Radiation Protection Module 4: Fluoroscopy, Barriers & MPED

What is the central goal of radiation protection?

Obtain diagnostic information with the least reasonable radiation exposure.

What three populations must radiation protection address?

Patients, personnel, and the public.

What is health physics?

Research, teaching, and operational practice relating to radiation safety, occupational protection, and population dose.

Which two concepts recur in health physics?

ALARA and the linear nonthreshold model.

What does ALARA stand for?

As Low As Reasonably Achievable.

Does ALARA prohibit medically justified X-rays?

No. It aims to avoid unnecessary radiation while obtaining medically necessary information.

What are the three cardinal protection principles?

Time, distance, and shielding.

What is the effect of doubling exposure time at a constant dose rate?

Exposure doubles.

What equation relates exposure to rate and time?

Exposure = exposure rate × time.

Which fluoroscopy features reduce time-dependent exposure?

Pulsed/intermittent operation, last-image hold, and a five-minute cumulative timer.

What is the inverse square law for radiation intensity?

I2 = I1 × (d1/d2)^2 for a point source.

Why should a technologist step away from the patient during exposure?

The patient is a major source of scatter, and greater distance reduces exposure.

Which distances are typically fixed for patients versus variable for technologists?

Patient imaging SID often 40 or 72 inches; technologist distance can be increased.

What approximate waist-height exposure rate is stated in the notes?

3 mGy/hr at a normal position.

Which two materials are common structural radiation shields?

Lead and concrete.

What is a half-value layer (HVL)?

Shield thickness that reduces beam intensity to 50%.

What is a tenth-value layer (TVL)?

Shield thickness that reduces beam intensity to 10%.

Approximately how many HVLs equal one TVL?

3.3 HVLs.

At 80 kVp, what is the concrete HVL in the notes?

1.1 cm.

At 80 kVp, what is the lead HVL in the notes?

0.19 mm Pb.

At 80 kVp, what is the concrete TVL?

3.6 cm.

At 80 kVp, what is the lead TVL?

0.64 mm Pb.

What lead-equivalent apron thickness is commonly stated?

0.5 mm Pb.

Why combine shielding with time and distance?

All three principles reduce unnecessary radiation exposure.

Which diagnostic imaging activity accounts for much occupational exposure?

Fluoroscopy.

What does effective dose (EfD) estimate?

Whole-body risk from partial-body exposure, using weighted organ and tissue doses.

Where is the dosimeter worn in the lecture example?

At the collar.

What effective-dose approximation is given for a collar badge?

Approximately 10% of its reading, under the described apron-wearing scenario.

If a collar badge reads 1 mSv, what whole-body effective dose does the lecture estimate?

0.1 mSv = 100 µSv.

What is NCRP Report No. 102 about?

Medical X-ray, electron-beam, and gamma-ray protection for energies up to 50 MeV; equipment and personnel/patient protection.

In what year was NCRP Report No. 102 published?

1989.

What is the tube-housing leakage limit given at 1 m?

No more than 1 mGy/hr under the specified maximum continuous-operation conditions.

What features should the protected control panel have?

An observation window, visible mA/kVp readouts, and audible/visible exposure indicators.

Why use a radiolucent table?

It minimizes unwanted absorption before the beam reaches the patient.

How accurate must SID indicators be?

Within 2% of indicated SID.

What is positive beam limitation (PBL)?

Automatic adjustment of X-ray field to image receptor size; no longer required per notes.

Why should collimation be tight?

It decreases patient dose and scatter production.

How closely must light and X-ray fields align?

Within 2% of SID.

What minimum total filtration is required above 70 kVp?

2.5 mm aluminum (inherent plus added).

Why is filtration important?

It removes low-energy photons that increase skin dose without improving the image.

What material is commonly used for filtration?

Aluminum.

What is reproducibility in X-ray quality control?

Consistent output with repeated exposures at the same factors.

What is the reproducibility variation limit in the notes?

5%.

What is linearity in X-ray quality control?

Output proportional to mAs across mA stations.

What is the linearity variation limit between mA stations?

10%.

How can QC failures increase dose?

They cause poor images and repeat exposures.

What adult fluoroscopy kVp range is listed?

75-110 kVp depending on body region.

What is the minimum SSD for stationary fluoroscopy?

38 cm (15 inches).

What is the minimum SSD for mobile fluoroscopy?

30 cm (12 inches).

How should the image intensifier be positioned relative to the patient?

As close as practical to reduce operator exposure.

How does increased SSD affect entrance skin exposure?

It reduces ESE.

What tabletop exposure-rate limit per mA at 80 kVp is listed?

21 mGy/min per mA.

What maximum fluoroscopy rate is listed without high-level control?

100 mGy/min.

What maximum fluoroscopy rate is listed with high-level control?

200 mGy/min.

How frequently does the fluoroscopy cumulative timer alarm?

Every five minutes.

What does DAP mean?

Dose area product, combining radiation dose and irradiated field area.

What units are given for DAP?

cGy·cm².

What happens to DAP when field area increases at constant dose?

DAP increases.

What is the primary protective barrier in fluoroscopy?

The image intensifier; 2 mm lead equivalent per notes.

What does a dead-man switch do?

Terminates X-ray exposure when released.

What protects personnel at the fluoroscopic Bucky slot?

Bucky slot cover and protective curtain, minimum 0.25 mm Pb equivalent.

What common mobile radiography SIDs are given?

100 or 120 cm.

What exposure cord length is specified for mobile radiography?

2 m (approximately 6 feet).

Where should the C-arm X-ray tube be positioned when possible?

Under the patient/table, with the receptor close to the patient.

What minimum source-to-end-of-collimator distance is given?

30 cm (12 inches).

What does a primary barrier stop?

The direct, unscattered primary X-ray beam.

What lead thickness is given for a 130-kVp primary barrier?

1.6 mm (1/16 inch) Pb.

How high should the primary barrier extend in the stated geometry?

7 feet (2.1 m) above floor.

What does a secondary barrier protect against?

Leakage and scattered radiation, not direct primary beam.

What lead thickness is specified for a secondary barrier?

0.8 mm (1/32 inch) Pb.

How far should secondary and primary barriers overlap?

Approximately 1/2 inch.

What radiation primarily reaches the control-booth barrier?

Secondary radiation.

What lead equivalence is given for the control-booth observation window?

1.5 mm Pb equivalent.

How should the control booth be secured?

Permanently attached to floor and extending 7 feet high.

What weekly exposure is expected in a well-designed facility behind the control booth?

At most about 0.02 mSv/week in actual practice.

What lead equivalence is specified for radiographic room doors?

0.8 mm (1/32 inch) Pb.

What does workload W measure?

X-ray production in mA-minutes per week.

What workload is described as a busy room?

Upwards of 500 mA-min/week.

What does use factor U measure?

Fraction of time the primary beam is directed at a barrier.

What are the listed primary-barrier use factors?

Full 1, partial 1/4, occasional 1/16.

What is the secondary-barrier use factor?

Always 1.

What does occupancy factor T represent?

Fraction of time an area beyond the barrier is occupied.

What other two factors influence barrier requirements?

Beam kVp/penetrability and source-to-occupied-area distance.

Who occupies a controlled area?

Radiation workers.

What weekly MPED is listed for controlled areas?

1 mSv per week.

Who occupies an uncontrolled area?

Members of the general public.

What weekly MPED is listed for uncontrolled areas?

0.02 mSv per week.

What product is evaluated for each barrier?

mA-minutes × U × T.