Lecture 3: IMAGING CONTRAST/DENSITY, RECORDING MEDIAS

Image Density:

• Defined as the degree of darkening on a film (not tissue density)

• Image of high density is dark, image of low density is light. When more photons reach an image receptors, density increases, with fewer photons, density decreases.

• Greater film density on right side of patient. Patient has pneumothorax on right.

• Tissue density is the compactness of molecules in atomic structure of different body parts.

Density

Factors Affecting Image Density

• mAs –

  • Directly proportional to density (↑ mAs = ↑ density)

• kVp –

  • Small increase → large increase in density

• Distance (TID/SID) –

  • Greater distance → fewer photons reach the imager → lower density

Distance and How it Affects Density

• The distance between the focal spot (target/source) of the x-ray tube and the recording medium.

  • TID: Target to Imager Distance

  • FFD: Focal Film Distance

  • SID: Source to Imager Distance

• Distance and density relationship follows the inverse square law.

Inverse Square Law

• Intensity of the beam inversely proportional to the square of the distance

• If distance doubles, the quantity of x-rays reaching the image receptor is reduced to ¼

  • This is due to the surface area being 4x as great

  • Intensity is ¼ it’s original value

Contrast

• The tonal range of densities from black to white

  • or the number of shades of gray in an image

• It provides visual evidence of the different absorption rates of various body tissues*

• kVp & Contrast

  • low kVp = high contrast = short scale of contrast (small range of densities)

  • high kVp = low contrast = long scale of contrast (wide range of densities)

I.e. Images with different contrast levels

Short and long scales of image contrast

Parts of the body with low contrast between its body parts will give us a better image

Image with high contrast is not desirable.

The factors that need to be considered when deciding kVp are: part thickness, scatter radiation, and field size

Structure absorption

• Dense structures absorb more photons photoelectrically and produce more Compton scattering

• →

• Less dense structures - Tissues that are thin or aged - result in decreased attenuation and produce a disproportionately darker image

  • Older bones with less calcium have less x-ray absorption

Structure Absorption

• The amount of energy absorbed by a structure depends on:

  • the atomic number (Z) of the tissues of that structure,

  • the mass density of the tissue

  • the energy of the incident photon

• Absorption increases as

  • Z #  increases

  • Mass Density increases

    • A material that is twice as dense as another will absorb twice as much energy from the same photon beam

  • Photon Energy decreases

    • Low energy photons are more readily absorbed in a material such as biologic tissue than are high-energy photons

Imaging Pathology

• Pathology may affect the body’s absorption characteristics

• Additive pathology

  • Adds physical density to body tissues

  • Examples: Edema, Paget’s disease, atelectasis, abscesses, pleural effusions 

• Destructive pathology

  • Lowers the body’s normal attenuation ability

  • Examples: Astrocytomas, necrotic areas, osteolytic metastatic disease, multiple myeloma 

Additive Pathology

This pleural effusion (A pleural effusion is a buildup of fluid between the layers of tissue that line the lungs and chest cavity.) in the left hemithorax is an example of an additive pathology.

Pleural effusion can result in diminished breathing capacity and possible lung collapse.

Additive pathology

Malignant mass in lung

Destructive Pathology

An Ewing sarcoma has destroyed part of the distal femur.

Recording Medias

• A medium is a source that may convey data or information

  • Media is the plural form of many different types of mediums

There are many different types of recording medias

• Processing film (older conventional method)

• Fluoroscopic screens

• Image intensifiers

• Electronic imaging devices (EPID)

• Photostimulate phosphor plates,

• Scintillation and piezoelectric crystals

• Flat panel detectors (FPDs)

• Digitally reconstructed radiograph (DRR)

Radiographic cassettes

Provides light tight conditions necessary for photostimulable plate or x-ray film to work

• Construction:

  • Front made of a material with a low atomic number

    • Cardboard

    • Plastic

    • Carbon fiber

• Back made of the material with a high atomic number

  • lead

  • copper

  • other metal

  • 
    

    • This is to prevent backscatter from reaching the film once it exits the cassette

      • Backscatter can cause unnecessary fog and reduce image quality

Radiographic cassette

Opens like a book, can only open cassette in a dark room for processing

Film markers

used for site identification on films

Computed Radiograph - CR cassette

• Photostimulable plates that converts x-rays into a digital image

• The plate is known as an imaging plate, storage phosphor plate or digital cassette

• A cassette similar to a regular cassette houses the digital imaging plate

Flat Panel Detectors (FPDs)

• An amorphous silicon imaging device attached to the linear accelerator

• KV & MV Imaging Systems (many linacs have both)

• Consists of:

  • FPD hardware

  • Workstation

  • Imaging software

• Converts the radiation beam exiting the patient to an electrical signal pixel by pixel to create an image

Benefits of Flat Panel Detectors (FPDs)

• Allows real time electronic imaging

• Can be acquired in seconds, uploaded to a computer and sent to the MD to review

• Is accessible to multiple workstations simultaneously, MD can review from anywhere

• Imaging characteristics can be manipulated via software to improve image quality (density, contrast via window leveling)

• Images can be archived and integrated in the patient’s electronic record

EPID- Electronic Portal Imaging Device

MV Imaging System

• A digital image taken with the therapy machine (MV)

• It is better quality than the analog images acquired by using a film and cassette

  • EPIDs are mounted on linear accelerators to capture images using the treatment beam itself (4–25 MV photons).

OBI – On Board Imaging System

KV Imaging System

• A separate kilovoltage X-ray tube mounted on the gantry, opposite a kV flat panel detector.

• Completely independent from the MV treatment beam and EPID.

• Optimized for 40–150 kVp photons

• Uses:

  • 2D kV radiographs (orthogonal pairs for patient setup)

  • kV fluoroscopy (real-time tumor or fiducial tracking)

  • kV cone-beam CT (volumetric imaging before treatment)

Thin-film transistor (TFT) technology in Flat Panel Detector technology

• TFT acts as switches turning each pixel on (light) or off (dark) depending on the amount of radiation reaching the pixel

  • A pixel (short for picture element) is a single point in a picture. On the monitor of a computer, a pixel is usually a square. Every pixel has a color and all the pixels together are the picture.

• More about this in the CT lectures

DRR – Digitally Reconstructed Radiograph

• An 2D image reconstructed from CT data to create a beam’s-eye-view (BEV) display of the treatment field

• These images resemble conventional radiographs

Which structures absorb more photons – more dense tissues or less dense?

• More dense tissues absorb more photons

  • Why?

    • There are more atoms in that tissue for photons to interact

What x-ray interaction is where the photon is absorbed by the atom?

Photoelectric effect

Which imaging system on the treatment machine using MV energy?

EPID – electronic portal imaging device

• What are the two major components of an x-ray tube?

• Which is the negatively charged side?

• Cathode and anode

• Cathode

What is the primary way to control scatter radiation?

Collumation

• What happens to the intensity of the beam when distance is doubles?

• What is the law that explains this relationship?

• It reduces by ¼

• Inverse square law

• What is the historical unit for exposure in air?

• What year did Wilhelm Conrad Roentgen create x-rays

• Roentgen

• 1895