Dose Limits, Shielding and Occupational Monitoring

Overview

• Radiation dose reduction in medical imaging is founded on the ALARA principle (As Low As Reasonably Achievable) and underpinned by regulatory dose limits.
• Strategies apply to both occupational (monitored, trained workers potentially exposed in the course of their duties) and public (individuals in uncontrolled areas who have not voluntarily chosen to be irradiated and may be unaware of exposure) protection.

The core methods of dose reduction are:

1. Time: minimising duration of exposure
2. Distance: maximising separation from the radiation source
3. Shielding: interposing attenuating material between source and receptor
4. Access control: restricting who can enter high-dose areas

These principles apply in both diagnostic radiology (X-ray, fluoroscopy, CT) and nuclear medicine (SPECT, PET, PET/CT), though the relative importance and practical implementation differ significantly between modalities.

Controlled vs Uncontrolled Areas

• The shielding design goal $P$ is expressed as air kerma ($K$, in mGy) at a reference point $0.3~\text{m}$ beyond the barrier.
• Effective dose $E$ (mSv) cannot be directly measured and depends on X-ray energy spectrum and individual posture, so $K$ is used as the practical surrogate for shielding design.
• Film and cassette design goals: A separate shielding design goal of $P < 0.1~\text{mGy}$ applies for the period of stored radiographic film.
• For loaded screen-film and CR cassettes awaiting use (more sensitive), $P \leq 0.5~\mu\text{Gy}$ for the storage period is recommended.

Method 1: Time Reduction

Principle

$$E \propto \dot{D} \times t$$

where $\dot{D}$ is the dose rate and $t$ is duration of exposure.

Diagnostic Radiology

• Technologists operate X-ray equipment from behind protective barriers and are rarely in the room during standard radiography or CT
• During fluoroscopy and interventional procedures, operators are present at the table; minimising fluoroscopy time (using last-image hold, pulsed fluoroscopy, appropriate frame rates) directly reduces staff dose
• Efficient technique, good preparation, and road-mapping reduce total fluoroscopy on-time

Nuclear Medicine

• Once a patient has been administered a radiopharmaceutical, the patient becomes a continuous, uncontrollable mobile radiation source
• Technologist time near patients in uptake rooms, injection rooms, and scanning suites should be minimised
• Efficient injection technique, pre-prepared doses, and fast radiopharmacy operations reduce handling time
• Patient waiting areas must be positioned and managed to minimise time staff spend near post-administration patients

Method 2: Distance

Inverse Square Law

$$I \propto \frac{1}{d^2} \qquad \frac{I_1}{I_2} = \frac{d_2^2}{d_1^2}$$

• Doubling the distance reduces intensity to one quarter.
• This is the most powerful, cost-free dose reduction tool.

Scatter in Diagnostic X-ray

At $1~\text{m}$ from a patient at 90° to the primary beam, scatter intensity is approximately 0.1-0.15% of the incident beam intensity for a standard fluoroscopy field area of $400~\text{cm}^2$.

Diagnostic Radiology

• Staff should stand as far from the X-ray tube and patient as practicable
• During mobile radiography, NCRP recommends staff stand at least $2~\text{m}$ from the X-ray tube and patient
• During mobile fluoroscopy, all individuals within $2~\text{m}$ of the patient should wear protective aprons
• In fluoroscopy, the spatial scatter pattern determines optimal staff positioning:
  • With the beam in lateral angulation, scatter is markedly higher on the X-ray tube side than the image receptor side
  • When the X-ray tube is beneath the patient (PA/AP orientation), head and arm doses to staff are minimised
• CT rooms are designed so the technologist workstation is separated from the gantry by structural shielding and distance

Nuclear Medicine

• Distance is the primary dose reduction strategy in nuclear medicine; higher photon energies (e.g. $511~\text{keV}$ from PET annihilation photons; up to $365~\text{keV}$ from SPECT radionuclides, though some emit higher-energy $\gamma$-rays at low abundance) often make complete shielding impractical
• Imaging rooms should be designed so the computer terminal and operator area are as far from the patient table as possible; the majority of a nuclear medicine technologist's annual whole-body dose is received during patient imaging
• Tongs, forceps, and syringe shields extend the effective distance between hands and radioactive sources during handling

Method 3: Protective Clothing (Personal Protective Equipment)

Limitations

• Protective aprons are effective at diagnostic X-ray energies (typically $\leq 140~\text{keV}$) where photoelectric absorption dominates, enhanced by high-$Z$ materials
• At PET energies ($511~\text{keV}$), lead aprons provide minimal attenuation; impractical thickness would be required, distance is preferred
• Aprons must be regularly inspected for cracks and defects; damaged aprons may provide false security
• Protective clothing supplements, it does not substitute for, distance and structural shielding

Dosimetry Placement

• Personal dosimetry (TLD, OSL, or electronic dosimeters) is required for workers in controlled areas.
• When aprons are worn, dosimeters are placed at collar level (above the apron) and sometimes at waist level (below apron) to estimate effective dose.

Method 4: Shielding Barriers

Fixed Structural Shielding

• Fixed shielding is designed into the room during facility construction or modification.
• It is the cornerstone of public dose reduction and reduces occupational exposure during routine operations.
• Shielding may be in walls, floors, or ceilings; in cabinets for radioactive source storage; incorporated behind image receptors; and in CT gantries.

Primary vs Secondary Barriers

• In fluoroscopy and mammography, the image receptor assembly is the primary beam stop ($U = 0$) → only secondary barriers required for room walls
• In CT, the detector array within the gantry is the primary barrier → all room walls are secondary barriers
• Tube leakage is limited by FDA regulation to $\leq 0.88~\text{mGy/h}$ at $1~\text{m}$ from the tube housing at maximum continuous current (typically $3\text{-}5~\text{mA}$) at maximum rated tube potential (typically $150~\text{kV}$)
• Interventional suites are secondary-barrier rooms but may require more shielding than general fluoroscopy rooms due to long fluoroscopy times, cine/digital fluorography sequences, and multiple X-ray tubes

Use Factor $U$

• The use factor $U$ is the fraction of workload during which the primary beam is directed at a given barrier.
• Example values: $U = 1$ for the chest bucky wall; $U = 0.02$ for a general unspecified wall; $U = 0$ for the ceiling and control area barrier in a radiographic room.

Shielding Materials

• The viewing window of a control booth must have attenuation equivalent to the adjacent wall and be large enough for unobstructed patient observation.
• The room configuration must never depend on the control booth barrier as a primary barrier, and there must be no unprotected direct line of sight from the patient or X-ray tube to the operator.
• The energising switch must be positioned so the operator cannot stand outside the shielded area to activate it.

Shielding Design Calculation Framework

The key design formula uses the dimensionless ratio:

$$\frac{NT}{Pd^2}$$

where:

• $N$ = average number of patients per week
• $T$ = occupancy factor of the adjacent area
• $P$ = shielding design goal (mGy/week)
• $d$ = distance from radiation source to the point of interest (m)

Required lead (or concrete) thickness is read from NCRP Report No. 147 charts for the appropriate room type and barrier.

• Occupancy factor $T$: The average fraction of time the maximally exposed individual is present in an adjacent area while the beam is on.
• The barrier must attenuate radiation to $P/T$.
• Full-time offices: $T = 1$; unattended storage areas: $T \approx 0.05$.

Point of closest approach (reference points):

• Wall: $0.3~\text{m}$ from the barrier surface
• Floor below: $1.7~\text{m}$ above the floor of the room below
• Ceiling/floor above: $\geq 0.5~\text{m}$ above the floor of the room above
• Workload $W$: Time integral of X-ray tube current (mA·min/week).
• NCRP Report No. 147 uses a normalised workload per patient $W_\text{norm}$ distributed across a range of kV values (e.g. $50\text{-}60~\text{kV}$ for extremities, $70\text{-}80~\text{kV}$ for abdomen, $>100~\text{kV}$ for chest), which is more accurate than older single-kV methods.

Conservative assumptions in design include: (1) no attenuation of the primary beam by the patient (patient typically attenuates by factor 10-100); (2) perpendicular beam incidence on barriers (maximum transmission); (3) ignoring other attenuating materials in the beam path.

Worked Example (Primary Barrier)

Chest bucky wall in a general radiographic room:

• $N = 120~\text{patients/week}$, $T = 0.2$, $P = 0.02~\text{mGy/week}$, $d = 2.4~\text{m}$

$$\frac{NT}{Pd^2} = \frac{120 \times 0.2}{0.02 \times (2.4)^2} = \frac{24}{0.1152} \approx 208$$

From NCRP 147 charts: approximately $1.3~\text{mm}$ lead required → specified as $4~\text{lb/ft}^2$ ($1.58~\text{mm}$) installed from finished floor to $2.1~\text{m}$ height, with $\geq 0.5~\text{m}$ lateral margins around the image receptor.

For the adjacent secondary barrier (same chest bucky wall): closest distance $= 1.5~\text{m}$:

$$\frac{NT}{Pd^2} = \frac{120 \times 0.2}{0.02 \times (1.5)^2} = 533$$

This yields approximately $0.6~\text{mm}$ lead → specified as $2~\text{lb/ft}^2$, installed continuously and seamlessly with the primary barrier.

CT Shielding

• All walls are secondary barriers; the in-gantry detector array is the primary barrier
• Scatter is highly non-isotropic: highest along the scanner axis (patient table direction), not in the gantry direction
• Assuming isotropic scatter is conservative and provides flexibility for future scanner reorientation
• Highly filtered beam at $120\text{-}140~\text{kV}$ produces highly penetrating scatter
• Three calculation methods: $\text{CTDI}_\text{vol}$-based, DLP-based (air kerma at $1~\text{m}$ per acquisition scaled by number of acquisitions), or manufacturer-provided isodose scatter maps
• High patient throughput with modern MDCT increases weekly workload; floors and ceilings may also require shielding evaluation

Nuclear Medicine and PET/CT Shielding

• PET/CT facilities have the greatest structural shielding demands in nuclear medicine; AAPM Report No. 108 provides PET/CT-specific shielding design methods
• The thick shielding required for $511~\text{keV}$ annihilation photons is generally more than sufficient for secondary radiation from the CT component of PET/CT
• For SPECT/CT, a shielding calculation must be performed for the CT system; SPECT rooms alone are often not shielded
• If a nuclear medicine imaging room is near a PET/CT or uptake room, additional shielding beyond personnel protection limits may be required to prevent high-energy photons from interfering with nearby imaging procedures
• Cone-beam CT in some SPECT/CT systems produces scatter distributions requiring specific shielding considerations

Weight management strategies for PET/CT facilities:

• Place PET/CT and uptake rooms on the ground floor (no shielding below) or top floor (no shielding above)
• Position against exterior or corner walls (eliminates shielding on one or two sides)
• Site low-occupancy rooms (storage, bathrooms, mechanical/electrical plant) immediately adjacent to uptake/PET rooms
• Place uptake rooms adjacent to each other (shared walls reduce total shielding mass)

Radiopharmacy: Requires shielded workbenches, storage containers, and potentially structural shielding; located near injection and imaging rooms to minimise transport distances.

Portable and Mobile Shielding

• Ceiling-mounted transparent lead acrylic shields in fluoroscopy suites protect the operator's head and upper body from scatter
• Freestanding mobile lead shields can be repositioned to optimise protection during interventional procedures; understanding the scatter distribution pattern (highest on the X-ray tube side during lateral angulation) guides positioning
• For mobile radiography in wards/operating theatres: maintain $\geq 2~\text{m}$ from tube and patient; use mobile shields between patient and attending personnel; during mobile fluoroscopy, all individuals within $2~\text{m}$ should wear protective aprons
• If a mobile system is routinely used in a specific location, a formal dose evaluation of adjacent areas should be performed to determine whether additional fixed shielding is necessary

Method 5: Restricting Access to Radiation Areas

Controlled Area Access Controls

• Locked doors, radiation warning signs, and training requirements for entry
• Interlocks on X-ray room doors prevent beam activation when the door is open
• The X-ray tube energising switch must be positioned so the operator cannot stand outside the shielded control area to activate it
• Radiation warning lights (beam-on indicators visible from outside the room) indicate when X-rays are being produced

Nuclear Medicine-Specific Access Controls

• Separate hot corridors or restricted zones for handling and transporting radiopharmaceuticals
• Physical barriers and signage limit public access to injection rooms, uptake rooms, and the radiopharmacy
• Patient waiting areas post-radiopharmaceutical administration are sources of continuous radiation; area design must ensure doses to staff and public remain within design goals
• Patient release criteria govern when patients administered therapeutic radiopharmaceuticals may return to public environments
• Rooms housing patients administered large therapeutic activities (e.g. high-dose I-131 therapy) may require structural shielding

Comparison: Diagnostic Radiology vs Nuclear Medicine

Post-Installation Quality Assurance

After shielding installation, a radiation protection survey by a qualified expert is mandatory:

• Visual inspection during construction to ensure barrier continuity (no gaps, voids, or misaligned lead sheets; screws penetrating lead must remain in place)
• Transmission measurements using an appropriate $\gamma$-ray source of suitable energy (e.g. Tc-99m) with a high-sensitivity, fast-response detector (e.g. GM survey meter)
• Alternatively, the installed X-ray source can generate primary and secondary radiation for barrier testing using a suitable high-sensitivity integrating survey meter
• Surveys verify that barriers achieve $P/T$ at all reference points

Summary: Key Principles

1. Time, distance, and shielding are the universal triad of radiation protection in all environments
2. In diagnostic radiology, structural shielding is the dominant tool; protective clothing and mobile shields supplement during fluoroscopy and interventional procedures
3. In nuclear medicine, distance is the primary protective tool due to higher and less readily attenuatable photon energies; facility shielding is essential but weight-intensive for PET
4. Controlled areas: $P = 0.1~\text{mGy/week}$; uncontrolled areas: $P = 0.02~\text{mGy/week}$ (air kerma design goals per NCRP 147)
5. Occupancy factor $T$ and use factor $U$ tailor shielding to actual usage patterns, avoiding over-engineering
6. Facility siting decisions (floor level, wall orientation, adjacent room types) can substantially reduce shielding mass, particularly critical for PET/CT
7. Access restriction through interlocks, physical design, and administrative controls protects members of the public who cannot be individually monitored
8. ALARA remains the overarching principle: dose reduction beyond regulatory limits is always sought where reasonably practicable