Recorded Detail Flashcards
March 13 = physics exam
Recorded detail
detail definition or sharpness in film/screen radiography
Known as spatial resolution in DR
The sharpness of structure lines in a radiograph
4 factors affecting recorded detail
Geometric
motion
IR
Object
Geometric factors
Focal spot size
SID
OID
Heel effect
Deal with the arrangement in space of xray beam, part being radiographed, and IR
Motion factors
X-ray source
IR
Patient
IR factors
Film speed
Intesifying screen
Quantum mottle
Object factors
Size
shape
Focal spot size
Smaller focal spot size, the greater the recorded detail and vice versa
Small focal spot range: 0.3mm to 1.0mm
Large focal spot range: 1.0mm-2.5mm
2 ways to describe focal spot size
Actual focal spot size - the actail area being bombarded by electrons
Effective, apparent, projected, or nominal focal spot - the way the actual area appears from the perspective of the IR
Line focus prinicle
effective focal spot will always appear smaller than the actual focal spot
Relationship between the actual and effective focal spot size is governed by this
Size of affected focal soot is affected by 2 factors
- Size of actual focal spot - smaller the actual, smaller the effective
Size of actual focal spot is governed by size of filament in cathode - Anode angle - smaller the anode angle, smaller the effective focal spot size
Dual focused xray tube
Required to give radiographer a choice of either a small or large focal spot,
Advantages + disadvantages of focal spot size
Small - better detail
use at less than 50 mAs
Lower heat capacity
Reduces tube life
Large- Detail not as sharp
use at any mAs
Higher heat capacity
Increases tube life
Effect of focal spot size on recorded detail
Umbra (shadow) – the distinctly
sharp area of a shadow or region
of complete shadow.
This will be the true image of the
object being radiographed.
Penumbra - the imperfect, unsharp shadow surrounding the
umbra.
It can also be referred to as
unsharpness, edge gradient or
geometric unsharpness.
Penumbra
As focal spot size increases, so does the size of the penumbra
Recorded detail decreases
Focal spot blooming
Over time or with increase in mA used, size of the focal spot can increase from its stated nominal value
Focal spot size evaluation
to determine if focal spot blooming has occured
Focal spot size testing can be performed by one of three devices
Pinhole camera - tiny hole of a specified shape is cut into a plate of metal allloy
Focal spot test tool - series of slits is radiographed, corresponding chart is used
Star or slit resolution patterns - chart if various shapes and/or lines that can
estimate focal spot size.
Effect of SID
Short SID’s will decrease recorded detail because the penumbra size increases
Long SID’s will increase recorded detail because penumbra is smaller
SID and recorded detail have a direct relationship
Effect of OID
Larger OID will cause larger penumbra before reaching IR, thereby reducing recorded detail
Shorter OID will increase recorded detail
Inverse relationship for OID and recorded detail
Calculating average penumbra size
penumbra = EFS x OID/SOD
SOD = SID-OID
Anode heel effect
The penumbra is always
larger on the cathode side of
the x-ray field due to the
heel effect.
Therefore, recorded detail is
slightly better toward the
anode side
Motion factors
Motion of patient, xray source, IR during exposure can cause motion blur
Motion blurs destroys recorded detail
Patient motion
Common cause of motion blur
Minimized by:
Suspend respiration
Communicate clear instructions
Utilize short exposure times
Immobilize when necessary
IR factors
Film speed - faster speed, decreased recorded detail
Intensifying screen - Faster speed will decrease recorded detail
- Crystal size/shape: smaller size or columnar shape will increase recorded detail
-Active layer: thinner layers increase recorded detail
-Film/screen contact: bad contact reduces recorded detail
Quantum mottle - causes blotchy appearance which reduces recorded detail
Quantum mottle
Speckled appearance caused by insufficient xray photons covering IR
Object factors - size
Thinner objects allow for a smaller penumbra than thicker objects
Recorded detail is better with thinner object
Object factors - shape
Objects similar in shape to xray beam (cone-shaped) have smaller penumbra size than any other shape
Results in better spatial resolution
Related terms
Visibility and resolution - not the same thing as recorded detail but are related
Visibility of detail
How well the radiographic image can be seen and affect mainly by optical density and contrast
Modulation transfer function
can be used to measure visibility of detail
Numbers from 0 to 1 obtained using fourier analysis
The closer to 1.0, better spatial resolution
DR image quality characteristics
MTF measures visibility of detail, which is clarity of the image
DR systems should deliver a MTF of 30% at 2 lp/mm and 60% at 60% at 1 lp/mm
Resolution
Ability of an imaging system to deliver recorded detail, visibility of detail, and events happening in time
3 types of resloution:
Contrast
Temporal
spatial
Contrast resolution
Ability of imaging system to distinguish structures that are similar in size or density as separate entities
Also known as low contrast resolution
Affected by contrast sensitivity of IR
Evaluation of contrast resolution
Use a scoring system from a phantom image
Contrast resolution and visibility of detail
Image of contrast resolution test tool can be used to create a contrast detail-curve
Further the curve is toward y-axis, better contrast resolution and visibility of detail
Contrast detail curve
Smaller the object to be observed, the more contrast in the image you need in order to see it
MRI has the best contrast resolution
Temporal resolution
Ability of an imaging system to show events that are happening close together in time as being separate events
Applicable to real-time modalities such as fluoro, CT, MR angiography, MRI
Affected by framte rate of acquisition and display rate
Frame rate
the number of frames/second or Hertz (Hz)
The faster the frame rate, the better the temporal resolution,
but the greater the patient dose.
Spatial resolution
The ability of an imaging system to create
separate images of closely spaced objects. Also known as High Contrast Resolution.
Is equivalent to recorded detail in DR
Spatial frequency
requires an image of a resolution pattern that creates a series of black and white lines
Frequency of 5 lp/mm means that each black and white line is only 0.1 mm wide
Point spread function
Can also be used to measure spatial resolution
A narrower peak on the
graph indicates better
spatial resolution
Line spread function
Creates a similar graph to Point Spread Function
but requires a slit camera instead of a pinhole
camera.
The slit camera has an aperture that is 10 μm
wide.
Edge spread function
Can also be used to measure spatial resolution
Closer the graph is to the center
line, the better the spatial resolution
Distortion
A misrepresentation of the true size, shape, or
spatial relationship of the part in the
radiographic image.
3 types of distortion:
Size - msirepresentation of true size. SID, OID, SOD
shape - Misrepresentation of true object shape. Elongation, foreshortening
spatial - Misrepresentation of true spatial relationship between objects in image
Size distortion or magnification
Misrepresentation of true size of the object in the radiographic image
Factors are:
SID
OID
SOD
Effect of SID on magnification
As SID increases, the amount of magnification decreases due to less beam divergence
Shorter SID will cause more magnification
Effect of OID on magnification
As OID increases, so does amount of magnification
A decrease in OID will cause less magnification
Magnification calulations
Amount of Size Distortion
* Image size/object size = SID/SOD
Magnification Factor – tells how many times bigger image is
than object
* Use either half of the above equation
Percent Magnification – tells what percent bigger is image
than object
* = (image size – object size)/object size x 100
Cieszynski’s Law of Isometry
Can be used to determine foreshortening of objects that are not parallel to IR
Cr angle should be 1/2 of the angle between the part and IR to minimize shape distortion
Spatial distortion
taking 3-dimensional object and capturing it in a 2-dimensional image
Can be corrected by taking 2 separate images at 90 degrees from each other
