Assessing distortion in radiography

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Distortion

Assessing distortion

•Distortion is defined as “a misrepresentation of the size or shape of structures being examined”

•Familiarity with the normal size and shape of structures is helpful when assessing images

•It is therefore the technologists responsibility and goal when positioning to ensure the anatomy of interest is properly represented on the image

 

Factors affecting size distortion

•In film/screen systems size distortion can only be magnification, this is due to the divergence of the x-ray photons

•Size distortion is controlled by SID and OID

•Minimizing magnification distortion will result in an increase in recorded detail

•Size distortion (magnification) is controlled by positioning the x-ray tube and body part to maximize SID and minimize OID

–Consider C-spine and CXR positioning

•Source-to-image-receptor distance

–The greater the SID, the smaller the magnification… “as SID increases, the percentage of the total distance that makes up OID decreases.”

•OID is the critical distance for magnification and resolution

–The technologist selects the appropriate SID when positioning

–SID is maximized to decrease magnification

•Object-to-image-receptor distance

–As stated, OID is the critical distance in both magnification and resolution

–Objects that are further from the IR will be magnified; this is the opposite of what is perceived visually

–The size of the projected object depends on where it is situated in the body; therefore obtaining two images at 90 degrees to each other will verify the positional relationship of structures

–OID is also an important consideration to establish skin entrance dosimetry limits

–OID varies with part size and position of the patient i.e. AP projection versus a lateral view

–The technologist must minimize OID to decrease magnification

•AP CXR will place the heart ~ 15 cm from the IR versus the PA position will place the heart ~ 5 cm from the IR

•Calculating size distortion

–Clinically it is important for the radiologist to know the amount of magnification produced

–This is especially important during:

•Chest x-rays

•Angiography

•Mammography

–Image size, object size, magnification factor and percent magnification can all be calculated

•SID = OID + SOD

•OID = SID – SOD

•SOD = SID – OID

 

• Calculating size

distortion (cont’d)

–Magnification Factor

•Magnification or size distortion can be assessed by calculation of the magnification factor M = SID/SOD

M = magnification factor

 

Example #1

–What is the magnification factor if the SID is 100 cm and the SOD is 75 cm?

•Example #2

–If the SID is 100 cm and the OID is 5 cm, what is the magnification factor?

Ohm’s Law Triangle Mnemonic Device Formula used to calculate object size

The magnification factor permits calculation of the actual size of an object that is projected as an image

O = I/M

O = actual object size

I = image size

M = magnification factor

 

•Example #3

–If a projected image measures 5 cm and the magnification factor is 1.02, what is the size of the actual object?

–Percent Magnification

•If the image size and the object size are known, the percent magnification (%M) may be determined

I – O x 100/O = % M

•Example #4

–If an object measures 5 cm and the image measures 6 cm, what would the percent magnification of the object?

Factors Affecting Shape Distortion

•Shape distortion is the misrepresentation by unequal magnification of the actual shape of the structure being examined

•Elongation (FIGURE 29-7 B.)

–Anatomy on the image appears “stretched” or longer compared to the normal shape

•Foreshortening (FIGURE 29-7 C.)

–Anatomy on the image appears “shorter and thicker” than it is normally

•Shape distortion also occurs because of the divergence of the x-ray beam.

•The projected shape is dependent on the anatomy’s orientation within the x-ray beam

•Alignment

–Shape distortion may be caused or avoided by careful alignment of the central ray with the anatomical part and the image receptor

–Central ray is at right angles to the anatomical part and image receptor

–Part and image receptor are parallel

–Central Ray

•The CR is the theoretical photons that exit from the exact center of the focal spot

•Ideally the CR is intended to be projected perpendicular to both the anatomical part and the image receptor

•Any structure that is not positioned at the CR will be distorted because of the divergence of the beam

•The farther the structure is from the CR the greater the distortion

–Anatomical part

•The long axis of the anatomical part or object is intended to be positioned perpendicular to the CR and parallel to the image receptor

•Elongation occurs when there is poor alignment of the x-ray tube and/or image receptor

•Foreshortening occurs only when there is poor alignment of the part with the image receptor

–Image Receptor

•B. and C. are ?       •D. and E. are?

•Angulation

–Angulation refers to the direction and degree the x-ray tube is moved from its normal position which is perpendicular to the image receptor

–Angulation of the x-ray tube;

•is designed to cause a controlled or expected amount of shape distortion to avoid superimposition of anatomical structures

•also changes the SID, which, unless compensated for by a new SID, will produce a decrease in density/IR exposure

•Direction

–The most angle placed on the x-ray tube is a longitudinal angle

•Cephalad

–The angle is directed towards the patient’s head

•Caudad

–The angle is directed toward the patient’s feet

–Transverse angles

•Trauma radial head : Axiolateral (45° towards shoulder)

•Trauma oblique c-spine: AP axial oblique (45 ° mediolateral AND [if possible] 15 ° to 20 ° cephalad)

•Degree

–Degree is simply a method of describing the exact amount of angulation

–Is usually stated as the angle between the CR and the image receptor plane; from the standard reference point of perpendicularity

TABLE 29-1 SID Compensa-tions for Common Tube Angulations

Tube Angulation Overhead Scale True SID
39.8” 40”
10° 39.4” 40”
15° 38.6” 40”
20° 37.6” 40”
25° 36.2” 40”
30° 34.6” 40”
35° 32.8” 40”

Effect on Image Appearance

•Size

–Magnification is the outcome of size distortion. See Carlton and Adler P. 454 FIGURE 29-12 comparing heart size in PA and AP CXRs

•Shape

–Foreshortening and elongation both attribute to shape distortion. See Carlton and Adler P. 454 – 455 FIGURE 29-13 depicting various degrees of distortion on common bones.

Additional Clinical Application / Discussion

* Demonstration of air-fluid levels. A coconut was used to demonstrate the effect of various positions and tube angles on air-fluid levels.

A. (A)Recumbent AP projection with beam perpendicular to the air-fluid level.

* Demonstration of air-fluid levels. A coconut was used to demonstrate the effect of various positions and tube angles on air-fluid levels.

A. (A)Recumbent AP projection with beam perpendicular to the air-fluid level.

 

* Demonstration of air-fluid levels. A coconut was used to demonstrate the effect of various positions and tube angles on air-fluid levels.

B. (B) Upright AP projection with beam parallel to the air-fluid level.

* Demonstration of air-fluid levels. A coconut was used to demonstrate the effect of various positions and tube angles on air-fluid levels.

B. (B) Upright AP projection with beam parallel to the air-fluid level.

 

* Demonstration of air-fluid levels. A coconut was used to demonstrate the effect of various positions and tube angles on air-fluid levels.

(C) The cassette was at a 45º angle to the plane of the floor with the beam parallel to the air-fluid level and therefore parallel to the floor.

 

* Demonstration of air-fluid levels. A coconut was used to demonstrate the effect of various positions and tube angles on air-fluid levels.

(C) The cassette was at a 45º angle to the plane of the floor with the beam parallel to the air-fluid level and therefore parallel to the floor.

 

* Demonstration of air-fluid levels. A coconut was used to demonstrate the effect of various positions and tube angles on air-fluid levels.

(D) The cassette was at a 45º angle to the plane of the floor and therefore at a 45⁰ angle to the air-fluid level, with the beam perpendicular to the cassette. Note the central ray must be parallel to the floor to demonstrate distinct air-fluid levels.

 

Demonstration of air-fluid levels. A coconut was used to demonstrate the effect of various positions and tube angles on air-fluid levels.

(D) The cassette was at a 45º angle to the plane of the floor and therefore at a 45⁰ angle to the air-fluid level, with the beam perpendicular to the cassette. Note the central ray must be parallel to the floor to demonstrate distinct air-fluid levels.

* Alignment problems during mobile chest radiography.

(A) A simple alignment problem correctly resolved.

(B) A difficult alignment problem correctly resolved.

 

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