Accurancy of kVp testTo check that the generator is producing the kVp set on the control panel. This can be carried out by using a meter or a kVp kVp test cassette. One of the most widely used is the Winconsin peak kilovoltage test cassette.Using a meter: kVpa. Frequency of test-When the equipment is first installed-Yearly -As necessaryb. Equipment required-kVp meters-Generator to be testedc. Method-Place the sensor in the middle of the x-ray beam at a FFD (SID) of 100 cm.Collimate-to a standard sufficient to cover the area of the sensor.-Take readings at 10 kVp intervals from 50 kVp to the maximum kVp available.-Used a standard mA and time throughout.-Repeat this procedure for each tube focus available.d. Evaluation-Compare each kVp reading with the appropriate setting of the kVp.-The measured kVp must be within + or-5 kVp or 5%.e. Action -If the measured kVp s do not fall within the acceptable limits, call an x-ray engineer.Using a kVp test cassette:a. Frequency of test-When equipment is first installed-Yearly -As necessaryb. Equipment required-Loaded kVp test cassette -Two sheets of lead rubber, one of 10 x 23 cm and the other 23 x 23 cm.-Test cassette calibration graphs for each kVp level used.-Densitometer c. Method-Place the cassette on the tabletop, face up (the side marked wiht kVp divisions) and the long dimension parallel to the cassette of the anode-cathode axis.-Set a 100 cm FFD (SID).Collimate-to the area of the cassette marked with the lowest kVp (60 kVp).-A Set of 60 kVp.-Set a mAs value that will give a density of approximately 1 in the region of the dots (you may need to experiment).Shiels-the remainder of the cassette with lead rubber.-Make an exposure.-Exposes each of the other section of the cassette (marked 80, 100 and 120 kVp) separately, using the appropriate each time.-The time setting should be adjusted each exposure, to maintain an optical density of 1 in the region of the dots each time. The mA remains the samefor each exposure.-Remember to adjust the lead rubber each time.-Process the film.-Repeat this process at low, mid and high mAs settings.Approximate mAs settingskVp Single phase units Three phase unit60 500 40080 75 40100 15 10120 12 8Joel E Gray et al, Quality Control in Diagnostic Imagingd. Evaluation-On the radiograph of the regions are identified as A (60 kVp), B (80 kVp), C (100 kVp), D (120 kVp).-Each kVp region contains two columns of dots.-The horizontal rows are numbered 1 to 10, 1 being at the darker end of the column.-One column of dots is of uniform density, in each region, and is called the reference column.-The dots in the other column, in each region, will show a density gradient, darkest at the top (120 kVp region) and lightest at the bottom (60 kVp region).-Look for density match between the two columns, in each region. This should be done with a densitometer.- If an exact match is not found an average must be determined using the following method.Example :Step No. Optical density of step Optical density of reference5 1,40 1,156 1,10 1,15Match step = 6 + 1,40 – 1,15 = 5 + 0,25 = 5,83 1,40 – 1,10 0,30- Whwn a match has been determined for each region, rafer to the calibration chart for the appropriate region (chart for each of the four kVp regions are supplied with the cassette).- The chart will have two graphs on it, one for a single phase unit and one for a three phase unit.- Using the appropriate graph, plot the match step level for each kVp region on the vertical axis, then the read off the corespondensing kVp s from the horizontal axis. These kVp s should, correspond to the kVp s used.- The measured kVp must be within + or 5 kVp or 5%.e. Action - If the measured kVp s do not fall within the acepted limits, call an X-ray engineer.
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