Radiation in imaging and nuclear medicine

The principles from Lesson 9 apply everywhere, but each area of the hospital that uses radiation has its own patterns of exposure and its own controls. This lesson walks through the main ones: general X-ray rooms, portable X-rays on the wards, fluoroscopy in the operating room and interventional suites, CT, nuclear medicine, and patients who carry radioactive material back to the units. For each, the key questions are the same: where does the radiation come from, who is near it, and how do time, distance and shielding apply.

The main source: scatter from the patient

In X-ray imaging, the worker is almost never in the main beam. The radiation that reaches staff is scatter: X-rays that hit the patient's body and bounce off in all directions. The patient becomes the source. Scatter is much weaker than the main beam, but it is spread around the room, and staff who stand near the patient during many procedures can accumulate a significant dose over time. Three facts follow:

• The more radiation goes into the patient, the more scatter there is. Lowering the patient dose, with good technique, short beam-on times and the smallest field size, lowers staff dose too.
• Scatter is strongest on the side of the patient where the beam enters, which in most fluoroscopy is the side nearest the X-ray tube.
• Scatter follows the inverse square law from the patient, so stepping back works.

General X-ray rooms

Fixed X-ray rooms are designed with shielded walls, doors and a control area behind a leaded window, so that the medical radiation technologist stands behind shielding during every exposure. Ontario's X-ray Safety regulation and the Healing Arts Radiation Protection Act require X-ray equipment to be registered and installed according to an approved plan, with shielding checked by a qualified expert. Lights and signs at the door warn when the beam is on, and nobody else should be in the room during an exposure unless they are holding or helping the patient and wearing protection.

Portable X-rays on the units

Portable X-ray machines bring imaging to patients who cannot be moved: in intensive care, the emergency department, operating rooms and wards like 4 West. Here, nurses, other patients and visitors can be close by. The doses are small for any one exposure, but the habits matter because the exposures add up.

• Before the exposure, the medical radiation technologist calls out a warning, such as "X-ray", so others can step away.
• Staff who do not need to be there move at least 2 m (6 ft) from the patient, out of the direct line of the beam. At that distance the scattered dose from one portable exposure is tiny.
• Staff who must stay close, for example to hold a patient's position, wear a lead apron and stand to the side, not in line with the beam.
• Other patients in a shared room are usually at least 2 m (6 ft) away; curtains give privacy but no shielding.
• Pregnant workers step out of the room.

At Northbridge, nurses on 4 West used to leave the room for every portable X-ray, even when it meant leaving an unstable patient alone. After Dr. Ward showed them the dose arithmetic from Lesson 9, the unit changed its practice: nurses step back to the foot of the bed or the doorway, about 2 to 3 m (6 to 10 ft) away, and stay in sight of the patient. Their doses, measured with badges for three months, stayed at background.

Fluoroscopy

Fluoroscopy uses a continuous or pulsed X-ray beam to produce live moving images. It is used in interventional radiology, cardiac catheterization labs, the operating room (for orthopedic and vascular surgery, often with a C-arm), endoscopy and pain clinics. Procedures can last an hour or more, with staff standing beside the patient, which makes fluoroscopy the main source of occupational radiation dose in most hospitals.

Controls in fluoroscopy work at several levels:

• Equipment and technique: pulsed fluoroscopy at the lowest pulse rate that gives an adequate image, last-image hold instead of more beam time, tight collimation to the area of interest, keeping the image receptor close to the patient and the tube as far away as practical, and the tube under the table so the strongest scatter goes toward the floor.
• Fixed and mobile shielding: table-mounted lead skirts that block scatter from below, ceiling-suspended leaded acrylic shields positioned between the patient and the operator's head, and mobile shields for staff who do not need to be at the table.
• Personal protection: lead or lead-equivalent aprons that wrap around the body, thyroid collars, and leaded glasses with side shields, since cataracts are the deterministic effect most often found in interventional staff. Aprons are checked for cracks every year and stored hanging, not folded.
• Distance and time: staff who are not needed at the table stand back; the operator keeps hands out of the beam; beam-on time is recorded and reviewed.
• Monitoring: collar badges outside the apron, with ring badges or electronic dosimeters for operators.

CT

CT scanners deliver higher patient doses than plain X-rays, but staff are rarely exposed, because the scan is run from a shielded control room. The exceptions are CT-guided procedures, where a physician may work beside the patient during scanning; the same apron, shield and distance rules as fluoroscopy apply. Family members who stay with a child during a scan wear an apron.

Nuclear medicine

Nuclear medicine uses radioactive drugs, called radiopharmaceuticals, given by injection, by mouth or by inhalation. The drug goes to an organ or a tumour, and a camera detects the gamma rays or, in PET scanning, the radiation from positrons. Some radiopharmaceuticals are used for treatment, delivering radiation directly to a cancer.

Unlike X-rays, these materials keep emitting radiation until they decay, and they can spread as contamination. Workers are exposed in two ways: external dose from vials, syringes and patients, and contamination of skin or surfaces from spills and from patients' body fluids. Common radiopharmaceuticals decay at very different rates.

Controls in the nuclear medicine department include syringe and vial shields made of lead or tungsten, tongs for handling vials, shielded workstations with leaded glass, drip trays and absorbent pads, disposable gloves changed often, personal and ring dosimeters, a ban on eating and drinking, and routine surveys of hands and work surfaces with a contamination meter. Radioactive waste is stored behind shielding until it has decayed, usually ten half-lives, after which it can be disposed of as ordinary waste once a survey confirms it is at background.

Decay arithmetic is simple and is used every day: after each half-life, half the radioactivity remains. A short calculation shows why technetium waste can be cleared in a few days while iodine-131 waste must be held for months.

# Radioactive decay: fraction remaining after time t
half_lives = {"Tc-99m": 6.0, "F-18": 1.83, "I-131": 8.0 * 24, "Lu-177": 6.7 * 24}  # hours

def remaining(t_hours, half_life_hours):
    return 0.5 ** (t_hours / half_life_hours)

print(f"{'nuclide':8}{'1 day':>10}{'1 week':>10}{'10 half-lives':>16}")
for name, h in half_lives.items():
    day = remaining(24, h)
    week = remaining(24 * 7, h)
    ten = 10 * h
    span = f"{ten / 24:.1f} days" if ten >= 24 else f"{ten:.0f} hours"
    print(f"{name:8}{day:10.4f}{week:10.4f}{span:>16}")
print(f"after 10 half-lives, fraction left = {0.5 ** 10:.5f} (about 1 in 1000)")

nuclide      1 day    1 week   10 half-lives
Tc-99m      0.0625    0.0000        2.5 days
F-18        0.0001    0.0000        18 hours
I-131       0.9170    0.5453       80.0 days
Lu-177      0.9017    0.4847       67.0 days
after 10 half-lives, fraction left = 0.00098 (about 1 in 1000)

Patients who carry radioactivity

Some of the highest nuclear medicine exposures happen outside the department, when patients return to a unit or stay in hospital after treatment. For diagnostic scans with technetium-99m, the patient's radioactivity is small and decays within a day; normal care with standard precautions is enough, though pregnant staff may limit close contact for the rest of the day.

Patients treated with large amounts of iodine-131 or lutetium-177 need more planning. Depending on the amount given and the rules that apply, the patient may stay in a shielded single room with its own bathroom, with the floor and surfaces covered, and with signs on the door. Nursing care is planned to limit time at the bedside, staff stay at a distance when they can, and care is shared among staff rather than given by one nurse. Body fluids are treated as contaminated; linen and waste are held and surveyed by the radiation safety staff before they leave the room. Pregnant staff and visitors do not enter. The patient is discharged when their measured dose rate falls below the release limit, with written instructions for home.

Patients who have had radioactive seeds implanted, such as iodine-125 seeds for prostate cancer, emit low levels of radiation that are mostly absorbed in their own bodies. Staff need no special precautions beyond knowing that a lost seed, found in a dressing or a urinal, must be picked up with forceps, placed in a shielded container and reported to the radiation safety officer, never thrown in the garbage.

In Canada, nuclear medicine departments hold a licence from the CNSC, which sets the conditions for handling, storing and disposing of radioactive material, for designating NEWs, and for releasing patients. Each licence names a radiation safety officer who runs the radiation protection program.

Spills and emergencies

A spill of a radiopharmaceutical is handled like a chemical spill with extra steps: warn others, cover the spill with absorbent material, keep people who may be contaminated in the area until they are checked, call the radiation safety officer, and do not clean up a large spill without the radiation safety staff. Contaminated skin is washed with lukewarm water and mild soap, not scrubbed. The area is surveyed after cleaning to confirm contamination is gone.