Recorded Detail in Radiography

Recorded Detail

Introduction

Assessing Recorded Detail

*Defined as the degree of geometric sharpness or accuracy of the structural lines actually recorded in the image

*Other names used when referring to recorded detail include;

-Definition

-Sharpness

-Spatial resolution (quantifies recorded detail)

-Detail

Assessing Recorded Detail

Unit of resolution – lp/mm

-Resolution tool with closely placed lines, alternating radiopaque and radiolucent, whose spacing is accurately known

-Line and corresponding space = one line pair

-At a normal reading distance the unaided eye may discern about 5 lp/mm

-Spatial resolution is limited to the maximum number of lp/mm that can be recorded by an imaging

*Radiographic images have less recorded detail than the anatomical part being imaged

*The art of radiography is to control the degree of unsharpness (penumbra) on the image

-We will explore aspects of acquiring an image that affect recorded detail of which the MRT has control

*Recorded detail is usually evaluated using a resolution test tool, however may be evaluated clinically by evaluating bony trabeculae

*Spatial resolution

-Spatial resolution is high when an imaging system can accurately display two small or closely placed objects

-The following are different measures of spatial resolution 1.Point spread function (PSF)

2.Line spread function (LSF)

3.Edge spread function (ESF)

-All of the above express the boundaries of an image. In conventional imaging this is called penumbra or image blur.

4.Modulation transfer function (MTF)

1.Point spread function (PSF)

*Based on the fact that tissues and organs consist of a number of minute points of anatomical detail

*Obtained with a pinhole camera and a microdensitometer

2.Line spread function (LSF)

*Produced by irradiating an aperture with a 10 μm slit and taking microdensitometer measurements of the line

*Will measure the absorption unsharpness caused by diffusion of light in intensifying screens

*A perfect imaging system should produce an image of a line 10 microns wide

3.Edge spread function (ESF)

*A sheet of lead is placed on a cassette and exposed

*Microdensitometer readings are taken across the black-and-white border

*Measure of the density gradient depends on unsharpness and density difference (contrast)

*A high contrast image may appear sharper than a low contrast image even though both images posses the same measured unsharpness

Spatial frequency

-High spatial resolution represents a high-frequency signal that is capable of imaging smaller objects

4.Modulation transferfunction (MTF)

•An objectivemeasurement of thecombined effectthateach aspect of theimagingsystemhason recorded detail

•Numericalvalues areobtainedfromtheLSF graph with anumerical process known as Fourier transformation

(obtaining anumberfrom acurve)

•Number ranges from0 to 1 (0% to 100%) with 1 being themaximum spatial frequency

4.Modulation transferfunction (MTF)

•Thereforethe MTFcan never be> 1

•The total MTF isobtained bycombining all of the component MTF values

–MTF_Total= MTF₁x MTF₂x MTF₃ etc.

Noise

*Imaging noise is the background information the image receptor receives

*Quantum noise indicates that there is insufficient incoming data / number of photons reaching the image receptor -Resulting in quantum mottle; a blotchy or mottled image

-Solution is to increase the number (quanta) by increasing the mAs

Effects on Image Appearance

*Resolution has a direct effect on image appearance

*Recorded detail describes the degree of sharpness of structural lines on a radiographic image

-Resolution is good when fine detail structures are demonstrated; detail will demonstrate sharp lines

-Resolution is poor or lacking demonstrating a blurry edge around the line i.e. motion unsharpness

*Recall that some detail is always lost

Factors affecting recorded detail (P.2)

*Carlton and Adler recommend addressing resolution problems in the following order;

1. Eliminate motion
2. Reduce OID
3. Reduce focal spot size
4. Reduce intensifying-screen phosphor size and concentration; and
5. Increase SID

Geometry

-The x-ray beam originates from a small point (focal spot); the further the photons move from the source, the farther the divergence

-Does this sound familiar? This is the basis of the inverse square law…

-As the collimated x-ray field size passes through the patient the amount of tissue exposed increases

Distance

Distance between the x-ray source (focal spot), the object (anatomy of interest), and image receptor are critical to consider due to their effect on recorded detail

SOD + OID = SID

As SID increases the recorded detail increases

Due to the more centrally located x-ray photons used to form the image

The central x-ray photons have a more parallel path when compared to the diverging x-ray photons at the periphery of the beam Carlton P. 434 FIGURE 28-14 (E. and F.)

*As OID decreases the recorded detail increases

*As OID increases the recorded detail decreases

-Due to the increased penumbra

-An increase in OID also results in magnification of the image

Focal Spot Size

-Focal spot size is controlled by the line focus principle

-Selecting a small focal spot size will result in a smaller effective focal spot

-The effective focal spot size is controlled by the size of the actual focal spot (length of the filament) and the anode angle

-Smaller effective focal spot size permit the best detail resolution

-Carlton’s definition of penumbra “the imperfect, unsharp shadow surrounding the umbra”

-Focal spot size controls penumbra

-Recall that the focal spot is not a point source rather a rectangular shape; this is the cause of penumbra Carlton P. 434 FIGURE 28-14 (A. and B.)

Penumbra

-Other factors that influence penumbra are OID, SID, and attenuation or absorption unsharpness

-Attenuation or absorption unsharpness

*The shape of anatomical structures result in increased penumbra because of the divergence of the x-ray beam

*Most shapes in the body are circular resulting in unequal attenuation and edges of structures imaged as gradual changes in density versus a sharp change

*Generally, larger objects have less recorded detail than smaller objects due to their increased distance to the image receptor

Image Receptor

-Film/screen systems

-There is an inverse relationship between film/screen speed and resolution

*Slow film/screen combinations demonstrate better recorded detail than faster speed systems

Film/Screen System combination

-There is an inverse relationship between film/screen speed and resolution

*Slow film/screen combinations demonstrate better recorded detail than faster speed systems

*Refer to our study of intensifying screens in Part 1: Unit 7

-Phosphor size, phosphor thickness, and phosphor concentration (packing density) are beyond the scope of this course

-Recall the importance of foam pads within the cassette to ensure film screen contact is maintained

-The result of poor film screen contact is loss of recorded detail due to increased divergence of light

*The light must travel a greater distance to reach the film

-Traditionally this is seen in fast imaging systems

-Recall the indirect relationship between relative speed of an imaging system and the mAs required

-As RS increases the mAs required to obtain a comparable diagnostic image decreases

mAs1 / mAs2 = RS2 / RS1

Motion

-Unsharpness of the image caused by movement of the object during exposure or movement of the equipment

*Patient motion may be voluntary or involuntary

– “…the image is spread over a linear distance and appears as a blurred series of densities in which no fine detail can be perceived.”

-Reduced by decreasing exposure time, providing the patient with clear instructions and using immobilization devices or supports for the patient