Advanced Biomedical Imaging Lecture 4 Dr. Azza Helal A. Prof. of Medical Physics Faculty of Medicine Alexandria University.

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?