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Chapter 39

Physicians rely on imaging more because: - x-ray exams diagnostic accuracy and efficacy has improved-more difficult procedures are being completed in x-ray
Two important variables in the estimation of patient dose: -Efficiency of x-ray production-Image receptor speed
Four ways to estimate patient dose: Entrance Skin Exposure (ESE)Skin DoseMean Marrow DoseGonadal Dose
Entrance Skin Exposure (ESE) -Most often referred to as patient dose-Looking at epidermal layer of skin-Easy and accurate to measure-Calculated at min. SOD-Usually meausred with TLD
Nomogram used by physicist to measure ESE in absence of a patient
PA Chest ESE 10 mrad
Extremity ESE 10 mrad
Lateral skull ESE 80 mrad
cervical spine ESE 110 mrad
thoracic spine ESE 180 mrad
abdomen ESE 220 mrad
Fluoro: cassette-loaded spot films ESE 200 mR per view
Fluoro: photo spots ESE 100 mR per view
Fluoro: digital fluoro ESE 200 mR per frame
Cineradiography ESE 1000 mR/s at 15 frams/s
Mean Marrow Dose -contains large number of stem or precursor cells and blood cells that could be depleted-only estimated (can't be done accurately)-important because bone marrow is the target organ believed responsible for radiation-induced leukemia
Average fluoroscopic examination ESE 4 R/min
Gonadal dose importance: because of possible genetic responses to medical x-ray exposure
US mean marrow dose from diagnostic x-ray examinations averaged over the entire population is: approximately 100 mrad/yr
Genetically significant Dose (GSD): the gonadal dose that, if received by every member of the population, would produce the total genetic effect on the population as the sum of the individual doses actually received
US GSD: 20 mrad/yr
Glandular Dose (Dg): approx. 15% of the ESE
Glandular Dose should not exceed: 100 mrad/view with contact mommography200 mrad/view with magnification without grid300 mrad/view with a grid
2 concerns related to patient dose in CT: skin dose and dose distribution during the scanning procedures
Dose increase in CT is contributed to: -some overlap of the margins of the x-ray beam occurs when each single section is made-some radiation scatter from the slice being made into the adjacent slice
Scan pitch ratio: -the relationship between the movements of the patient table and the x-ray beam collimation-1:1 pitch ratio complarable with conventioanal CT-Higher pitch ratios = lower patient dose
Signal-to-noise ratio affected by: -too few photons (mAs) used-the slection of pixel size-the selection of slice thickness
ESE for CT scan: approx. 5000 mrad per scan
potential response to radiation exposure it dependent on: Time (second-tenth week)Dose (no exact dose info known)
Major organogenesis: Second - Tenth week of pregnancymain developmental period of organ systems
If pregnant femal must be examined, use: tight collimationhigh kVp techniquesshielding
Factors affecting unnecessary dose: unnecessary examsrepeat examsradiographic techniqueintensifying screenspatient positioning and shielding
unnecessary exams: -routine examss when there is no precise medical indication-mass screening for TB-hospital admissions-pre-employment physicals-periodic health exams
types of specific area shielding: contact shieldsshadow shieldsbreast shieldsthyroid shields
Ways to reduce patient dose: communication, positioning, projection, immobilization, technical factors, collimation, shielding, compression, grids (increase), screens, processing
Created by: 509420642