radiology

1. Image Quality

2. Optical Density
 It is the degree of darkness or blackening of the xray image. It is the log of the ratio of the
incident light intensity to the transmitted light intensity.
OD = log10 (Incident light intensity/transmitted light intensity)
 Diagnostic ODs : 0.2 to 3.0
 Factors affecting OD:
 mAs : is the controlling or primary factor (larger mA values or longer exposure times produce
darker images)
 kVp ( increasing the kVp, increases the density to the image)
 SID
 Filtration
 Beam restriction
 Body part thickness ( inversely related)

6. Radiographic Contrast
 It is made up of the total amount of contrast acquired from both the subject contrast
and film contrast.
 Film contrast is represented as the slope of the characteristic curve.
 Subject contrast depends on differential absorption of the xray beam.
 Factors affecting the subject contrast:
 Tissue thickness
 Tissue type and atomic number
 Tissue density
 kVp
 Contrast agents
 Scatter radiation

8. Characteristic Curve
 A plot of the optical density as function of logarithm of the exposure
is called the characteristic curve.
 It shows the speed , contrast and latitude of a particular film .
 The four regions of the characteristic curve are the :
Base plus fog
Toe region
Shoulder region
Maximum density (Dmax)

9. • Base plus fog : initial film density
before exposure to xrays
• Toe: represents area of low exposure
levels
• Straight line: range used in radiology
• Shoulder : any additional exposure
does’nt produce much additional
blackening ( high exposure levels)
• Dmax :additional exposure will
result in less density .

10. Speed
 The ability of an xray film to respond to an xray exposure.
 The position of the toe will determine how soon the straight line portion will
begin , which is an indication of the speed of the film.
 The Speed point of a film describes the exposure required to produce an
optical density of 1.0 above base plus fog.
 A faster film requires less exposure and lower mAs to produce the same
optical density.

12. Film Contrast
 It is the difference in optical density between two areas on the image.
 Films with exposure between OD 0.5 and 2.5 exhibit contrast in the diagnostic
range.
 Films with optical density in the toe or shoulder portion demonstrate a loss of
contrast.
 Films with steeper straight line portions have higher contrast.

15. Latitude
 It is the range of exposures that produce an acceptable radiograph with
densities in the diagnostic range.
 Latitude and contrast are inversely related.
 Wide latitude images are termed long grayscale contrast or low contrast.
 Narrow latitude images are termed short grayscale contrast or high contrast.
 Higher speed films have higher contrast and narrower latitude ; slower speed
films have lower contrast and wider latitude .

17. Radiographic Noise/ Quantum Mottle
 It is the random speckled appearance of an image.
 It is noticeable when the number of xray photons forming the image is too
low.
 High speed screens require fewer photons and produce images with noise.
 The technical factor that influences the quantum mottle is mAs.
 Increasing the mAs will effectively eliminate the quantum mottle.

18. Technical Factors
 mA: the quantity of number of xray photons reaching the image receptor
determines the noise or a snowy appearance of the image. (25 to 1000mA)
 Exposure time: longer exposure times result in more xray photons striking the
patient . Exposure times should be selected to allow for the shortest
exposure time to minimize the patient motion artifacts.
 Kilovoltage :changes in kVp alter the penetration or quality of the xray
beam.kVp is the primary controlling factor of radiographic contrast. High kVp
images have low contrast and smaller differences between black and white .

19.  Distance : the distance between the xray source and the image receptor (SID)
influences the quality of the beam and the radiographic contrast. Doubling
the distance decreases the xray intensity by a factor of 4. This is called the
inverse square law.
I2 = I1( D1/ D2)2
 Optical density, mAs , SID: changes in the SID will change the optical density
unless the mAs is altered . The mAs is the primary controlling factor for
density. mAs and distance is directly proportional. If the distance is doubled ,
mAs must be increased by a factor of 4.
 kVp and image density: a higher kVp means less xray quantity is needed to
produce the same optical density. A 15% change in kVp is equivalent to
changing the mAs by a factor of 2.

21. Geometric Factors
 Two primary geometric factors: Recorded detail and Distortion
 Recorded Detail/definition/sharpness/spatial resolution : it is the degree or
amount of geometric sharpness of an object recorded as an image.
 Detail is defined as the smallest separation of two lines or edges which can be
recognized as separate structures on the image.
 Spatial resolution is determined by lines per millimeter(lp/mm). A system
with lines 0.5mm wide can image two objects 0.5mm apart as distinct
objects. Imaging systems with better resolution can resolve more lines per
millimeter.

22. Factors Affecting Detail
 Following measures must be taken when the image resolution is less than
optimal:
 Eliminate patient motion
 Increase SID
 Reduce focal spot size
 Reduce object image distance (OID)

24.  When OID increases , the resolution is degraded. For this reason the affected
side or part of interest is typically placed as close as possible to the image
receptor. If an improvement in detail is needed , the first factor to consider is
the OID.
 When SID increases , the resolution is improved.
 A small target angle produces an effective focal spot that is smaller than the
actual focal spot. The smaller the effective focal spot , the lesser the focal
spot blur.
 Xrays diverging from different parts of the focal spot will blur the edge of an
image. This blur or unsharp shadow along the edge of the image is called
penumbra .

25.  Larger focal spots produce greater amounts of penumbra or blur. The size of
the focal spot is a major controller of image resolution because it controls the
amount of focal spot blur. Amount of blur is calculated by the formula:
focal spot blur = focal spot size x OID/ SOD
 The penumbra will decrease when the focal spot size decreases , the OID
decreases and the SID increases.

27.  Screen speed and resolution: thicker layers with large crystals (means a high
speed screen , most of the xray photons will be converted to light) have
poorer spatial resolution because the light spreads sideways and blurs out the
edges of an image .
 Slow speed screens for extremity imaging to provide the best resolution to
demonstrate the fine bony markings. High speed screens used for abdomen
imaging where fine detail is not needed.
 Motion Blur: Motion affects the recorded detail because it appears as a
blurred series of densities where no fine detail can be visualized. Movement
of the patient or xray tube during exposure results in blurring.
 Patient motion is usually the cause of motion blur.

28.  Procedures for reducing the motion blur :
Use the shortest possible exposure time
Restrict patient motion
Use a large source to image receptor distance (SID)
Use a small object to image receptor distance (OID)

29. Distortion
 It is the misrepresentation of the size or shape of an object.
 Size distortion is termed magnification.
 Shape distortion is termed either elongation or foreshortening.

30. Magnification / Size Distortion
 Magnification results from the represented object appearing larger on the
final radiographic image.
 The greater the SID the smaller the magnification of the anatomical part.
 The structures with the largest OID will be projected with a larger size than
structures that have a small OID.
 Magnification factor: mF= SID/SOD
 Decreasing the SOD , increases the magnification. Objects that are closer to
the source have a high magnification factor.

34. Shape Distortion
 It depends on the alignment of the xray tube , the body part and the image
receptor.
 Shape distortion displaces the projected image of a structure from its actual
position.
 If the image is shorter in one direction than the object, the image is said to
be foreshortened. If it is longer in one direction than the object , it is said to
be elongated.
 Foreshortening occurs only when the part is improperly aligned with the tube
and image receptor.
 Elongation only occurs when the tube is angled.

35.  The central ray is the line connecting the focal spot to the center of the
image receptor.
 The central ray must be perpendicular to the image receptor and body part
being examined to eliminate shape distortion.
 Another alignment issue that must be corrected is incorrect centering which
occurs when the tube is off centered to the image receptor , when the image
receptor is off center to the tube and when the anatomical part is incorrectly
positioned in relation to the tube and image receptor.

37.  Beam angulation: Sometimes shape distortion is used to reduce
superimposition of overlying structures by angling the central ray.The amount
of angulation is used to create a controlled or expected amount of shape
distortion. This will result in foreshortening or elongation of the structures
overlying the body part of interest.
 Tube angle direction: the xray tube is commonly directed with the
longitudinal axis of the table, this avoids issues with grid cut off. The
longitudinal angles are either cephalic or caudal.

38. THINK TANK
1. A high contrast film has a ----------- lattitude. (wide / narrow)
2. Radiographic contrast depends on--------.
3. ---------- is the degree of blackening on the film.
4. The primary controlling factor for magnification is -------------.
5. When a larger focal spot size is used , how will it affect the focal spot blur
(increased/ decreased / no change)