{
"id": "radiography-safety",
"modality": "Radiography",
"category": "Safety",
"title": "Radiography Safety: Physics, Dose and Protection",
"minutes": 9,
"slides": [
{
"id": 1,
"diagram": "title",
"heading": "Radiography Safety",
"bullets": [
"50 of 200 scored questions (25%)",
"Physics, radiobiology, units, patient and staff protection"
],
"narration": "Welcome to Radiography Safety. This is the second biggest section on the radiography boards, 50 of the 200 scored questions, a full 25 percent. We'll cover how x-rays are made and interact, the inverse square law, dose units, radiobiology, and how you protect your patients and yourself, including fluoroscopy limits and dose limits. There are lots of numbers here, and the boards love to test them, so let's take them one at a time.",
"svg": ""
},
{
"id": 2,
"diagram": "xray-tube",
"heading": "How x-rays are made",
"bullets": [
"Thermionic emission at the tungsten filament",
"Over 99% heat, under 1% x-rays",
"Bremsstrahlung makes most of the beam",
"Max photon energy (keV) = kVp",
"Tungsten: Z 74, K-shell 69.5 keV"
],
"narration": "Electrons boil off the heated tungsten filament by thermionic emission, get focused by the focusing cup, and are accelerated by the k V p into the anode target. Over 99 percent of that energy becomes heat, and under 1 percent becomes x-rays. Bremsstrahlung, or braking radiation, makes most of the beam. The maximum photon energy in k e V equals the k V p, not the m A. Tungsten has an atomic number of 74 and a K-shell binding energy of 69.5 k e V.",
"svg": ""
},
{
"id": 3,
"diagram": "interactions",
"heading": "Photon interactions",
"bullets": [
"Photoelectric: inner shell, photon absorbed",
"Photoelectric rises with Z cubed, low kVp",
"Compton: outer shell, fog and staff dose",
"Coherent: below 10 keV, no ionization"
],
"narration": "Photoelectric absorption hits an inner shell electron, and the photon is totally absorbed. That creates contrast and most patient dose. Its probability rises with atomic number cubed and falls with energy, so it dominates at low k V p. Compton scatter hits an outer shell electron and the photon scatters with less energy. It causes image fog and most occupational exposure. Coherent scatter happens below about 10 k e V, with no ionization. Pair production never happens in diagnostic radiography.",
"svg": ""
},
{
"id": 4,
"diagram": "inverse-square",
"heading": "Beam and inverse square",
"bullets": [
"Quality: kVp and filtration, measured by HVL",
"Quantity is directly proportional to mAs",
"Double the distance = one quarter intensity",
"Double SID = four times the mAs",
"+15% kVp roughly doubles receptor exposure"
],
"narration": "Beam quality, or penetrability, is controlled by k V p and filtration and measured by half value layer. Beam quantity is directly proportional to m A s. The inverse square law says intensity varies with one over distance squared, so double the distance and you get one quarter the intensity. The exposure maintenance formula goes the other way: double the source to image distance and you need four times the m A s. And the 15 percent rule: raising k V p 15 percent roughly doubles receptor exposure.",
"svg": ""
},
{
"id": 5,
"diagram": "si-units",
"heading": "SI units",
"bullets": [
"Exposure: coulombs per kilogram (C/kg)",
"Absorbed dose: gray; 1 Gy = 100 rad",
"Equivalent dose: sievert; 1 Sv = 100 rem",
"Weighting factor: x-rays 1, alpha 20",
"Effective dose adds tissue weighting factors"
],
"narration": "SI units are what the boards use. Exposure is measured in coulombs per kilogram of air, and air kerma, in gray, has largely replaced it. Absorbed dose is in gray, and one gray equals 100 rad. Equivalent dose is in sievert, and one sievert equals 100 rem. The radiation weighting factor is 1 for x-rays and 20 for alpha particles, so for x-rays a gray equals a sievert. Effective dose adds tissue weighting factors to estimate whole body risk.",
"svg": ""
},
{
"id": 6,
"diagram": "radiosensitivity",
"heading": "Radiobiology",
"bullets": [
"Bergonie and Tribondeau: immature, dividing cells",
"Lymphocytes most sensitive; nerve, muscle least",
"Most damage is indirect, via free radicals",
"Stochastic: no threshold (cancer, genetic)",
"Deterministic: threshold (erythema, cataract)"
],
"narration": "The law of Bergonie and Tribondeau says cells are most radiosensitive when they are immature, undifferentiated and rapidly dividing. Lymphocytes are the most sensitive blood cells, while nerve and muscle cells are the least. Most x-ray damage is indirect, through radiolysis of water making free radicals that hit DNA. Stochastic effects, like cancer and genetic effects, have no threshold. Deterministic effects, like skin erythema around 2 gray and cataracts, have a threshold, and severity rises with dose.",
"svg": ""
},
{
"id": 7,
"diagram": "collimation",
"heading": "Reducing patient dose",
"bullets": [
"Highest practical kVp, lowest mAs",
"Collimate; light field within 2% of SID",
"Total filtration 2.5 mm Al above 70 kVp",
"PA instead of AP spares breast, thyroid, eyes",
"Electronic cropping does not reduce dose"
],
"narration": "To cut patient dose, use the highest practical k V p with the lowest m A s. Collimate to the area of interest, and know that the light field must line up with the x-ray field within 2 percent of the source to image distance. Total filtration must be at least 2.5 millimeters of aluminum equivalent above 70 k V p. Use P A instead of A P when you can, to spare the breasts, thyroid and eyes. And electronic cropping after the exposure does not reduce dose. Only collimation does.",
"svg": ""
},
{
"id": 8,
"diagram": "fluoro",
"heading": "Fluoroscopy limits",
"bullets": [
"Normal mode max 88 mGy/min (10 R/min)",
"High-level max 176 mGy/min (20 R/min)",
"SSD at least 38 cm fixed, 30 cm mobile",
"5-minute timer alerts; it does not stop the beam",
"Receptor close, tube far; magnification adds dose"
],
"narration": "Fluoroscopy limits are tested constantly. In normal mode, tabletop air kerma rate is capped at 88 milligrays per minute, which is 10 roentgens per minute. High level control is capped at 176 milligrays per minute, or 20. Source to skin distance must be at least 38 centimeters on fixed units and 30 on mobile C-arms. The 5 minute timer only alerts you, it does not stop the beam. Keep the receptor close and the tube far, and remember magnification mode raises patient dose.",
"svg": ""
},
{
"id": 9,
"diagram": "dose-limits",
"heading": "Staff protection and limits",
"bullets": [
"Time, distance, shielding; distance is best",
"Aprons at least 0.25 mm Pb equivalent",
"Occupational 50 mSv/yr; cumulative 10 x age",
"Lens 150 mSv/yr; skin, extremities 500 mSv/yr",
"Public 1 mSv/yr; fetus 0.5 mSv per month"
],
"narration": "Protect yourself with time, distance and shielding, and distance is the most effective. Protective aprons must be at least 0.25 millimeters of lead equivalent. Under N C R P report 116, the occupational effective dose limit is 50 millisieverts per year, with a cumulative limit of 10 millisieverts times your age. The lens limit is 150 millisieverts per year, and skin and extremities 500. The public limit is 1 millisievert per year, and the embryo of a declared pregnant worker is limited to 0.5 millisieverts per month.",
"svg": ""
},
{
"id": 10,
"diagram": "summary",
"heading": "Lock these in",
"bullets": [
"Max keV = kVp",
"Double distance = 1/4 intensity",
"Lymphocytes: most radiosensitive blood cells",
"Occupational 50 mSv/yr; fetus 0.5 mSv/month",
"Fluoro normal max 88 mGy/min (10 R/min)"
],
"narration": "Let's lock it in. Maximum k e V equals k V p. Double the distance, and you get one quarter the intensity. Lymphocytes are the most radiosensitive blood cells. The occupational limit is 50 millisieverts per year, and a declared pregnancy limits the fetus to 0.5 millisieverts per month. And the normal mode fluoroscopy maximum is 88 milligrays per minute, or 10 roentgens per minute. Now hit Practice this chapter and test yourself.",
"svg": ""
}
]
}