Shielding for Imaging Systems
Diagnostic Xray system shielding is covered by NCRP147 and AAPM TG108. The methodology used for shielding imaging systems shares many similarities with linac vault shielding and NCRP151.
CT Shielding
\begin{equation} B = \frac{P \ d^2}{T \ K^1 \ N} \end{equation}
 B is the maximum shielding transmission factor.
 P are the shielding design goals.
 Controlled areas: 0.1mGy/week, 5mGy/year
 Uncontrolled areas: 0.02mGy/week, 1mGy/year.
 d is the distance from the bore center to the point of interest.
 T is the occupancy factor.
 K^{1} is the air kerma per patient at a distance of 1 meter.
 This may be determined using the CTDI_{100} method, the DLP method, or the isodose map method.
 N is the number of patients per week.
Note that there is no use factor for a CT scanner. This is because CT rooms have no primary barriers because of the image detectors attenuation of the beam.
Key point: NCRP147 specifies shielding design goals in air kerma rather than dose equivalent or effective dose. This is because air kerma is readily measurable and the relationship between air kerma and effective dose depends on a variety of factors including orientation and posture of the individual.
Determining the thickness of shielding material required to reach a given transmission factor is more complicated than in MV shielding calculations because the attenuation coefficients of materials vary strongly with energy spectrum for kV photons.
\begin{equation} \textrm{barrier thickness } = \frac{1}{\alpha \gamma} ln(\frac{(\frac{NTK^1}{Pd^2})^{\gamma} + \frac{\beta}{\alpha}}{1 + \frac{\beta}{\alpha}}) \end{equation}
Methods for determining Air Kerma per patient (K^{1})
CTDI_{100} Method
This method uses measured values of
CTDI_{100} and an empirically derived κ value to determine K^{1}.
\begin{equation} K^1 = \kappa \frac{L}{p}CTDI_{100} \end{equation}
 P is the pitch
 L is the scan length
 κ is the scatter fraction per centimeter for head or body phantom
 κ_{head} = 9 x 10^{5} cm^{1}
 κ_{body} = 3 x 10^{4} cm^{1}
Dose Length Product (DLP) Method
Many modern CT scanners display doselengthproduct (DLP) for their scans. This provides a simple approach to determining K^{1}.
\begin{equation} K^1 = \kappa \textrm{DLP} \end{equation}
 κ is the scatter fraction per centimeter for head or body phantom
 κ_{head} = 9 x 10^{5} cm^{1}
 κ_{body} = 3 x 10^{4} cm^{1}
Isodose Map Method
The simplest method for determining
K^{1} is to simply use a vendor supplied isodose map for the scanner.
If using this technique, care must be taken to correct for differences in acquisition technique used to create the map such as slice thickness, mAs, and kVp.
PET/CT Shielding
Shielding of a PET/CT scanner is the topic of TG108. TG108 focuses on F18 based PET scanning because it is the most common radionuclide used in PET imaging. Additionally, because of F18’s relatively long half life (110 minutes), shielding calculations based on F18 are also adequate for other, shorterlived radionuclides such as O15 (T_{1/2} = 2 minutes), N13 (T_{1/2} = 10 minutes) and C11 (T_{1/2} = 20.4 minutes).
\begin{equation} B_{Uptake \ room} = \frac{10.9 \ P \ d^2}{T \ N_w \ A_0 \ t_u \ R_{tU}} \end{equation}
\begin{equation} B_{Scanner \ room} = \frac{12.8 / P / d^2}{T \ N_w \ A_0 \ F_u \ t_I \ R_{tI}} \end{equation}
 B is the maximum shielding transmission factor.
 P the shielding design goal.
 Controlled areas: 0.1mSv/week, 5mSv/year
 Uncontrolled areas: 0.02mSv/week, 1mSv/year.
 d is the distance from the center of the patient to the point of interest (m).
 T is the occupancy factor.
 N_{W} is the number of patients per week.
 A_{0} is administered activity (MBq).
 t_{U} and t_{I} are the time in uptake room and time in imaging room respectively (hr).
 F_{U} is the uptake decay factor.
 \( F_U = e^{\frac{.693 \ t_u}{T_{1/2}}}\)
 R_{tU} and R_{tI} are the dose reduction over uptake and imaging times.
 \(R_t = 1.443 \times (\frac{T_{1/2}}{t}) \times [1e^{\frac{0.693 \ t}{T_{1/2}}}]\)
 R_{t} = 0.91 for 30 minutes
 R_{t} = 0.83 for 60 minutes
 R_{t} = 0.76 for 90 minutes
Key point: Typical PET/CT shielding requirements will be dominated by the 511keV photons that result from positronelectron annihilation from the PET radionuclide.
Room Type  Typical Shielding 

High Energy Linear Accelerator Primary Barrier 

High Energy Linear Accelerator Secondary Barrier 

High Energy Linear Accelerator Door 

Ir192 HDR Suite 

PET/CT Room 

CT Room 

Radiographic Suite 

Knowledge Test
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