{"modality":"CT","category":"Safety","title":"CT Safety: Radiation, Dose and Protection","minutes":8,
"slides":[
{"id":1,"diagram":"title","heading":"CT Safety","bullets":["21 of 165 scored questions (12.7%)","X-ray production, dose factors, dose metrics, protection"],
"narration":"Welcome to CT Safety. This section is 21 of the 165 scored questions on the CT boards. We'll cover how x-rays are made, the factors that raise or lower patient dose, the dose numbers on your scanner, and how you protect yourself. Let's go."},
{"id":2,"diagram":"xray-tube","heading":"How the beam is made","bullets":["Thermionic emission at the cathode filament","Over 99% heat, under 1% x-rays","Bremsstrahlung makes most of the beam","Max photon energy (keV) = kVp"],
"narration":"Electrons boil off the cathode filament by thermionic emission and are accelerated by the kVp into the rotating tungsten anode. Over 99 percent of that energy becomes heat, which is why CT tubes need big heat capacity. Bremsstrahlung, or braking radiation, makes most of the beam. And remember: the beam is polyenergetic, and its maximum photon energy in keV equals the kVp."},
{"id":3,"diagram":"interactions","heading":"Photon interactions","bullets":["Photoelectric: inner shell, total absorption, about Z cubed","Compton: outer shell, main scatter to staff","Iodine K-edge 33.2 keV, barium 37.4 keV"],
"narration":"Two interactions matter most. Photoelectric absorption hits an inner-shell electron and the photon is totally absorbed. It rises sharply with atomic number, creates contrast, and adds patient dose. Compton scatter hits an outer-shell electron, dominates in soft tissue at CT energies, and is the main source of scatter dose to you. Iodine and barium light up because their K-edges, 33.2 and 37.4 keV, sit near the beam's mean energy."},
{"id":4,"diagram":"inverse-square","heading":"Inverse square law","bullets":["Intensity is proportional to 1 over distance squared","Double the distance = one quarter the intensity","Scatter is highest at the gantry opening"],
"narration":"The inverse square law is a board favorite. Intensity changes with one over the distance squared. Double your distance and you get one quarter the intensity. Triple it, one ninth. Scatter is highest right at the gantry opening in the scan plane, so every step back counts."},
{"id":5,"diagram":"kvp-mas","heading":"kVp and mAs","bullets":["Dose is directly proportional to mAs","Halve mAs: half the dose, about 41% more noise","Dose rises roughly with kVp squared","Lower kVp boosts iodine contrast"],
"narration":"Dose is directly proportional to mAs. Cut mAs in half and you cut dose in half, but noise goes up by about 41 percent, because noise follows one over the square root of mAs. Dose climbs roughly with the square of kVp, so dropping from 120 to 100 or 80 kVp saves a lot of dose and boosts iodine contrast, which is great for small patients and CTA."},
{"id":6,"diagram":"pitch","heading":"Pitch, collimation and filters","bullets":["Pitch = table travel per rotation / beam width","Higher pitch, same mA = lower dose","Over-beaming and over-ranging waste dose","Bowtie filter spares the periphery"],
"narration":"Pitch is table travel per rotation divided by beam width. At the same mA, raising pitch lowers dose, but watch out: many scanners hold effective mAs constant and simply raise the mA. Very narrow beams waste dose through over-beaming, and helical scans add over-ranging at each end, which adaptive collimators reduce. Bowtie filters strip low-energy photons and spare the periphery."},
{"id":7,"diagram":"tcm","heading":"Tube current modulation","bullets":["mA varies by angle (x-y) and along z","Localizer must cover the whole scan range","Center the patient at isocenter","GE: higher noise index = lower dose"],
"narration":"Tube current modulation matches mA to patient thickness, both around the patient and along the z-axis, using the localizer. Two rules: the localizer must cover the entire scan range, and the patient must be centered at isocenter. Off-center patients get the wrong mA and wrong bowtie filtering. And a classic trap: on GE, a higher noise index means lower dose."},
{"id":8,"diagram":"ctdi","heading":"Dose metrics","bullets":["CTDIw = 1/3 center + 2/3 periphery","CTDIvol = CTDIw / pitch","DLP = CTDIvol x scan length (mGy-cm)","Effective dose = DLP x k factor"],
"narration":"Know these formulas cold. CTDI weighted is one third the center plus two thirds the periphery, measured in a 16 centimeter head or 32 centimeter body phantom. CTDI volume equals CTDI weighted divided by pitch. DLP equals CTDI volume times scan length, in milligray centimeters. A longer scan raises DLP but not CTDI volume. Effective dose is DLP times the k factor. And remember, CTDI volume is a scanner output index, not the patient's actual dose. SSDE adjusts for patient size."},
{"id":9,"diagram":"time-distance-shielding","heading":"Protecting yourself","bullets":["Time, distance, shielding","Holder: non-pregnant family member, never staff","Dosimeter at the collar, outside the apron","CT fluoro: needle holder, low mA, short bursts"],
"narration":"Your three cardinal principles are time, distance and shielding. Stay out of the room with the door closed during exposure. If someone has to hold a patient, use a non-pregnant family member, never staff, in a lead apron and away from the scan plane. Wear your dosimeter at the collar, outside the apron. During CT fluoroscopy, use a needle holder, low mA, and short bursts."},
{"id":10,"diagram":"summary","heading":"Lock these in","bullets":["Max keV = kVp","Double distance = 1/4 intensity","Halve mAs = half dose, about 41% more noise","CTDIvol = CTDIw / pitch; DLP = CTDIvol x length","Fetal dose under 50 mGy: no anomalies (ACOG)"],
"narration":"Let's lock it in. Maximum keV equals kVp. Double the distance, one quarter the intensity. Halve the mAs, half the dose and about 41 percent more noise. CTDI volume is CTDI weighted over pitch, and DLP is CTDI volume times length. And for pregnancy, ACOG says fetal doses below 50 milligray are not associated with anomalies. Now hit Practice this chapter and test yourself."}
]}