what is the formula for grid ratio?
height over distance
what is the formula for selectivity?
% primary radiation transmitted over % of scatter radiation transmitted
what is the formula for Grid conversion factor?
mAs with grid over mAs without grid
name formula used for calcuting mAs when you change 1 grid ratio to another grid ratio
mAs1/mAs2=GCF1/GCF2
formula for grid conversion factor
new grid factor/original grid factor=CF
what is the GCF for no grid?
1
what is the GCF for 5:1 ratio?
2
what is the GCF for 8:1 ratio?
4
what is the GCF for 12:1 ratio?
5
what is the GCF for 16:1 ratio?
6
What is selectivity?
Ratio of primary radiation to scattered radiation transmitted through the grid
what is the formula for contrast improvement factor?(k)
Image contrast with a grid/image contrast with OUT a grid
lowering KVP (blanks) patient dose, and (blanks) image contrast
increases, increases
primary grid used in radiography
focused grid
ideal condition used for parallel grids
long sid
Ratio of primary radiation to scattered radiation transmitted through the grid
selectivity
to maintain adaquate exposure, as grid focus increases, patient dose (blank blank blank)
stays the same
a moving grid moving back and forth several times during x-ray exposure. The total distance of drive is approximately 2 cm (motor driven)
reciprocating grid
moving in a circular fashion around the grid frame, coming to rest after 20 to 30 seconds (springlike and uses a powerful magnet to activate)
oscillating grid
better grids for reducing scatter
focused grids
xrays which exit the patient and interact with the IR
image forming xrays
xrays which exit the patient
remnant xrays
what is the prime benefit to restricting volume of irradiated tissue?
reducing patient dose
what does proper collimation do for the radiographic image?
improves image contrast
which are the ideal xrays for diagnostic image formation?
those that do not interact with patient
2 benefits of collimation
reduces patient does and improves contrast resolution
what happens with increase in scatter?
looses contrast and appears grey and dull
3 factors influencing intensity of scatter
kvp, field size, and patient thickness
what happens to compton and photoelectric interactions with increase in KVP?
absolute number of compton interactions decrease, but number of photoelectric interactions decrease much more rapidly
what is the overall effect of increasing kvp?
relative number of xrays that undergo compton interactions increases
what effects does lowering kvp do to image and patient dose?
lowers scatter, raises contrast, raises patient dose
what phenomenon requires an increase in mAs?(and causes an increase in patient dose)
fewer xrays reach image receptor
what % of xrays incident on the patient reach the image receptor?
1%
what makes a high kvp technique preferable to a low kvp technique?
reduces patient dose
what happens to scatter as field size increases?
scatter radiation increases
what may be done to maintain same optical density when an exposure is made with smaller field size, compared to larger field size?
exposure factors may have to be increased
what % of scatter is produced in an extremity at 70kvp?
45% scatter
what % of scatter is caused by a 30 inch abdomen?
nearly 100% scatter
a definition of contrast
ability to distinguish and image soft tissues
two types of devices which reduce the amount of scattered radiation
beam restrictors and grids
the simplest of all beam restriction devices
aperture diaphragm
basically a lead or lead lined metal diaphragm that is attached to the xray tube head
aperture diaphragm
how the opening of the diaphragm is usually designed
just less the side of the image receptor used
the shape of an extension cone or cylinder
usually circular
most commonly used beam-restricting device in radiography
variable aperture collimator
which types of grids are more effective in cleaning up sactter radiation?
high ratio grids
what makes high ratio grids more effective at cleaning up scatter?
angle of scatter allowed by high ratio grid is less than that permitted by low-ratio grids
what does high-ratio grids do?
increase patient dose as they improve image contrast
define grid frequency
number of grid strips per centimeter
How does grid frequency compare in radiographic images?
show less distinct grid lines on a radiograph compared with grids with low frequency
advantages of aluminium over plastic in grids
nonhygroscopic(does not absorb moisture) easier to manufacture
in the contrast improvement forumula, what is in the numerator?
Image contrast with grid
what is in the denominator in the constrast improvement factor (k)?
image contrast without grid
how does the k value vary by grid ratio?
higher for high-ratio grids
bucky factor. the amount of increase required to maintain od with use of a grid
patient dose with grid/patient dose without grid
what happens as bucky factor increases?
radiographic technique and patient dose increase proportionately
images all structures in parts of the body with approximately equal fidelity
conventional radiograph
special procedure to remedy superimposition of structures, which obscures area (plane) of interest, bluring out structures above and below
conventional tomography
special feature of tomography
rod connecting tube and image receptor which move in reciprocating fashion in opposite directions
the area between the tube and IR in tomography which does not ‘move’, which is at the point of interest
fulcrum
principle advantage of tomography
improved contrast resolution
which two procedures have reduced the need for conventional tomography? (both have excellent resolution)
MRI and CT
what does ct stand for?(cat?)
computerized axial tomography
examples of anatomy which conventional tomographic applications
spine, chest, intravenous pyelogram, wrist
tomographic images produced with conventional radiography equipment
modifed tomography
how does the tube move in modifed tomography? (using conventional radiography equipment)
strait line
how does the tube move in dedicated tomographic equipment?
in an arch
most common procedure for conventional tomography?
kidney during an IVU
the distance the tube moves during an exposure
amplitude
when must the tube be moving during a tomograph?
before, during and after exposure
when table height changes
grossman principle
when fulcrum level changes
plainographic
disadvantages of tomography
high patient dose, requires several exposures, single nephrotogram can expose patient to 1000 mrad (10mGy)
the farther from the object plane (fulcrum) an anatomical structure is, how will it appear?
the more blurred it will be
what controls the thickness of a tomographic section?
the tomographic angle
the larger the tomographic angle, what happens to the tomo section thickness?
the thinner the tomographic section
used when subject contrast is so low that thin-section tomography would result in a poor image. best in chest and renal examinations
zonography
procedure which deliberately increases OID
magnification radiography
technique that is used principly by vascular radiologists and neuroradiologists and frequently in mammography
magnification radiography
magnification factor formula
SID/SOD=image size/object size
