paper is to show what measures have been taken to protect patients, occupational workers as well as environment from PET-CT radiation hazard through a cost effective design that satisfy the national regulatory demand. In this paper, AAPM (American Associations of Physicists in Medicine) Task group 108 analysis for PET and PET-CT shielding requirements is followed for our PET-CT facility shielding design. From theoretical calculation as shielding requirement, 1.1 cm Pb thickness or, 13 cm concrete thicknesses are found. Practically, all walls and ceiling are of 30.48 cm (1 foot) thick made of concrete with density 2.35 gcm-3 for more safety. As x-ray from CT is not taken into account for shielding analysis, Bangladesh Atomic Energy Commission (BAEC) conducted an extensive radiation survey at controlled, supervised and public area for CT. The report that is found meets the national regulatory requirements.
Bangladesh is a developing country and is now facing many challenges, especially in health sector. Government has given the priority in cancer diagnosis and management due to the current trend of cancer disease in this region. Under this consideration, nuclear tof PET-CT (time of flight positron emission tomography-computed tomography) machine for imaging purpose is newly installed at Institute of Nuclear Medical Physics (INMP) under Bangladesh Atomic Energy Commission (BAEC). Here, CT part is incorporated with PET to get fused image. This machine can diagnose cancer at molecular level. PET image resolution is very high (4 mm) imported from Philips (USA) of model Ingenuity TF; timing resolution and coincidence window are 600 ps and 5 ns respectively. In this machine, LYSO (Lu1.8Y0.2SiO5: Ce) crystals are used. The number of crystal detectors and PMTs are 28,336 and 420. PET-CT is a sophisticated device, so shielding pattern is totally different from other imaging device. 128 slice CT (80 - 140 kV) component incorporated with PET, gives comparatively lower dose than the 511 keV annihilation photons dose associated with positron decay from PET scan [
FDG Radionuclide
The radiotracer used for PET scan is fluoro-2-deoxyglucose (FDG). FDG is labeled with F18, whose half life is only 109 min. Before scanning, typically 5 to 10 mCi (185 - 370 MBq) of F18-FDG is injected rapidly into a saline drip running into a vein of a patient who has been fasting for at least 6 hours. After about an hour of injection, patient is undergone for PET scan. And it takes about 20 to 30 minutes. FDG chemical structure and positron annihilation process are shown in
Regulatory Limit
Maximum permissible dose per year (P) for radiation worker is 20 mSv (averaged over five consecutive years) and 50 mSv in a single year. For ALARA
principle it is taken as 1/10 of maximum value. So, P value is 5 mSv/yr. But, according to Bangladesh Atomic Energy Regulatory Act 2012 and Nuclear Safety & Radiation Control Rules 1997, P value should be 0.1 mSv/yr for shielding calculation in consideration with public dose. In controlled area, dose rate for radiation worker is 10 μS v∙hr−1 and 0.50 μSv∙hr−1 for that of uncontrolled area. Layout of PET-CT facility at INMP is shown in
F18-FDG administered dose varies from patient to patient. Bangladeshi people are small in size compared with other country patients. It depends on patient mass, uptake time and the acquisition mode etc. [
D • = Γ A r 2 (1)
where, Γ → Effective dose rate constant.
Since the patient itself becomes the radioactive source after the radiopharmaceutical has been administered, one has to consider the entire time that the subject remains in the clinic [
The dose D, accumulated over time t is
D = 1.44 × D • × T 1 / 2 × ( 1 − e − 0.693 × t T 1 / 2 ) (2)
and track decay of activity from moment-to-moment.
A ( t ) = A 0 × ( e − 0.693 × t T 1 / 2 ) (3)
Corresponding lead, concrete and iron thickness for broad beam transmission factors B at 511 keV is obtained from the
Another alternative way to measure thickness value is
I = B 0 × I 0 e − μ t (4)
or
I I 0 = e − ln 2 / HVL (5)
where, µ is attenuation co-efficient whose value is 1.92 cm−1 for lead in case of broad beam of energy 511 keV [
AAPM Task Group 108 recommends a realistic effective dose rate constant Γ
Thicknessa,b | Transmission factors | ||
---|---|---|---|
Lead | Concrete | Iron | |
0 | 1.0000 | 1.0000 | 1.0000 |
1 | 0.8912 | 0.9583 | 0.7484 |
2 | 0.7873 | 0.9088 | 0.5325 |
3 | 0.6905 | 0.8519 | 0.3614 |
4 | 0.6021 | 0.7889 | 0.2353 |
5 | 0.5227 | 0.7218 | 0.1479 |
6 | 0.4522 | 0.6528 | 0.0905 |
7 | 0.3903 | 0.5842 | 0.0542 |
8 | 0.3362 | 0.5180 | 0.0319 |
9 | 0.2892 | 0.4558 | 0.0186 |
10 | 0.2485 | 0.3987 | 0.0107 |
12 | 0.1831 | 0.3008 | 0.0035 |
14 | 0.1347 | 0.2243 | 0.0011 |
16 | 0.0990 | 0.1662 | 0.0004 |
18 | 0.0728 | 0.1227 | 0.0001 |
20 | 0.0535 | 0.0904 | |
25 | 0.0247 | 0.0419 | |
30 | 0.0114 | 0.0194 |
aThickness in mm for lead. bThickness in cm for concrete and iron.
is 0.092 μSv∙m2∙MBq−1∙hr−1 for F18 in patient [
P / T = 1 mSv ⋅ yr − 1 / 1 = 1 mSv ⋅ yr − 1
where, occupancy factor T = 1 [
At 3.5 m distance, with initially 420 MBq in patient, the dose rate is
D • = Γ A r 2 = 0.092 μ Sv ⋅ m 2 / MBq ⋅ h × 420 MBq ( 3.5 m ) 2 = 0.0032 mSv / hr
Assuming 40 PET patients per week and 2080 patients per year, the annual dose D in the office for a 45 min scan is then
D = 2080 pat / yr × 1.44 × 0.0032 mSv / hr × 1.83 hr × ( 1 − e − 0.693 1.83 h × 0.75 hr ) = 4.3 mSv / yr
So, broad beam transmission factors B at 511 keV is
B = 1 mSv ⋅ yr − 1 / 4.3 mSv ⋅ yr − 1 = 0.23
Concrete wall shielding thickness for B value 0.23 is found to be from
There is a lead equivalent glass window in between control console and PET-CT room. This glass thickness is calculated from
B = I I 0 = B 0 × e − μ t = 0.23 , ln 0.23 = − μ t , t = − 1.46 − 1.92 = 0.76 cm = 7.6 mm , t ≈ 8 mm
where, μ = 1.92 cm − 1 as mentioned earlier [
From theoretical calculation as shielding requirement for our PET-CT facility, 11 mm Pb thickness or, 13 cm concrete thickness are found shown in
Sl. No. | Parameter name | Technique used | Thickness value | Designed value |
---|---|---|---|---|
1 | Concrete barrier | Transmission factor | 13 cm | 30.48 cm |
Buildup factor | - | |||
2 | Lead equivalent glass | Transmission factor | 11 mm | 10 mm |
Buildup factor | 08 mm |
10 µSv/hr according to BAERA regulations. From measurement, radiation levels are found to be near background level.
In Equation (4), B0 is not needed if the beam is well collimated. In case of PET-CT shielding design, comparatively large thickness is required. So beam geometry is poor and considered to be broad-beam resulting significant number of scattered and back scattered radiation. Therefore, buildup factor B0 is applicable [
PET-CT machine is highly sophisticated imaging device. So the shielding calculation for PET-CT facility is completely different from those of other diagnostic and therapeutic facilities. In this case, the patient itself continuously emits high energy annihilation photon radiation during the whole procedure. In our facility, all shielding walls are made of concrete with thickness more than double of calculated value. The main entrance door is made of 4 mm thick lead sheet which is sandwiched in between two layer of wood. During the machine in operation, radiation survey reports meet our regulatory requirements.
The authors declare no conflicts of interest regarding the publication of this paper.
Uddin, M.F., Khatun, R., Akter, S., Jamil, H.M., Monika, A.N., Rahaman, M.A., Das, R.P., Sharmin, R.A., Rahman, M.M. and Ahasan, M.M. (2019) Radiation Shielding Analysis and Design of tof PET-CT Facility at Institute of Nuclear Medical Physics under Bangladesh Atomic Energy Commission, Bangladesh. International Journal of Medical Physics, Clinical Engineering and Radiation Oncology, 8, 1-8. https://doi.org/10.4236/ijmpcero.2019.81001