Radiography: Image Quality and Digital Principles

• Radiographic imaging involves X-ray image production.

Classes of Radiation

• Primary radiation exits the X-ray tube.
• Absorbed radiation stays in the patient.
• Remnant radiation strikes the image receptor.
• Scatter radiation is a type of non-diagnostic radiation.

The Latent Image

• It is formed in the detector after the beam strikes the detector.
• The latent image must be processed to convert it to a visible radiograph.

X-rays Produced

• When electrons strike the target, 99% heat and 1% X-ray is produced.

Attenuation of Radiation

• Attenuation is the loss of radiation energy as it passes through an absorbing material, like the human body.
• Different materials absorb radiation energy differently based on density and atomic number; this is known as differential absorption.
• The degree of attenuation can be high or low.
• High attenuation occurs in radiopaque matter.
• Low attenuation occurs in radiolucent matter.
• High attenuation materials are considered radiopaque, so X-rays do not pass through easily.
• Low attenuation materials are considered radiolucent, so radiation easily transmits through.

Classes of Diagnostic Radiographic Imaging

• Digital or computerized imaging.
• Fluoroscopic imaging.

Types of Digital Imaging Detectors

• Computed Radiography (CR) uses photostimulable phosphor (PSP).
• The latent image forms on the PSPs.
• Flat-panel (Digital Radiography (DR) includes direct and indirect capture.
• Direct capture converts X-rays directly to electrons.
• Indirect capture converts X-rays to light, then to electrons.

Definitions

• IR refers to the image receptor or detector, which is the medium used to capture the image.
• Other terms for IR include image plate and cassette.
• Image receptors detect the remnant radiation from the patient and convert it into chemical or electrical changes.
• CR refers to digital acquisition that uses storage phosphors plates or PSP to produce images.
• DR refers to a digital method of imaging that converts X-ray energy into a digital electronic signal projected on a display monitor.

PSP Capture

• Photostimulable Storage Phosphor (PSP) Technology is used in CR radiography.
• The PSP is housed in an imaging plate (IP).
• The exposed IP is placed in a reader for electronic processing of the latent image into a manifest image displayed on a monitor.
• PSP capture ultimately creates a digital image through computer software.
• A PSP plate or computer reader opens the IP, removes the PSP, scans, reads, and erases the PSP.
• After scanning, the PSP is returned to the IP and the IP is ejected from the reader.

Digital Radiography (DR)

• Uses solid-state electronics and thin-film transistor (TFT) technology.
• The detector is permanently encased in a rigid cassette known as the "flat panel detector" (FPD).
• Radiation exposure to the detector creates an electronic signal proportional to exposure.
• No reader is required.
• Image display occurs in seconds.
• Two classes of DR technology are indirect and direct.

Direct Capture

• In direct capture, the X-ray beam strikes the photoconductor, which includes amorphous selenium (aSe).
• The photoconductor converts X-rays to electrons.
• Signals are sent to a computer for processing and then to a monitor for display.

Indirect Capture

• In indirect capture, the X-ray beam strikes a scintillator, which includes cesium iodide (Csl).
• The scintillator converts X-rays to light.
• Light strikes a photodiode that converts the light to electrons.
• Signals are sent to a computer for processing and then to a monitor for display.

Automatic Rescaling

• The image on the monitor is automatically adjusted by the computer to show a perfect image.
• Computer software optimizes image quality by varying the brightness and contrast of the image based on detector exposure.
• The computer can adjust images with 50% underexposure.
• The computer can adjust images with 100% - 400% overexposure.
• Automatic rescaling is no excuse for exposure technique selection inaccuracies.

Exposure Index (EI)

• It helps the technologist identify that the initial exposure was correct; all images will display a numeric representation of total X-ray exposure to the receptor or detector.
• The EI number indicates whether the initial technique was correct, whether it was optimal, or whether over- or underexposure occurred.
• EI values can vary greatly among manufacturers (vendors).
• EI does NOT represent patient exposure.
• The EI number can be used to assess image quality in terms of image noise and IR exposure.
• The operator has a responsibility to understand what number means in terms of optimum exposure and image quality.

Deviation Index (DI#)

• Established by AAPM.
• Similar to the EI number, the DI Number can be used to assess image quality in terms of image noise and IR exposure.
• The same DI values are used by all manufacturers (vendors).
• Compares actual exposure to optimum Target Exposure (ElT).

Advantages of the Digital Image

• Image data is captured in digital format.
• This data can be copied and modified.
• Digital images are distributed worldwide.
• They are archivable for storage, retrieval, and display.
• Pixel refers to one cell in the matrix (digital image).
• Matrix is determined by the number of pixels in the rows and columns and expressed by number of pixels along length and width.

Brightness

• It is the intensity of light a monitor emits.
• Brightness represents individual pixels in the image (replaces the term density in film-base imaging).
• Controlling factors are that the user can alter the brightness of the digital image after exposure and that brightness is partly controlled by mAs.
• Controlling brightness on the display: increasing window level decreases brightness.

Contrast

• It is the density difference between two adjacent areas of a radiograph, or how the densities of two adjacent areas differ.
• This defines the quality of the x-ray beam.
• Low contrast images have many density levels, or shades of gray.
• High contrast images have few density levels, and are mostly black and white.
• The user can alter the contrast of the digital image after exposure.
• Contrast is partly controlled by kVp.
• Controlling contrast: increasing window width decreases contrast.

Noise

• It refers to random background information due to constant flow of current in the circuit.
• Noise does not contribute to image quality.
• On the digital image, noise results in a quantum mottle.
• A noisy image is usually an indication of under exposure.

Spatial Resolution

• It is described as the recorded sharpness or detail of structures on the image with which an object's structural edges are shown.
• It is measured as the minimum separation between two objects at which they can be distinguished as two separate objects in the image.
• Unit: Line pairs per millimeter (lp/mm)

Technical Exposure Factors

• They are set by the technologist to produce the voltage and current needed to create X-ray production.
• Two factors are set at the console: milliamperage (mA) and time in seconds, and kilovoltage peak (kVp).
• One factor is set at the X-ray table: Source-to-Image Distance (SID).
• mA refers to the amount of radiation produced by the X-ray tube.
• The product of the mA (milliampere) x seconds = mAs.
• mA is a measure of the current passing through the X-ray tube.
• Time is the duration of the exposure.
• mA controls the quantity of the X-ray photons produced.
• mAs is calculated as mA x time.
• Regardless of the mA and time combinations, the same mAs value will yield the same exposure (mAs reciprocity law).
• kVp measures the electrical pressure (potential difference) forcing the current through the tube.
• The X-ray beam is polyenergetic or heterogeneous, so the energy is not uniform.
• kVp controls the penetrating ability of the beam.
• Object-to-Image Distance (OID) is the distance between patient and image receptor.
• Source-to-Image Distance (SID) is the distance between the X-ray tube target and the image receptor.
• Source-to-image distance is the measure of the distance between the point of x-ray emission in the x-ray tube (focal spot) and the image receptor, displayed as Source-to-Image Distance (SID).
• X-ray production is similar to a point light source.
• It behaves according to the laws of light and intensity as a function of distance.
• The Inverse Square Law is I1/I2 = D22/D12.

Distortion

• Refers to any misrepresentation of the true size or shape of the patient's anatomy demonstrated on the radiographic image.
• Two types of distortion are size distortion and shape distortion.
• Size distortion is an increase in shape size, also called Magnification.
• The image is always slightly larger than the actual object size.
• It is controlled by using longer SIDs and minimum OIDs.
• Use of deliberate size distortion is accomplished by increasing the OID.
• Deliberate size distortion helps overcome natural OID, for example, when imaging the joints.
• Shape Distortion refers to any misrepresentation of the true shape of the patient's anatomy.
• It is controlled by alignment of central ray, patient's anatomy, and IR.
• Deliberate shape distortion may be deliberate to deal with superimposed structures.
• Use of deliberate shape distortion is accomplished by angling or rotating the patient relative to the central ray of the x-ray beam.
• It helps overcome superimposition of anatomic structures and is useful when imaging joints.
• Tissue in the human body is made up of different densities or thickness.
• X-rays will pass through or will be stopped depending on the thickness of the body part.

Beam Modification

• The x-ray beam can be modified before and after it enters the patient.
• Beam alterations can improve image quality and reduce dose.
• Modification of remnant radiation is a scatter control process.
• The primary method of beam modification or Scatter control is use of collimation, also called beam restriction, and use of a grid.

Scatter Control

• Interaction of x-rays with any matter produces scatter radiation, which provides little diagnostic information to the image.
• Reduction in scatter radiation yields a decrease in IR exposure, which detracts from image quality if scatter is excessive.
• Grids are used to reduce the amount of scatter radiation reaching the image receptor and intercept a portion of the remnant radiation.
• They improve image quality.
• Grids necessitate an increase in exposure to compensate for the grid, and are placed between the patient and the detector.

Motion

• Motion distortion is the most common cause of image unsharpness.
• Motion is mainly caused by voluntary and involuntary patient motion.
• Involuntary motion is not under the control of the patient, such as a heartbeat or peristalsis.
• Voluntary motion is under patient control, such as walking.

X-Ray Production Objectives

• Describe the Bremsstrahlung x-ray production process.
• Describe the characteristic x-ray production process.
• Identify the information contained in an x-ray spectrum.
• Identify the x-ray emission spectrum, characteristic x-ray spectrum, and Bremsstrahlung x-ray spectrum.
• Identify factors affecting the x-ray emission spectrum, including mA & mAs, kVp, Filtration, Target Material, and Voltage Waveform.

How X-rays are Produced

• Projectile electrons, produced by thermionic emission in the cathode, are accelerated by the high voltage to the anode, and are stopped at the anode to interact.
• Electrons interact with orbital electrons or nuclear field of the target atoms.
• As a result, 1% X-ray production occurs as Bremsstrahlung rays (60% of diagnostic x-rays) or characteristic radiation.

Anode Heat

• Heat is produced at the anode.
• The kinetic energy of the electrons is converted to heat.
• Electrons interact with outer-shell electrons at the target.
• The outer shell electrons become excited, raise to a higher energy level, and then drop back to their normal level.
• When returning to normal levels, they emit infra-red radiation.
• Increasing the tube current increases heat produced.
• Doubling x-ray tube current will double the heat produced.

Interactions in the Anode

• Two main possibilities exist for a projectile electron to interact with atoms in the anode: interact with the nucleus (Bremsstrahlung Radiation) and interact with an orbital electron (Characteristic Radiation).

Characteristic Radiation

• An incident photon ejects an inner shell (K or L) electron and an outer shell electron fills the vacancy.
• It results in a "cascade effect."
• The characteristic x-ray photon that is emitted has an energy of photon emitted equals that of the difference of the two shells' binding energies.
• The atom will fill an inner-shell vacancy by pulling a higher orbital electron down from its shell.
• The dropping electron loses potential energy.
• That energy is emitted as a characteristic x-ray, which equals the binding energies for the two shells.
• An orbital electron will then have sufficient energy to ionize or remove an orbital electron from an inner electron shell, which creates a vacancy and the atom becomes unstable as a result.
• A higher outer-shell electron will then fill the inner shell vacancy.

Bremsstrahlung Radiation

• The positive attraction of the nucleus pulls the electron toward it as the electron passes, which slows the electron and changes its direction.
• The lost of kinetic energy as the electron slows is emitted as an x-ray.
• Bremsstrahlung is produced by interaction of electrons with the force field of the nucleus.
• Bremsstrahlung Interactions happens when electrons are slowed down in the anode. electrons are negative, so it nucleus its positive.
• As a result, electrons get close to the nucleus and are slowed by the force field of the nucleus - slows its change direction.
• As the electron slows it loses energy that is emitted as an x-ray photon.
• The closer a projectile electron passes to the nucleus, the more it will be decelerated, so the more energy it will lose.
• Bremsstrahlung radiation accounts for most of the x-rays in the beam and is responsible for the heterogeneous or poly-energetic nature of the x-ray beam.
• This contributes to differential absorption by tissues of the body and provides for subject contrast in the remnant beam.
• Subject contrast includes the ability to see different structures in the body, e.g. heart and lungs.
• Without subject contrast, it would be impossible to produce diagnostic images with x-rays.
• With x-rays of many energies subtle tissues, such as fat pads and bone marrow, can absorb different portions of the beam so that they are demonstrated.
• Computers can modify many aspects of the image, but they restore information that is not present in the remnant x-ray beam in the first place.

Explore radiography principles including grid use, causes of image unsharpness, motion artifacts, and X-ray tube dynamics. Understand heat production, digital radiography with cesium iodide, automatic rescaling, and the importance of the Exposure Index (EI) and Deviation Index (DI).