Lab 2 Flashcards
What is the purpose of a grid?
To absorb scatter radiation that has been produced in the patients body before it reaches the IR
T/F
Dose to the patient always increases when there is grid cut off
False
What are the 4 causes of grid cut off?
- Lateral descentering w/wo grid
- Upside down focused grid
- Off focus grid cut off
- Combination of all or some of the above
What is the most common type of grid cut off?
Lateral descentering
What is laterally descentering a result of?
Not being in detent laterally
Where is an upside down focused grid most common in?
Portable x ray or CR plates
What is off focus grid cut off?
Using the wrong SID for the grid
What is the result on the image from an upside down focused grid error?
-Dark band of central exposure
-Severe cut off (whiteness) on the edges
What is the result on the image from lateral descentering without tilt?
-Uniform loss of primary beam across the image resulting ina uniformly light radiograph
What is the hardest grid cut off error to detect?
Lateral decentering without tilt
What does OD stand for in regards to grid cut off?
Optical density
What is optical density?
Amount of darkness in the image
What is the result on the image from lateral decentering with tilt?
-All incident rays will be angled across entire image, resulting in lower optical density
What will be the result on the image for an off focus grid cut off error?
-Fairly even exposure with lack of OD
-Cut off more towards the edges of the image
What is the result on the image for an off centered and off focused grid error?
-An uneven exposure with a gradient of dark to light
What is the objective of grid alignment testing?
To test the alignment of the radiographic grid in the bucky mechanism with respect to the central ray of the x ray tube
What equipment is needed for a grid alignment test?
-Grid alignment test tool
-Lead strips
-Densitometer
What tool is this?
A grid alignment test tool
If the grid is aligned correctly, how should the holes in the test tool appear?
Center hole will have the highest OD, The OD should fall off symmetrically on either side of the center hole.
Based on the results from the grid alignment test, is the grid misaligned towards the window or the door of Room 2?
Grid misaaligned twards the window in room 2
According to SC35, what is the frequency of grid alignment testing?
Annually
What are the 3 factors that may cause beam-grid misalignment in a moving grid?
- Improper centering of the x-ray beam to the bucky.
- Misalignment of the grid within the bucky mechanism.
- Malfunction in the reciprocating action of the grid - delay between initiation of grid motion and beginning of x-ray exposure may be out of sync.
Define the following term
Grid lattitude
The margin of error in centering the x ray beam to the center of the grid before significant grid cut off appears in the resulting image
What is the beam grid alignment specified by the manufacturer?
+or- 1 inch
Where is the grid lattitude measurement listed?
Either on the front of the grid or with the literature supplied by the manufacturer
What would the histogram for this mammography image look like?
-2 spikes for the the super white and black areas
What would the histogram for this mammography image look like?
No major spikes
What is the purpose of timer accuracy testing?
To ensure accurate exposure time
What materials are needed for a timer accuracy test?
Digital exposure timer
According to S.C. 35, the actual time of exposure should be within how many seconds of the time selected by the operator.
Plus or minus 10% + 1ms
According to the HARP Act, the actual time of exposure should be within how many seconds of the time selected by the operator?
10%
As per S.C. 35, the irradiation time meter accuracy test should be performed how often to ensure that it is working within limits?
Annually
