Lecture 6: Image Quality in Radiography

Components of every image

Contrast

resolutionVisibility of recorded detail : dependent on -

Intensity/brightness

Recognizability aka geometric integrity dependent on -

Sharpness of recorded detail - edge sharpness

Radiographic Contrast - consists of:

film contrast

Film contrast

Inherent property of film bc films are designed with differing speeds and inherent contrastDetermined by slope of the straight line portion of the characteristic curve

Film contrast Light transmittance formula

Percent transmittance = incident photons/transmitted photons thru filmFor every 0.3 change in optical density, the %T inversely changes by a factor of 2Every time it doubles, there is a new gray tone

Film contrast Characteristic Film Curve

Film Gamma

= maximum slope of curve

Film latitude

Subject contrast = image contrast resulting from patient interactions

I_small

Subject contrast =

I_large

Depends on:

Thickness differences

Subject contrast Thickness differencesDifferent parts of the same material that attenuates x-ray beams differentlyThicker parts =

increased

attenuationThinner parts =

decreased

Subject contrast Density differencesGreater density results in a

greater

Subject contrastDensity differencesEx: bone is more dense than soft tissue so it absorbs

more

Subject contrast Atomic Number DifferencesHigher atomic number tissues attenuate more x-rays

Bc higher atomic number indicates larger nuclei with more protons and neutrons which are more things a x-ray photons can interact with

Subject contrast Atomic Number DifferencesHigher atomic number tissues attenuate more x-raysPrimarily results from

photoelectric attenuating

Subject contrast

Higher atomic number tissues attenuate more x-rays.

Photoelectric reactions probability

Probability of photoelectric reactions is (atomic number)^3.

kVp effect on photoelectric interactions

If we decrease kVp, percentage of photoelectric interactions increase.

Cubed power

The cubed power.

Quality of radiation (kVp)

Increasing kVp decreases subject contrast.

Lower kVp effect

Lower kVp = higher contrast.

Higher kVp effect

Higher kVp = lower contrast.

Contrast relationship with kVp

Lower kVp = higher contrast; Higher kVp = lower contrast.

kVp and penetrability

Increasing kVp increases punching power, changing the ratio of penetrability.

Changing mAs effect on subject contrast

Changing mAs does not affect subject contrast.

Higher kVp yield

Higher kVp yields a lot of gray tones because contrast is lower.

Mid-gray tone value at 100 kVp

Much darker at 100 kVp indicating an overall darker image even though there is a lot of gray tones.

Fog & scatter

Fog reduces.

Scatter Radiation

Scatter radiation that reaches film produces unwanted density increases with increasing part thickness.

Scatter example

More scatter when x-raying the abdomen than x-raying the foot.

Field size effect on scatter

Increasing field size increases scatter.

Energy (kVp) of x-rays

Increased energy (kVp) of x-rays increases scatter radiation.

Protection from scatter radiation

Can protect yourself and staff from scatter radiations by using lead in the walls.

Grid system

Grid system is for image quality, not safety.

DRange

Extended CR density range imaging plate does not have a Dmax but rather a straight line.

CR/DR imaging

CR/DR imaging continues to record the exposure way beyond the limits of film.

mAs in film

mAs in film does not affect film contrast.

mAs in digital

In digital, spatial resolution is constant but contrast resolution is improved.

Increasing kVp association

Increasing kVp is associated with increased scatter radiation.

Fogging and small volume anatomy

In small volume anatomy (like the foot and ankle), there is not visible fogging even with an increase in 25.

Umbra vs Penumbra

Umbra is the shadow; Penumbra is the object size.

Sharpness optimization

Sharpness optimized by decreasing beam penumbra by minimizing object-image distance (OID).

Maximizing sharpness

Maximizing sharpness by maximizing source-image distance (SID).

Decreasing focal spot size

Decreasing focal spot size increases sharpness.

OID and SID relationship

Keeping OID constant increases SID.

OID effect on sharpness

Keeping SID constant decreases OID which increases penumbras and decreases sharpness.

Object distortion causes

Object distortion can be caused by object thickness.

Thicker objects distortion

Thicker objects are more distorted than thinner objects.

Object position distortion

Distortion occurs if object plane and image plane are not parallel.

Object shape distortion

Object shape can cause distortion.

Object motion

Movement of the patient can cause image blurring.

Radiographic mottle (aka noise)

Random fluctuations or unwanted variation of film density following a uniform exposure.

Number 1 cause of radiographic mottle

Short exposure time.

Quantum Mottle

Most important source of mottle/noise; result of statistical fluctuation.

Statistical fluctuation formula

+/- square root of average number of photons.

Reducing quantum mottle for film

Use of high or low mAs.

mAs

Low or high.

kVp settings

Slower or fast.

Image fog in digital imaging

Setting kVp too high yields image fog.

Minimum kVp

Should provide at least partial penetration through all tissues of interest.

Optimal kVp increase

15% increase in kVp generally does not double image receptor exposure.

Optimal kVp effect on patient exposure

Increasing kVp by 15% will only increase patient exposure by 25 to 40%.

Patient factors affecting technique

Thickness of part.

Body composition measurement

Should be measured.

Patient habitus

Primarily soft tissue, bone, or both?

Pathology

Sthenic, hypo or hyperstenic, asthenic.

Controlling Optical Density

Different variables; mAs - direct proportionality.

SID

Via inverse square law.

kVp effect on optical density

Has a disproportionate OD effect.

Changes in OD with mAs

Only affects brightness, not image contrast.

Optical density control

Controlled by mAs.

Contrast control

Controlled by kVp.

Detail control

Controlled by focal-spot size.

Distortion control

Controlled primarily by patient positioning.

Basics of Subject contrast for Chest

High.

Subject contrast for Lung

Low mass density.

Subject contrast for Bone

High mass density.

Basic rule for chest imaging

Use high kVp (to decrease the high subject contrast) and low mAs combos.

Basics of Subject contrast for Abdomen

Low.

Basic rules for abdomen imaging

Use low kVp and high mAs combos.

Basics of Subject contrast for Extremities

Intermediate to high.

Basic rules for Extremities imaging

Despite the high subject contrast, kVp should still be lowered due to part thickness.

Visible change in OD

mAs values must be changed by 30%.

kVp settings change

Must only be changed by 4%.

kVp

5

Thickness kVp variation

2

Image quality subheadings

Visibility and Recognizability (geometric integrity)

Factors affecting visibility of recorded data

Contrast, Brightness, and Noise

Pixel levels within anatomy

should be neither completely white or black

High contrast visibility

Greater Visibility

End result of subject contrast and display contrast

Image Contrast

Thicker parts attenuation

increased attenuation

Thinner parts attenuation

decreased attenuation

Greater density of tissue

results in greater beam of attenuation

Factors affecting subject contrast

Thickness difference, Density difference, Atomic number difference, and Quality of radiation

Higher atomic numbered tissue effect

more attenuated the xrays

Changing mA x time

does not

Factors affecting image contrast

Film, Digital, Fog & Scatter