Advanced Biomedical Imaging
Lecture 4 Dr. Azza HelalDr. Azza Helal
A. Prof.A. Prof. of Medical Physics of Medical PhysicsFaculty of MedicineFaculty of Medicine
Alexandria UniversityAlexandria University
Radiographic Image QualityRadiographic Image Quality
Image quality parameters (photographic properties)
DensityDensity
ContrastContrast
Resolution Resolution (Image details)(Image details)
Signals / Noise Signals / Noise (Grainy image)(Grainy image)
Lack of artifactsLack of artifacts
1. Density1. DensityOverall blackening of the image.
If the image is too dark, it has too
much density, overexposedoverexposed..
If the image is too light, it lacks density,
underexposedunderexposed..
2. Contrast2. Contrast Difference between densities of adjacent structures
Controlled by energy of the beam.
µα1/E3
Depends on tissue and type of filmDepends on tissue and type of film
High contrast is too black & white.
Low contrast is too gray.
Contrast & DensityContrast & Density
Contrast & density are controlled by exposure factors and film processing.
X ray controlX ray control
X-ray Exposure FactorsX-ray Exposure Factors
Settings controlled by the operator used to produce the radiograph.
Will influence the amount of patient exposure
Type of x-ray machine and screen film combination will impact the exposure factors.
Increasing Increasing voltage (kV)voltage (kV) increases maximal increases maximal energy of x-rays produced, & ↑ penetration of energy of x-rays produced, & ↑ penetration of
beam,beam, film exposure, film exposure, making the film darkermaking the film darker, ↓ , ↓ pt dose & ↓ exp time but pt dose & ↓ exp time but ↓ contrast↓ contrast. (obese pt) . (obese pt)
Increasing Increasing mAsmAs increases the amount of x-rays increases the amount of x-rays produced, exposure, and signals, produced, exposure, and signals, making the making the
film darkerfilm darker, ↓ exp time., ↓ exp time.
Film dose is α KvFilm dose is α Kv44 mAS. mAS.
Exposure factorsExposure factors
A A small focal spotsmall focal spot produces sharper images. produces sharper images.
A A large focal spotlarge focal spot can tolerate more heat but can tolerate more heat but increase image unsharpness. (see lecture 3).increase image unsharpness. (see lecture 3).
Exp timeExp time: : ↓ by ↑ Kv & large focal spot ↓ by ↑ Kv & large focal spot (↑blurring)(↑blurring)
mAsmAs
Product of mA times the exposure time
100 mA @ .10 seconds = 10 mAs
200 mA @ 0.05 seconds = 10 mAs
As long as the product of the mA and time are the same, the exposure should be the same if the Ma and timer calibration is accurate.
mAs determines the quantityquantity of x-ray photons.
mAs establishes the densitydensity on the image.
Density is the blackness of the image.
It takes a change of 25% to 30% in mAs to make a significant change in the density of the image.
Change the mAs and Change the Density of Change the mAs and Change the Density of the Imagethe Image
The center image has the mAs decreased by 25%. Density is decreased.decreased.
Image on the right has the mAs increased by 25% above the image on the left. Density is increasedincreased.
Wrong mAsWrong mAs
This image is under under exposedexposed.
Not enough mAs was used & density reduced
This image is overexposedoverexposed for the patella.
Too much mAs was used & density increased.
30-50 Rule30-50 Rule
Relatively large changes in mAs are needed to change the density of the image.
It takes a change of 20 to 30% to make any change in the density of the image.
It needs a change by 50% to make a significant change.
This is the 30-50 Rule
Changing mAsChanging mAs
As mAsmAs increased;
the density of the image
changes from under
exposed to over exposed.
Over exposedOver exposed
This was taken with 60 mAs.
To correct the density of the
image, 30 mAs would be used.
So mAs reduced by 50%.
Under exposedUnder exposed
This image was taken at 30 mAs.
To darken the image, 60 mAs would be used.
To correct the density of the films double the mAs.
Grid Ratio and FrequencyGrid Ratio and Frequency
The higher the ratio and frequency, the more radiation is
needed to produce the image.
Low grid ratio is limited to low kVp. A 5:1 grid is
limited to below 90 kVp.
High ratio grids can be used up to 125 kVp.
Grid Ratio and FrequencyGrid Ratio and Frequency
Low frequency grids have prominent grid lines on image.
They are used in true Buckies.
The grid moves during the exposure to blur the lines.
High frequency grids produce almost invisible lines and
do not need to move during the exposure.
For imaging of lumbar spine without the grid:
– 8 mAs @ 74 kVp used
– 15-16 mAs @ 74 kVp used to produce the image
with the 5:1 grid. (doubling mAs)
– The conversion factor would be 2 for adjusting the
mAs for the 5:1 grid
Grid Ratio and TechniqueGrid Ratio and Technique
The 5:1 grid required doubling the mAs.
The conversion factor is determined by dividing the
new mAs by the old mAs.
What would be the conversion factor for the 10:1 What would be the conversion factor for the 10:1
grid? Old mAs = 8 New mAs = 40grid? Old mAs = 8 New mAs = 40
Grid Ratio and TechniqueGrid Ratio and Technique
No Grid UsedNo Grid Used
Lumbar Spine needs the use of a
grid. why?
There is too much scatter radiation
due to the size of the body part
Exposure factors needed to be
adjusted to visualize this area.
Low Ratio & Frequency GridLow Ratio & Frequency Grid
For the A-P Lumbar Spine, a 5:1 with 80 grid lines will remove much of the scatter radiation.
Going from Non-Grid to a 5:1 ratio requires doubling the mAs.
Grid lines are very noticeable.
High Ratio & Frequency GridHigh Ratio & Frequency Grid
A 10:1 grid with 100 lines will
provide excellent clean-up of
scatter radiation.
Grid lines are nearly invisible.
But, requires 5 times more
radiation.
Grid Cut-offGrid Cut-off
When high frequency and ratio focused grids are used,
the lead strips must be perfectly aligned with the primary beam.
A minor misalignment will result in the grid removing primary radiation.
Misalignment of more than 2° will result in grid cutoff.
Grid Cut-offGrid Cut-off
Is the density of the image of
both knees the same?
This is an example of grid
cut-off.
Some of the primary beam is
being removed by the grid.
Grid Cut-off Grid Cut-off causescauses
If grid lines are not parallel to primary radiation due to
– tube being too close
– too far from the grid,
– If the tube is angled against the grid lines.
– If grid is not not perpendicular to perpendicular to the beam the beam (mos(most common problem).
– If grid is backwardsbackwards, only
center of beam will pass
though the grid.
So proper alignment So proper alignment
must be maintained.must be maintained.
Air-gap Vs GridsAir-gap Vs Grids
Using air gap not grid,
Radiation exposure
reduced five folds
With no loss of image
quality.
3. Resolution3. Resolution
Resolution (image details): Minimum distance
between two points that can be recognized as
being separate.
Noise (grainy image): variation in no of x
ray photons absorbed
4. Noise4. Noise
Artifact is the appearance of signal in an image
location not representative of actual properties of
object.
5. Lack of artifacts5. Lack of artifacts
QuestionsQuestions
1. What are main parameters that control image
quality?
2. Enumerate causes of grid cut-off?
3. Define contrast, density and artifact?