Density, Contrast, Distortion & Detail Lab Test #3
Dec 24, 2014
Density, Contrast, Distortion & Detail
Lab Test #3
Contrast and the 15% Rule
Objective: To demonstrate the influence of the 15% rule on contrast
1st exposure at recommended technique
Used as control
Contrast and the 15% rule
Apply the 15% rule twice and ↑ kVp ↓ mAs accordingly
All other factors stay the same
The math doesn’t quite add up on this one, so I think the images are from 2 different groups
Contrast and the 15% rule
Again, this image doesn’t match my lab sheet, I believe we lowered the kVp 15% and shot another on just for S&G
At this point, we should compare the 2 images to determine which has the longest scale of contrast
See next slide
Contrast and the 15% rule
Low kVp=High Contrast= Short scale High kVp=Low Contrast= Long scale
More B&W More Shades of Gray
Density- Control Image
Objective: To demonstrate the effect of select factors on density
Control image of knee phantom taken @ 5mAs, 60kVp, & 40” SID.
Density- Filter
With no change in tech factors, a copper sheet is taped to the face of the collimator
Filters have an inverse relationship on density thus:
↑ Filtration ↓ Density
Density- Grid
Copper filter removed No change in technical
factors Grid is introduced to IR Result: Adding a grid
decreases density this is an inverse relationship
(+) ↑ Grid ↓Density
Density- SID
Grid removed No change in technical
factors SID increased from 40”
to 60”
Density-Tissue Thickness
Grid is removed and hand phantom is substituted for knee
No change in technical factors
Overall, the hand the tissue thickness of the hand is much less than that of the knee
↑ tissue thickness ↓density
Density- Collimation-Control
Torso phantom is substituted for hand
14x17 collimation Use technique
recommended by console + 20 mAs
This image will be compared with the highly collimated image.
Density- Collimation
Collimation is increased to a 4x4 square.
Repeat exposure of torso with no change in technical factors
↑collimation ↓density
Density- Anode Heel Effect
Use a foot phantom and a 14x17 cassette
2 mAs, 60 kVp, 40” SID Leave collimation open
lengthwise to 17” Orient toes over the
anode side
Density- Anode Heel Effect
All technical factors remain the same
Foot is moved to opposite end of cassette to place toes on cathode side
In practice, the thickest tissue should be placed at the cathode end of tube
In theory, placed thicker tissue at anode end would effect density
A visible change in density would only be appreciated with film/screen
Size Distortion: Control
Objective: To demonstrate the various types of distortion
Control image of hand shot at 40” SID
Technique: Pre-programmed
Size Distortion: 6” OID
2nd image 6” OID is created with
sponges All other factors remain
the same Compare with control
image to note distortion (magnification)
Size Distortion: 12” OID
Image #3 OID is ↑ to 12” Collimation is exactly the
same No other factors have
been changed Note magnified
appearance of hand Such distortion can mask
pathology
Distortion: OID + SID
OID is ↓ to 6” SID is ↓ to 20” All remaining tech factors
are unchanged Result: Size distortion
(magnification in this case) can be caused by OID, SID or both
Shape Distortion: Control
Control exposure of knee phantom w/recommended tech factors
This image will be used for comparison with others
Shape Distortion: Angled Tube
2nd image of knee phantom
Tube is angled All tech factors remain
the same
Shape Distortion: Angled IR
3rd exposure of knee phantom
IR is angled All tech factors are
unchanged from control
Note closed joint spaces
Shape Distortion: Angled Anatomy
4th exposure of knee phantom
All tech factors remain constant except…
Phantom is angled Result: Shape distortion
can be caused by the angulation of tube, IR or anatomy being imaged.
Density & mAs
Objective is to demonstrate the effects of overexposure & underexposure on CR images
First image is made @ 60kVp, 10mAs w/40” SID
Density & mAs 2
mAs is ↑ 50 All other technical factors
remain the same LGM#’s of all images will be
compared with control LGM represents the # of
photons reaching IR to form the latent image
LGM is proportional to mAs
Density & mAs 3
↑ to 100 mAs No other technical factors
are changed At this stage, LGM#’s
appear to be ↑ as mAs is ↑ (a direct relationship).
Density & mAs 4
↑ to 200 mAs Remaining tech factors
unchanged LGM#’s increase with
each increase in mAs mAs is THE controlling
factor of density
Detail and Distortion: Motion
Objective: To demonstrate the effect of motion on detail
Technical factors: The programmed technique for a foot but ↓ mA
Decrease in mA is necessary to ↑ exposure time (applicable in next image)
1st image of top taken on 10x12 w/no motion
Detail and Distortion: Motion
2nd image- top is spinning while exposure is taken
Technical factors are unchanged
The lower mA ↑ exposure time, allowing the motion to be caught on film
Detail and Distortion: Motion
Additional image of this experiment
Compared to the 1st image (stationary), the lead letter attached to top is blurred.
Result: Increased motion decreases detail
Motion↑↓ Detail Although motion is generally a
detriment to good films, it can be used to the RT’s advantage
Ex: Using breathing technique to blur ribs
Quantum Mottle
Definition: a lack of sufficient incoming data to process an image; AKA quantum noise
No idea what this image has to do with anything, but that’s how it was labeled
That would be a cool name for a band though
Contrast- Control Image
Objective: To demonstrate how selected factors effect contrast
66kVp, 10 mAs, 40” SID on table top
Contrast 66kv
Skull phantom is replaced w/step wedge
Exposure taken with no changes in technical factors
Compare this image w/second exposure using a higher technique
Contrast: ↑kVp ↓mAs
↑ to 86kVp Compensate by ↓mAs to
¼ of its original value All other factors remain
the same
Contrast- 86kv
Again, step wedge takes the place of skull phantom
No change in technical factors
Compare shades of gray in adjacent densities
Comparison of Contrast
Low kVp High kVp
Contrast: +Grid
3rd image of skull Grid is introduced mAs ↑ 4x to
compensate All remaining factors
unchanged
Contrast: Scatter
4th image of skull Collimation open wide
to expose IR to scatter All remaining factors
unchanged Compare this series of
images to note differences in adjacent densities
Contrast Collimation
Skull is now positioned laterally
This exposure will use bucky instead of table top
100 kVp @ 20mAs
Contrast: Collimation
Repeat exposure of lateral skull
Collimated to 3”x3” area
All remaining factors are unchanged
Short vs Long Scale Contrast: kVp
Objective: To demonstrate short & long scale contrast
Elbow phantom @ 46kVp & 5mAs
Stepwedge 2 Contrast ????
I think this is supposed to be part of a contrast lab but I couldn’t match it up with anything
Also, it was posted twice in the images file…so there’s nothing to compare it to as far as I can tell
Short vs Long Scale Contrast; kVp
2nd image of elbow phantom
↑ to 70kVp ↓to 3mAs
Density Chart
Variables Effects Density Relationship
Filtration ↑↓ ↓↑ Inverse
Grid ↑↓ ↓↑ Inverse
Tissue Thickness ↑↓ ↓↑ Inverse
SID ↑↓ ↓↑ Inverse
Collimation ↑↓ ↓↑ Inverse
Anode Heel Effect Cathode: ↑Anode: ↓
*In theory the anode heel effect does cause a change in density but this is not evident in digital imaging
N/A unless film screen technology is involved
Contrast ChartVariable Effect Contrast Relationship
kVp ↑↓ ↓↑ inverse
mAs Ø Ø none
SID Ø Ø none
OID ↑↓ ↑↓ direct
Filtration ↑↓ ↓↑ inverse
Collimation ↑↓ ↑↓ direct
Tissue Thickness ↑↓ ↓↑ inverse
Contrast Media + ↑ direct
Grid ratio ↑↓ ↑↓ direct
Focal spot size Ø Ø none
Film Processing *+/- developing time and temp beyond optimal
↓ Always ↓ contrast
Detail Chart
Variable Effect Detail Relationship
SID ↑↓ ↑↓ direct
OID ↑↓ ↓↑ inverse
Tissue Thickness ↑↓ ↓↑ inverse
Focal Spot Size ↑↓ ↓↑ inverse
Motion ↑↓ ↓↑ inverse