RADIOLOGY EXPOSURE - UNIT 4
Magnification Math: What is the percent of magnification for chest at 72" SID, 3" OID, and MF = 1.043?
% of MF = (MF - 1) x 100 % of MF = (1.043 - 1) x 100 %of MF = .043 x 100 % of MF = 4.3%
Fixed Matrix
-(fixed) same matrix size = pixels DECREASE, FOV DECREASE -sm to lg = pixel size INCREASE -lg to sm = pixel size DECREASE
Central Ray of Magnification
-Center anatomy of interest to CR and IR -Middle of beam least distortion due to beam divergence increase blur and and distortion around edges
Factors of Receptor Unsharpness
-DR -CR -Film
Factors of Pixel Size
-DR = DEL size -CR = sampling frequency: differ for different brands and IR sizes (determined pixel pitch and size)
Spatial Resolution Synonyms
-Detail Sharpness -Recorded Detail -Detail -Definition -Spatial Frequency
Sources of Blur
-Focal Spot -Motion -Receptor -Pixel Size -Digital Image Processing
Factors of Geometric Unsharpness
-Focal Spot Size -SID -SOD -OID
Spatial Resolution - 3 types
-Geometric (FS, OID, SID) -Receptor (CR vs. DR vs. Film) -Motion (Patient and Eqiupment)
Define Matrix
-Image is divided into cells arrange in columns and rows -matrix size = # of rows X # of columns -field of view (FOV) = size of image receptor ex: 512 matrix = 512 x 512 = 262,144 pixels (same FOV - larger pixels) ex: 1024 matrix = 1024 x 1024 = 1,048,576 pixels (same FOV - smaller pixels = better SR)
Factors of Size Distortion
-SID -SOD -OID (also affect penumbra)
Distortion - 2 types
-Size Distortion -Shape Distortion
Fixed Sampling Frequency
-analog/film: does not change -pixel size stays the same -small plate = SF INCREASE = # of pixels DECREASE -large plate = SF DECREASE = # of pixels INCREASE
Shape Distortion - 2 types
-elongation = longer than actual (larger in one dimension) -foreshortening =shorter than actual (smaller in one dimension)
What conditions make the anode heel affect more noticable?
-long exposure field -short SIDs -tube w/ small target angles
Size Distortion -2 types
-magnification = image overall larger in both dimensions -minification = image overall smaller in both dimesions
Factors of Motion Unsharpness
-patient motion -involuntary -voluntary -equipment motion
How to minimize Blur
-small focal spot -small OID -larger SID
Pixel size (pixel pitch)
-the space from the center of one pixel to the center of the next pixel (measured in microns um, equal to pixel pitch) -the minimum # of pixels required to demonstrate 1 lp is 2 pixels
Lack of unsharpness
-unsharpness -blur -penumbra
Spatial Frequency Math: 1)What is the pixel size if FOV is 17" (431.8mm) and matrix size is 1024? 2) What if the FOV was decreased to 12" (304.8mm) with the same matrix size of 1024? 3) What if the matrix size increased to 2,048 for the same FOV?
1) Pixel Size = FOV / Matrix ps = 431.8mm / 1024 ps = 0.4217mm 2)Pixel size = FOV / Matrix ps = 304.8mm / 1024 ps = 0.2977mm 3) Pixel Size = FOV / Matrix ps = 431.8mm / 2048 ps = 0.2108mm
The human eye can see up too ______ lp/mm
5 lp/mm (1 line = .1mm) (1 space = .2mm)
Cieszynski's Rule/Law of Isometry
Angle CR 1/2 of the angle formed by the part and IR will minimize shape distortion
Describe the effect the anode heel affect has on the xray beam.
Beam intensity and IR exposure is greater under the cathode end and less under the anode side of the tube ex: femur xray -cathode end = over the hip -anode end = over the knee
As focal spot increases (select all that apply):
Detail Sharpness = DECREASE IR Exposure = NO EFFECT Contrast = NO EFFECT Size Distortion = NO EFFECT Pixel Size = NO EFFECT
If the CR is perpendicular to long axis of PART, but not the IR, the image will be...
Elongated (and magnified)
As pixel density increases, pixel size increases
False
As pixel size increases, spatial resolution increases.
False
If matrix size is maintained and FOV is increased, pixel size will decrease.
False
If pixel density is increased, pixel size is increased.
False
If the CR reader uses a fixed sampling frequency, pixel size will stay the same when using a smaller IR size with CR receptors.
False
If the matrix of a digital image is increased, pixel size decreases.
False
Increasing the OID or decreasing the SID will increase the SOD
False
It takes at least 1 pixel to demonstrated one line pair.
False
As SID increases (select all that apply):
Film Density = DECREASE Detail Sharpness = INCREASE Size Distortion = INCREASE Contrast = NO EFFECT
Spatial Frequency Math: What is the spatial frequency (lp/mm) for a pixel size of 148 um?
First convert microns to mm: 148 um = 0.148 mm SF = 1 / (2 x PS) SF = 1 / (2 x 0.148) SF = 1 / 0.296 SF = 3.38 lp/mm
If the CR is perpendicular to IR, but not the part, the image will be...
Foreshortened (and magnified)
As OID increases (select all that apply):
IR Exposure = DECREASE Detail Sharpness = DECREASE Contrast = INCREASE Size Distortion = INCREASE Pixel Size = NO EFFECT
Define Pixel (pixel size)
Individual cells or boxes "picture element" (each pixel = different numerical value = different gray tones)
Magnification Math: 1cm marker placed on patient. On image marker is measured to 1.25cm. What is the magnification factor?
MF = Image Size / Object Size MF = 1.25cm / 1cm MF = 1.25
Magnification Math: What is the magnification factor for chest at 40" SID and 4" OID?
MF = SID / SOD (sod=sid-oid) MF = 40" / 36" (40" - 4" = 36") MF = 1.1111
Magnification definition and formula
Magnification = image overall larger (both dimensions) MF = SID/SOD (sod=sid-oid) MF = Image Size / Object Size -Image Size = Object Size -Object Size = Image Size / MF % of M = (MF - 1) x 100
Magnification Math: MF for cerebral angiogram was 1.5. Size of blood vessel measured at 3mm on image, what is the actual size of blood vessel?
Object size = Image Size / MF OS = 3mm / 1.5 OS = 2mm
Penumbra definition and formula
Penumbra = unsharpness P= Focal Spot x OID / SOD (sod=sid-oid)
Pixel Size, Matrix, and FOV formula
Pixel size = FOV / Matrix
Receprocity Math: Patient Motion X-ray has good EI but needs to be repeated. To maintain original mAs to maintain EI. 50 mA x 1s = 50 mAs
Receprocity: -400 mA x 1/8s = 50 mAs -100 mA x 1/2s = 50 mAs -200 mA x 1/4s = 50 mAs -300 mAs x 1/6s = 50 mAs
Geometric Properties of Image Quality - 2 types
Recognizability -Spatial Resolution -Distortion
Spatial Frequency Math: A radiograph imaging system has a spatial resolution of 3.5 lp/mm. How small an object can it resolve?
SF = 1 / (2 x PS) 3.5 = 1 / (2 x PS) 3.5 x 2 = 1 / (PS) 7 = 1 / PS 1mm / 7 = PS PS = 0.143 mm = 143 um
A screen-film mammography imaging system operating in the magnification mode can image high-contrast microcalcifications as small as 50 um. What spatial frequency does this represent?
SF = 1 / (2 x PS) SF = 1 / (2 x 0.05) SF = 1 / 0.1 SF = 10 lp/mm
Spatial Frequency Math: What is the spatial frequency (lp/mm) for a pixel size of 0.3mm?
SF = 1 / (2 x PS) SF = 1 / (2x0.3) SF = 1 / 0.6 SF = 1.6 lp/mm
Guidelines for Focal Spot
Small Focal Spot -small parts -low mAs -small IR Large Focal Spot -large parts -high mAs -long IR
Spatial Frequency definition and formula
Spatial Freq (Spatial Resolution) = the sharpness of edges on the exposed image and the visibility of small objects (lp/mm) -small objects = higher SF -large objects = lower SF -Increase # of lp/mm = Increase SF SF = 1 / (2 x PS)
If matrix size is maintained and FOV is decreased, matrix size will decrease.
True
If pixel pitch (size) is increased, spatial resolution is decreased.
True
If pixel size is decreased, matrix size will increase.
True
If the image matrix becomes smaller and the FOV does not change, pixel size will become larger
True
If the magnification is increased due to geometric factors, detail sharpness is decreased.
True
If the sampling frequency is increased, pixel size is decreased and spatial resolution is increased.
True
Pixel size determine the smallest detail that can be demonstrated in a digital image.
True
Photographic properties of Image Quality - 3 types
Visibility -brightness -contrast -Noise
Inverse Square Law Math: A radiograph is taken at 70 kvp, 10 mAs, and 80" SID. How much does the radiation exposure to the IR change if the SID is changed to 40" with no other changes?
i1 = 1 ; i2 = IR exposure ; SID1 = 80" ; SID2 = 40" i1 / i2 = (SID2 / SID1) ^2 i1 / i2 = (40" / 80") ^2 i1 / i2 = 1 /4 i2 = 4x i1 exposure (increase 4x)
Inverse Square Law Math: A radiographer's exposure dose during a fluoro exam is 10mR at 1 ft from patient's side. What would happen to the radiographer's exposure if he or she stepped back to 3ft?
i1 = 10 mR ; i2 = New exposure ; D1 = 1ft ; D2 = 3ft i1 / i2 = (SID2 / SID1) ^2 10mR / i2 = (3 / 1 ) ^2 10mR / i2 = 9 /1 i2 = 1.11mR
Inverse Square Law Formula
i1/i2 = (D2 / D1) ^2 i1 / i2 = (SID2 / SID1) ^2
Inverse Square Law Math: A radiograph is taken at 70 kvp, 10 mAs, and 40" SID. How much does the radiation exposure to the IR change if the SID is change to 80" with no other changes?
i1= 1 ; i2 = IR exposure ; SID1 = 40" ; SID2 = 80" i1 /i2 = (SID2 / SID1) ^2 i1 / i2 = (80" / 40") ^2 i1 / i2 = 4 i2 = 1/4 exposure (decrease 4x)
Direct Square Law Math: Radiography taken using 70 kvp, 20 mAs, and 80" SID. what technical factors should be used to maintain the exposure index if the SID is changed to 40"?
mAs1 = 20 ; SID1 = 80" ; SID2 = 40" mAs1 / mAs2 = (SID1 / SID2) ^2 20 / mAs2 = (80" / 40") ^2 20 / mAs2 = 4 mAs2 = 20 / 4 mAs2 = 5 mAs to maintain EI
Direct Square Law (mAs) Formula
mAs1/mAs2 = (SID1/SID2)^2 (to maintain IR exposure, increase mAs 4x)
Penumbra Math: How much penumbra if image taken at 40" SID, 5" OID, and 1.5mm focal Spot?
p= 1.5mm x 5"/35" (40" - 5" = 35") p= 7.5mm / 35 (1.5mm x 5 = 7.5mm) p= .21mm
Penumbra Math: How much penumbra if image taken at 72" SID, 5" OID, and a 1.5mm focal spot?
p= 1.5mm x 5"/67" (72" - 5" = 67") p= 7.5mm / 67 (1.5mm x 5 = 7.5mm) p= .11mm
pixel density
the number of pixels per mm
