Rad 100 Unit 2 New chapters Combined (9,10,14,21)

5 X-ray interactions

coherent scattering, Compton scattering, photoelectric effect, pair production, photodisintegration

Coherent/Classical/Thompson scattering

energies below approximately 10 keV interact matter this way.

the incident x-ray interacts with a target atom, causing the atom to become excited.

target atom releases this excess energy as a scatter wavelength equal to the incident x-ray.

different directions.

Coherent scattering is of ____ importance to diagnostic radiology

Little

Compton scattering

scatters the x-ray but reduces its energy and ionizes the atom as well.

the incident x-ray interacts with an outer-shell electron and ejects it from the atom, thereby ionizing the atom

scattered x-ray is equal to the difference between the energy of the incident x-ray and the energy of the ejected electron. The energy of the ejected electron is equal to its binding energy plus the kinetic energy with which it leaves the atom.

COMPTON EFFECT

Ei =Es + (Eb +EKE)

where Ei is energy of the incident x-ray, Es is energy of the scattered x-ray, Eb is electron binding energy, and EKE is kinetic energy of the scattered electron.

The probability of Compton scattering is…

inversely proportional to x-ray energy (1/E) and independent of atomic number.

Compton Scattering most likely to occur

With outer-shell electrons

With loosely bound electrons

As x-ray energy increases, Compton scattering…

Increased penetration through tissue without interaction

Increased Compton scattering relative to the photoelectric effect (So Compton scattering becomes more dominant over the photoelectric effect)

Reduced Compton scattering (≈1/E)

As atomic number of absorber increases, Compton scattering…

No effect on Compton scattering

As mass density of absorber increases, Compton scattering…

Proportional increase in Compton scattering

Compton scattering does what to image contrast

reduces it

recoil electron

ejected electron from Compton scattering

X-ray fog

unwanted exposure to the image receptor caused by scattered x-ray

photoelectron

electron that is ejected from the surface of a material (like a metal or gas) when a photon of light hits it with enough energy via the photoelectric effect

PHOTOELECTRIC EFFECT EQUATION

Ei =Eb +EKE

where Ei is the energy of the incident x-ray, Eb is the electron-binding energy, and EKE is the kinetic energy of the electron

The photoelectric effect is total ________.

x-ray absorption

Photoelectric Effect

The x-ray is not scattered, but it is totally absorbed. Creates useless characteristic x-rays inside the patient that does not contribute to the image

The probability of the photoelectric effect is…

inversely proportional to the third power of the x-ray energy (1/E)³.

is directly proportional to the third power of the atomic number of the absorbing material (Z³)

Photoelectric effect is most likely to occur

With inner-shell electrons

With tightly bound electrons

When x-ray energy is just higher than electron binding energy

As x-ray energy increases, photoelectric effect…

Increased penetration through tissue without interaction

Less photoelectric effect relative to Compton scattering

Reduced absolute photoelectric effect (≈1/E)³

As atomic number of absorber increases, photoelectric effect

Increases proportionately with the cube of the atomic number (Z³)

As mass density of absorber increases, photoelectric effect…

Proportional increase in photoelectric absorption

pair production

An x-ray photon interacts with the nuclear field and disappears, producing a positron (+) and negatron/electron (−).

involves only x-rays with energies greater than 1.02 MeV

very important for positron emission tomography (PET) imaging in nuclear medicine.

Pair production and photodisintegration does not occur during…

x-ray imaging

Photodisintegration

occurs with x-rays greater than 10 MeV. the nucleus is raised to an excited state and instantly emits a nucleon or other nuclear fragment.

does not occur in x-ray imaging

Differential absorption occurs because..

Compton scattering, photoelectric effect, and x-rays transmitted through the patient.

radiopaque

anatomic structures with high x-ray absorption characteristics; bone

radiolucent

penetrate the body and are transmitted to the image receptor with no interaction whatsoever. They produce the dark areas of an x-ray image; air in lungs

Differential absorption increases

as the kVp is reduced.

What increases differential absorption?

• ↓ kVp → ↑ differential absorption → ↑ contrast

• But ↑ patient dose

What does photoelectric effect depend on?

• Atomic number (Z³)

• ↓ with increasing energy (1/E³)

What does Compton scattering depend on?

• Not atomic number

• ↓ with increasing energy (1/E)

Low vs. high x-ray energy?

• Low: Photoelectric dominates

• High: Compton dominates

What happens when x-ray energy increases?

• ↓ interactions

• ↑ transmission

• ↓ mAs needed

Why does bone show on an x-ray?

• Higher Z + mass density

• More x-rays interact than in soft tissue

Why are lungs visible?

• Mainly mass density differences

• Air has much lower density than soft tissue

How does mass density affect interactions

• ↑ density → ↑ absorption & scattering

• ↓ transmission

What causes radiographic contrast?

• The product of image receptor contrast and subject contrast.

What causes loss of contrast/fog?

Compton scattering

What are positive contrast agents?

• Barium & iodine

• High atomic numbers → absorb more x-rays

What is a negative contrast agent?

• Air

• Used with barium for double-contrast exams

What is attenuation?

Reduction of x-rays by absorption + scattering

What does exponential attenuation mean?

• A percentage of x-rays is removed with each thickness

• Not a fixed number

Medical image quality

The fidelity or exactness with which anatomy and tissues are reproduced on the image.

Spatial resolution

Ability to image small, high-contrast objects or distinguish two separate objects.

What mainly limits spatial resolution in digital imaging?

Pixel size.

How does pixel size affect spatial resolution?

Smaller pixels improve spatial resolution.

Spatial frequency

A measure of how closely spaced line pairs can be and still be resolved.

What are the units of spatial frequency?

Line pairs per millimeter (lp/mm).

Line pair

One high-contrast line plus an equal-width interspace.

What does a higher spatial frequency mean?

Smaller objects and better spatial resolution.

What does a lower spatial frequency represent?

Larger structures, such as the liver, kidneys, or brain.

Examples of high-spatial-frequency anatomy

Bone trabeculae, breast microcalcifications, and contrast-filled vessels.

How do you estimate object size from spatial frequency?

Object size in mm = 1 ÷ (2 × lp/mm).

Modulation transfer function (MTF)

Describes how well an imaging system reproduces object contrast at different spatial frequencies.

What would the MTF of an ideal imaging system be?

1.0.

What is the limiting spatial resolution on an MTF curve?

The spatial frequency at 10% MTF.

What happens to MTF as spatial frequency increases?

MTF decreases because small objects are reproduced less faithfully.

Why does DR have a cutoff spatial frequency?

Objects smaller than the pixel size cannot be resolved.

Bar pattern test tool

A high-contrast line-pair test object used to evaluate spatial resolution and construct an MTF curve.

Contrast resolution

Ability to distinguish structures with similar contrast or many shades of gray.

Which modality is considered to have the best contrast resolution?

MRI, although its contrast depends strongly on the RF pulse sequence.

Dynamic range

The number of gray shades an imaging system can reproduce.

What determines dynamic range in digital imaging?

Bit depth of each pixel.

How are shades of gray calculated from bit depth?

Number of shades = 2^bit depth.

12-bit dynamic range

4,096 shades of gray.

14-bit dynamic range

16,384 shades of gray; typical of digital radiography in this chapter.

16-bit dynamic range

65,536 shades of gray; used in digital mammography.

How many gray levels can the human visual system see at one time?

About 30.

Postprocessing

Computer manipulation of a digital image to extract or display more information.

Window and level

Postprocessing tools that expand a selected part of the grayscale into visible shades.

What is a major advantage of window and level?

It allows visualization of gray shades that the eye could not distinguish all at once.

DMIST finding

Digital mammography performed better for younger, denser breasts and about equally for older, less-dense breasts.

Signal in a radiographic image

Image-forming x-rays that represent anatomy.

Noise in a radiographic image

Random background information that contains no useful anatomic information.

Principal source of noise on an x-ray image

Compton-scattered x-rays reaching the image receptor.

Signal-to-noise ratio (SNR)

Relationship of useful image signal to unwanted image noise.

How does noise affect contrast resolution?

More noise lowers contrast resolution.

How does increasing mAs affect SNR?

It increases SNR but also increases patient radiation dose.

Dose creep

Unnoticed increase in patient dose because DR can still produce diagnostic-looking images after excessive exposure.

Technique creep

Adjusting digital technique toward higher kVp and lower mAs to reduce dose while maintaining image quality.

How does digital receptor response relate to radiation intensity?

It is approximately linear over a wide exposure range.

Does changing exposure directly change digital image contrast?

No. Digital image receptor response is linear, so image contrast does not change with changing radiation exposure.

What is the main problem with very low digital exposure?

Low SNR and quantum mottle or quantum noise.

Quantum mottle

Mottled image noise caused by too few x-rays interacting with the image receptor.

Detective quantum efficiency (DQE)

Measure of how efficiently an image receptor absorbs and uses x-rays.

What does higher DQE allow?

Fewer x-rays can produce an adequate image, reducing patient dose.

What digital technique trend can reduce patient dose?

Use higher kVp with a compensating reduction in mAs.

How can quantum mottle be reduced according to Chapter 21?

Use higher mAs, lower kVp, or a slower image receptor.

Fast image receptor

Produces higher noise and lower contrast resolution.

Slow image receptor

Produces lower noise and higher contrast resolution.

Magnification

Image enlargement compared with the actual object.

Magnification factor (MF) using size

MF = image size ÷ object size.

Magnification factor (MF) using distance

MF = SID ÷ SOD.

How can magnification be minimized?

Use a large SID and a small OID.

Magnification radiography

An examination in which image magnification is intentionally planned.

Distortion

Unequal magnification of different parts of the same object.

What three conditions contribute to distortion?

Object thickness, object position, and object shape.

How does object thickness affect distortion?

Thicker objects show more distortion because OID varies more across the object.

What happens when the object plane and image plane are parallel?

Distortion is minimized.