Introduction to Radiologic and Imaging Sciences and Patient Care
Image Receptor Characteristics
Digital image receptors have inherent characteristics that determine spatial resolution limitations.
These characteristics include:
Matrix size
DEL size
DEL pitch (physical distance between adjacent DELs, measured from center to center)
Sampling frequency (rate at which the signal is sampled)
Higher matrix sizes, smaller DELs with shorter distances between them, and higher sampling frequencies yield higher spatial resolution.
Radiologic technologists cannot alter these inherent characteristics.
Contrast Resolution
Contrast resolution is the ability to distinguish between subtle differences in image receptor exposures (signal values) and differentiate them from noise in the image.
It is affected by image receptor contrast and subject contrast.
Image Receptor Contrast
Image receptor contrast is the range of signal values acquired by the image receptor.
It is controlled by:
Detective quantum efficiency (DQE): measure of the sensitivity and accuracy of the image receptor
Grayscale bit depth: the maximum number of shades of gray each detector element can handle
Dynamic response: the ability of a detector material to sense the incoming radiation
Fluoroscopic Imaging
Fluoroscopic examinations involve a combination of imaging processes.
The fluoroscopic image is a dynamic (moving) image, unlike static radiographic images.
The fluoroscopic examination is divided into:
Viewing a physiologic event in real-time
Archiving images for later review
Modern fluoroscopic units have the x-ray tube located either over or under the x-ray table.
Opposite the tube is the image intensifier unit or flat-panel detector (FPD), which intercepts the attenuated beam as it exits the patient.
The image intensifier or FPD is the image receptor in this case.
X-ray photons reaching the image intensifier or FPD are transformed into an electronic image, which is displayed on a monitor.
Radiologists can view physiologic events and observe abnormalities in function.
Image Archiving Methods
Spot imaging: the fluoroscopic unit changes to radiographic mode for the exposure duration; images are processed, viewed, and stored.
Digital fluoroscopy: has replaced traditional filming and allows for easy manipulation of fluoroscopic images after conversion to a digital signal.
Summary of Key Concepts
The task of the radiologic technologist is to capture images produced by x-rays in a format allowing for storage and repeated viewing.
The basic mechanism of x-ray production has not changed significantly.
A beam of x-rays, produced mechanically by passing high voltage through a cathode ray tube, traverses a patient and is partially absorbed.
An image receptor intercepts the x-ray photons that exit the patient.
Various IR systems are used in radiography, including CR, DR (direct and indirect conversion), and fluoroscopic imaging systems.
A good-quality image must have a proper balance of:
Image signal properties (IR exposure and contrast) affecting the visibility of the image
Image resolution properties (spatial resolution and contrast resolution) affecting the sharpness and accuracy of the image
The radiographer must be able to manipulate technical factors such as mAs and kVp.
Central Ray Angulation and Body Part Rotation
Changing the orientation of the body part undergoing radiography also affects the relationship of the beam, objec t, and IR.
If the object of interest is superimposed on another object, the resulting image is difficult to evaluate.
By rotating or obliquing the body, the object of interest can be projected free from the interference of the overlying object.
Frequently, a combination of part rotation and central ray angulation is used to best demonstrate the anatomic details free from superimposition by overlying structures.
Ideally, the goal of a radiologic technologist is to place the anatomic part parallel to the IR and have the central ray aligned perpendicular to the IR.
Controlling Object-to-Image Receptor Distance (OID)
Control over OID depends on the radiographer's knowledge of anatomy and positioning.
Distortion
Distortion is the misrepresentation of the true size or shape of an object.
Size Distortion
Magnification is the only possible size distortion in radiography, due to the divergent property of x-ray photons.
Magnification can be controlled by proper SID and OID.
Source-to-Image Receptor Distance (SID)
The size of the recorded image varies with the SID.
If the SID decreases, magnification increases.
Magnification decreases as SID increases.
The use of standardized SIDs allows the radiologist to assume a specific magnification factor is present on all images.
Noting any deviation from the standard SID is extremely important.
Object-to-Image Receptor Distance (OID)
Varying the OID also influences the magnification.
If the object is moved closer to the receptor, magnification decreases.
If the object is moved farther from the receptor, magnification increases.
Magnification decreases as OID decreases.
The best image is produced with a small OID and a large SID.
Shape Distortion
The misrepresentation of the shape of an object on an image is called shape distortion.
It is controlled by the alignment of the beam, part, and IR.
Influencing factors include central ray angulation and body part rotation.
Central Ray Angulation
The beam of radiation diverges from the source in an approximately pyramidal shape.
The photons in the center of the beam travel along the straightest pathway (central ray).
The most accurate representation of an object results from the passage of photons in a straight line through the area of interest.
When the central ray is angled, the relationships among the beam, part being imaged, and IR are altered.
Objects may appear elongated or foreshortened.
Central ray angulation is used in many radiographic examinations to help demonstrate specific anatomic details.
Focal Spot Size
The width of the beam is controlled by the selection of the small or large focal spot.
The small focal spot is used when fine detail is required, as in the radiography of small bones.
The large focal spot is used for most general radiographic examinations.
Source-to-Image Receptor Distance (SID)
The distance from the source of x-ray radiation to the IR is a major influence on the spatial resolution of the image.
Penumbra (image unsharpness) obscures the true edge (umbra).
If the flashlight is positioned closer to the object, then as distance decreases, the penumbra around the true shadow increases.
In radiography, the greater the SID, the better the spatial resolution will be.
SIDs are standardized so that the degree of penumbra is at least a known factor.
The most common SID is 40 inches, but chest radiography uses a 72-inch SID.
The SID of an examination should always be indicated to allow for the calculation of image unsharpness.
Object-to-Image Receptor Distance (OID)
When the object is moved closer to the IR, penumbra decreases and image sharpness increases.
As the object is moved farther from the receptor, penumbra increases and sharpness decreases.
The smaller the OID is, the better the spatial resolution will be.
Many of the objects that must be radiographed are structures located deep within the body.
Getting them close to the IR is often impossible.
Beam Modification
One of the primary purposes of beam modification is scatter control.
Scattered radiation reaching the IR produces nondiagnostic exposures referred to as fog.
Removal of fog results in the loss of some specific IR exposures.
Anything that decreases scatter increases subject contrast.
Digital Image Receptor Systems
With digital image receptor systems, the traditional relationship between displayed image contrast and exposure variables does not exist.
Displayed image contrast is determined by the look-up table (LUT), which is applied through digital processing.
The proper LUT provides the proper grayscale for the displayed image, regardless of variations in kVp and mAs, resulting in consistent images.
The information is displayed in a range of brightness, which can then be adjusted through the manipulation of the window width.
Sufficient differences in exposure to the receptor are still necessary.
The radiographer must set proper exposure factors to create an acceptable image quality with minimum exposure of the patient.
Grids
Grids absorb scatter radiation, improving subject contrast.
Grids are available in a diverse range of ratios and frequencies.
These choices allow the radiographer to select how much scatter radiation is to be eliminated, and thereby impact subject contrast.
Due to the high sensitivity of digital image receptors to scatter radiation, the use of a grid is much more critical than in film-screen radiography.
When possible, the highest grid frequency available should be used with digital imaging to avoid certain grid errors.
Image Resolution Factors
Spatial Resolution
The sharpness with which an object's structural edges are represented on an image is referred to as spatial resolution.
It is also described as sharpness of detail, definition, and recorded detail.
Sharpness of detail is complemented by visibility of detail.
Good image quality requires a proper balance of the two.
The chief factors affecting spatial resolution include temporal resolution, beam geometry, and image receptor characteristics.
Temporal Resolution
Temporal resolution is the relationship between the duration of signal acquisition (exposure time) and dynamic motion of the anatomy.
The most common cause of image unsharpness is motion.
Voluntary motion can be controlled by careful instructions, suspension of respiration, short exposure times, and immobilization devices.
Involuntary motion is best controlled by the shortest exposure time possible.
Equipment motion can be decreased by the use of short exposure times.
Beam Geometry
Beam geometry is affected by the focal spot size, distance, and distortion.
The fundamental problem is attempting to represent a three-dimensional object on a two-dimensional image.
Lessening the effect of this inherent loss of detail is possible by adjusting the factors over which the technologist has control: focal spot size, SID, object-to-image receptor distance (OID), alignment, and angulation.
Kilovoltage Peak (kVp)
kVp still controls subject contrast and should be selected for the desired level of latent image contrast (pre-processing).
When the kVp is too low, most of the photons do not reach the image receptor because they are absorbed in the patient.
With high kVp, subject contrast is decreased because both thick and thin body parts are penetrated, which reduces the degree of differential attenuation.
Amount of Irradiated Material
The amount of irradiated material is determined by the thickness of the body part being imaged and the size of the irradiated field.
As body part thickness increases, more x-ray photons will be attenuated.
Conversely, as thickness decreases, so does attenuation.
Subject contrast is reduced with an increase in scatter production.
Type of Irradiated Material
The atomic number of the irradiated material and its tissue density influence subject contrast.
Materials with a higher atomic number absorb a greater percentage of the x-ray photons.
Tissue density is determined by how tightly the atoms of a given tissue are packed together.
When the difference in the densities of adjacent tissues is great, subject contrast is increased.
Contrast Media
Contrast media are substances that attenuate the beam to a different degree than the surrounding tissue.
Contrast media increase subject contrast by introducing greater differences in atomic number variations than those that exist naturally.
Examples include barium and iodine compounds and air.
Technical factors, particularly kVp, must be adjusted for adequate penetration.
Milliampere-Seconds (mAs)
mAs alters IR exposure (signal value) and therefore influences subject contrast.
If an image is grossly overexposed or underexposed, then contrast is affected.
For digital systems, the exposure indicator should be in the acceptable range to assure that the image receptor received the correct exposure.
No increase in mAs can compensate for inadequate penetration.
Digital Image Contrast
Digital image contrast must be considered in terms of subject contrast, raw image contrast, and displayed image contrast.
Displayed image contrast is determined by computer processing and can be altered by the adjustment of window width, independent of the technical factors used during image acquisition.
Displayed digital images with relatively few gray tones possess high contrast, short-scale contrast, and narrow window width.
Images with greater numbers of gray tones possess low contrast, long-scale contrast, and wide window width.
Exposure Indicator Deviation Index Control Limits for Clinical Images
Table 7.3
Details the exposure limits
Table 7.2
Details the Exposure indicators
Scatter Radiation
If scatter radiation reaches the IR, it is not carrying useful information.
Scattered photons that strike the IR degrade the quality of the image by contributing unwanted exposure known as fog.
By restricting the size and shape of the primary beam to the area of interest, we are decreasing the probability of the production of scatter radiation.
A decrease in scatter causes a decrease in IR exposure (signal value).
Grids
A grid is a device that is designed to remove scattered photons exiting the patient before they reach the IR.
A grid consists of thin radiopaque lead strips interspersed with radiolucent spacing material.
The grid is placed between the patient and the IR to intercept scattered photons.
Increasing the lead in a grid increases its ability to remove scatter from the remnant beam.
Grid ratios commonly range from 5:1 to 16:1, with the higher ratio grid able to prevent more scattered photons from reaching the IR.
Grid frequencies are generally 85 to 200 lines per inch.
Decreasing the amount of scatter enhances the radiographic contrast.
All other factors being equal, if the same exposure factors are used with a 5:1 grid and a 10:1 grid, the 10:1 grid produces an image with less IR exposure (signal value).
Higher Fill Factor
A higher fill factor percentage results in increased detector sensitivity.
Sensitivity is determined by how much charge a receptor produces per incident x-ray photon before the collected signal is amplified.
Evaluating IR Exposure (Signal Value)
With digital IRs, the key factor that must be considered when evaluating proper IR exposure (signal value) is the exposure indicator.
Additional considerations include exposure latitude, automatic rescaling, and window leveling.
Exposure Indicator
Exposure indicator is a numeric representation of the quantity of exposure received by a digital IR.
The target exposure indicator for all systems designates the middle of the image receptor's operating range as the optimal exposure.
The deviation index (DI) would be used in conjunction to determine if the appropriate radiographic technique factors were used during an examination.
The DI can be defined as a comparison of the target value with the actual exposure recorded by the IR.
Inverse Square Law
mA and time
mA and time are all directly proportional to beam intensity
Beam Modification
Anything that changes the nature of the radiation beam, apart from the factors already discussed, is referred to as beam modification.
The beam may be modified before it enters the patient (primary beam modification) or after it exits the patient (scatter control).
The amount of attenuating material required to reduce the intensity of a beam to half the original value is referred to as the half-value layer.
Beam restriction involves the use of devices, such as a collimator, to confine the x-ray beam to the area of interest, thereby reducing exposure to body parts other than those under examination.
Patient Factors
Various patient factors affect IR exposure (signal value).
Patient size and thickness, the predominant atomic numbers of the materials, pathologic conditions, anomalies, temporarily compressed tissues, and a number of other techniques all change the subject density of the tissues being examined.
As subject density increases, IR exposure (signal value) decreases, and vice versa.
Kilovoltage Peak
In addition to the number of x-ray photons produced, the relative strength of the photons must be considered.
The kVp setting determines the highest energy level, or the peak, possible for the photons within that beam.
The relationship between kVp and IR exposure (signal value) is not as simple as that of mAs.
Increasing kVp 15% will approximately double IR exposure (signal value).
Decreasing kVp 15% will approximately halve IR exposure (signal value).
kVp can be changed while maintaining the same IR exposure (signal value) as follows
Image Quality Factors
Exposure Maintenance Formula
Inverse Square law
Four factors for x-ray production
Vacuum
Source of electrons
Method to accelerate electrons rapidly
Method to stop the electrons