Radiography Flashcards
As high speed electrons strike the tungsten target in an x-ray tube, ________ x-rays are produced
Bremsstrahlung and characteristic
The inherent filtration in a diagnostic x-ray tube results from ___________ and is usually specified in terms of ________.
Components of the x-ray tube itself;
mm Al equivalent
Typical x-ray transformers for general radiography have a power rating of about _______.
100 kW
What is the relationship between the mean free path (MFP) and the half-value layer (HVL)?
MFP = 1.44 x HVL
The increase in x-ray beam intensity from the target (anode) end to the filament (cathode) end of an x-ray tube is due to ___________.
The heel effect = attenuation of x-rays from interactions within the target material
What design feature(s) is(are) used in x-ray tubes to manage the potentially excessive heat production in the anode?
Angled anode, rotating anode, and use of materials with high heat-storage capacity like tungsten
How does adding filtration affect patient dose?
Patient dose is reduced since lower energy x-rays (which may not penetrate the patient) are removed. Adding filtration (beam hardening) can be a skin dose sparing method for larger patients
** This applies to fixed technique and AEC
The heel effect can be reduced by ______.
Increasing the source to image distance (SID)
How do you calculate the grid ratio?
= height of the septa divided by the width of the interspace material
Common values for grid ratios for radiographic units are ____
8-12
What is the nyquist spatial frequency of a detector?
1 / (2 * detector element size)
**Note: the spacing between detector elements is sometimes referred to as the pitch
What are typical focal spot sizes for radiography x-ray tubes?
0.6 mm - 1.2 mm
How do you calculate magnification factor?
M = (a+b)/a
a = source to object distance
b = object to detector distance
How do you calculate the length of the geometric blur of an image?
Lg = Lf * (a/b)
Lf = focal spot length
a = source to object distance
b = object to detector distance
What are some possible grid related artifacts to be aware of?
1) Grid lines visible = moving grid bucky not function properly or grid frequency too large for detector pixel spacing
2) High intensity in center of image, fades to sides = incorrect focal length for grid. Image acquired at wrong SID for grid
How do you calculate Deviation Index (DI)?
DI = 10log10( EI / EI_t)
EI = exposure index
EI_t = target exposure index
How to calculate scatter to primary ratio?
SPR = S/P
S = energy deposited by scatter
P = energy deposted by primary beam
What is the space-charge effect?
Collection of electrons around filament. Negative charge buildup from electrons limits tube current unless tube potential is increased
