Consideration needs to be made to the radiation dose to both the patient and radiologist in CT fluoroscopy, given the relatively high and continuous radiation exposure involved.
Radiation exposure to the patient
The patient surface dosage may range between 2 and 10 mGy/sec, with exposure times lasting up to 200 seconds. There is potential for radiation-induced deterministic effects to the skin in long, high-dose CT fluoroscopic procedures. For safety reasons, departments should monitor the patient dose based on departmental phantom studies and cease the procedure if the deterministic threshold is reached.
Radiation exposure to the operator
Consideration needs to be made to the radiation exposure to the operators during CT fluoroscopy. The radiation exposure to personnel can be substantial and comparable to patient exposures at fluoroscopy during cardiac catheterization and interventional radiology.
The most significant area of radiation exposure to the radiologist is the hands. The radiologist’s hands will be close to the scan plane during image acquisition as they manipulate the needle under real-time imaging. This should be measured using a thermoluminescent dosimeter disc or ring. The absorbed dose to the hands in direct beam is approximately 1.1mGy per second. Other at risk organs from a radiation exposure perspective include the thyroid and lens of the eye. Departments will usually have a radiation safety threshold for staff on a per annum basis. Deterministic and stochastic effects could be developed if the radiation exposure exceeds 500 millisieverts per year and 50 mSv per year, respectively.
Strategies to minimize exposure to the operator
Time and technique
- minimize CT fluoroscopic scanning time. Consider incremental needle insertion outside the gantry, with intermittent short CT fluoroscopic ‘flashes’ to check for needle tip location
- use of lower-exposure CT fluoroscopic techniques (use lower mAs)
- reduce section thickness (e.g. use 2 mm or 5 mm thickness, rather than 10 mm)
- keep fingers out of the primary beam
- maximize distance from the primary beam and patient (inverse square law)
- shielding and other protective devices
- use purpose-built needle holders to increase the distance of the hands from the primary beam. A significant dose decrease can be achieved beyond 4cm from the beam (for example, a hand dose of 1.15 mGy per second in direct beam is 19 microsieverts 4 cm outside of the direct beam).
- use a lead or tungsten-antimony drape placed over the patient, adjacent to the scan plane. A lead apron placed 2.5 cm caudal to the scan plan reduces exposure by approximately 70%.
- use of a vertically fenestrated gantry shield/drape can reduce dose by up to 30%
- use of personal lead protection e.g. lead glasses (greater than 0.1mm lead reduces eye dose by 50%), lead aprons, thyroid shields (reduce dose by > 95%), lead gloves
- use the distal side of the gantry, where shielding can be provided by electronic racks inside the gantry.
- 1. Jerrold T. Bushberg, John M. Boone. The Essential Physics of Medical Imaging. (2011) ISBN: 9780781780575
- 2. Kyle, Sam. "Physics Notes", 2018