|Adjusting kVp Thickness Optimal Settings
These are recommended techniques; the final results will depend on many factors.
If films are too dark (overexposed) decrease time, Halving mAs values will reduce the film blackening/density by half, or reduce kV by 10 and keep time the same.
If films are too light (under exposed) increase time, Doubling mAs value will double the film blackening/density, or increase kV by 10 and keep time the same.
Adjusting for kVp thickness:
- 40-79 kVp -add/subtract 2 kV for each 1 cm increase/decrease in thickness.
- 80-100kVp-add/subtract 3 kV for each 1 cm increase/decrease in thickness.
- Increasing kVp by 10 will increase penetration/film density by 2 (doubling) at 40-79kVp.
- Reducing kVp by 10 will decrease penetration/film density by 1/2 at 40-79kVp.
- Increasing kV by 13 will increase penetration/film density by 2 (doubling) at 80-100kV
- Reducing kV by 13 will decrease penetration/film density by 1/2 at 80-100kVp.
Using optimal kVp settings ensures high contrast films:
- 2-4 cm 50 kVp
- 5-8 cm 60 kVp
- 9-11 cm 70 kVp
- 12-15 cm 80 kVp
- 16-20 cm 90 kVp
- 21 + cm 100 kVp
For grid use:
Double mAs when using grid for same film density as non-grid use. When doubling mAs or increasing kV by 10 does not produce a doubling in film density, x-ray unit re-calibration may be required or possibly a weak input line power source condition exists.
- If your films are too gray, A) your kVp is too high. B) Fixer is old. C) Screens are aged.
- If your films keep getting lighter & lighter (losing contrast) your problem is the processing chemicals, CHANGE CHEMICALS.
- If your films are coming out tacky (sticky): a. weak fixer b. adjust dryer heat
- If your processor is running hot (over 95 degrees) your films will be dark!
- Your films will be a little dark (20%) just after changing your chemical, until chemical is seasoned by running 10 films or new chemical sits for two days.
Use the right cassettes:
- If you are using a detail cassette and should be using a REGULAR CASSETTE your films will be light.
- If you are using a regular cassette and should be using a DETAIL CASSETTE your films will be to dark.
- When going from Regular Cassettes to Detail Cassettes multiply your technique by 2.
- When going from Detail Cassettes to Regular Cassettes divide your technique by 2.
- For every 5 cm of thickness increase, double your mAs or time.
- When you increase your kVp by 15%, cut your mA by 1/2 or time for same film density.
- When you decrease your kV by 15% double your mAs or time for same film density.
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Modifying kVp Factors
- Emaciated animals- reduce kVp by 10%
- Juvenile animals - reduce kVp by 10%
- Grid removed - reduce kVp by 10%
- Barium study - increase kVp by 5 - 10%
- Pleural effusion - increase kVp by 5 - 10%
- Ascites - increase kVp by 5 - 10%
- Obesity - increase kVp by 10%
- Plaster casts - increase kVp by 10%
Dr. Edwin Frost at Dartmouth University made the first x-ray in the United States February 3, 1896.
Geoffrey Hounsfield developed a workable computed tomography machine for body imaging in which he won a Nobel Prize for in 1979.
The use of clinical Magnetic Resonance Imaging (MRI) in the United States began the summer of 1985.
"Advances in imaging technology and radiation therapy have dramatically improved the diagnosis and treatment of serious diseases. But accurate and complete interpretation of all the data within your studies is critical to providing the information you need to support the direction of your treatment."
~American College of Veterinary Radiology
Troubleshooting Technique Chart Failure
· Aged chemicals
· Incorrect processing temperature
· Changed processing time
· Failure to extrapolate correctly
· Inappropriate selection of trial exposure factors
· Unmatched x-ray film and intensifying screens
· Faulty exposure timer
· Variation in line voltage
· Non-linear kVp/mAs machine settings
· Inconsistent measuring method
· Sight developing instead of time and temperature method
· Film fog (faulty safelight or light intrusions)
"Action and reaction, ebb and flow, trial and error, change - this is the rhythm of living. Out of our over-confidence, fear; out of our fear, clearer vision, fresh hope. And out of hope, progress."
Commonly Seen Film Faults
Technique is the most common cause of film faults which can be contributed to several different variables.
Inadequate number of projections
Lack of collimation
No anatomical markers
No patient identification or date
Poor positioning or centering
Incorrect use of a grid
Poor film screen contact
A number of faults can occur that are detrimental to the diagnostic value of a radiograph, which is why it is important to understand the exposure factors that affect radiograph quality. The most common cause of poor contrast in radiographs is inappropriate exposure factors. Through quality assurance and equipment care, these faults can be avoided, or rectified quickly preventing unnecessary repeat radiographs and extended anesthesia for the patient.
In last month’s issue “Physics of Radiology” we discussed that kVp (kilovolt peak/kilovolt potential) gives variable "speed" to the electrons determining the penetration of the x-ray beam, which affects the efficiency of x-ray production, and determines the scale of contrast in the image. The mAs (milliampere seconds) determines the number of x-rays produced per unit time and the number of x-rays reaching the film determines the degree of blackening of the film.
In order to take a quality radiograph that details as much diagnostic information as possible the following factors must be considered:
• The type of tissue in the region of interest
• The type of x-ray machine (specifically the type of
• The type of film or screen system being used.
The type of tissue in the region of interest constitutes the subject contrast which is what you want to display. For example, you want to display the differences between the bone cortex, the bone marrow cavity and the surrounding muscle; or display the difference between the liver, the falciform fat, the intestinal wall and the intestinal lumen; or display the difference between the air filled region of the lung, the pulmonary vessels and the heart. The major way to depict these is in the selection of the kVp used to image the area of interest.
The correct kVp will produce differential x-ray absorption of soft and dense anatomic structures. Increasing kVp increases the penetrating power of the x-ray beam. If kVp is too low, the image will lack density resulting in a whitewashed or sooty appearance. If kVp is too high the image will be over exposed and too dark.
In small animals and in the distal extremity region of large animals where bone is the major structure of interest, a relatively low kVp (60-70) is sufficient to penetrate the bone and soft tissue. The low kVp will be very effectively absorbed by the bone mineral and provide good contrast. This level of kVp will provide a rather “contrasty” image which most find pl