Chapter 18

Technique Charts

 

 

 

 A technique chart is a table with predetermined x-ray machine settings that enables the radiographer to select the correct machine settings based on the thickness of tissue and the anatomical portion of the body to be radiographed. Use of these machine settings will regularly produce a diagnostic radiograph. Technique charts become of value only when the radiographer has developed confidence in them. The technique chart prevents unnecessary wastage of time and film due to the use of inappropriate exposure factors. Use of a technique chart does away with the necessity of sight developing.

It is often thought that a combination of mA, time, and kVp settings that produces a satisfactory radiograph when used with one machine will produce the same satisfactory radiograph when used with another machine. This is not true. A part of the problem lies with basic inherent differences in the x-ray machines, while different types of accessory radiographic equipment and processing procedures also contribute greatly to production of a different quality of radiograph. Items that contribute to this difference are:

Table 18-1

VARIABLES TO CONSIDER IN DEVELOPMENT OF A TECHNIQUE CHART:

1. Variables that affect exposure factors

  • a. Focal-film distance
  • b. Filtration
  • c. Speed of intensifying screens
  • d. Type of film
  • e. Developer time
  • f. Developer temperature
  • g. Developer chemistry
  • h. Grid type
  • i. Incoming line voltage

2. Variables that do not affect exposure factors

  • a. Object-film distance
  • b. Focal spot size
  • c. Size cassette (film)
  • d. Collimation (if larger than 5" x 7")
  • e. Direction of x-ray beam

18.1 Formulation of a Technique Chart:

As many factors as possible must be standardized before formulating a chart such as:

Several technique charts may need to be developed. The individual charts are based on

 Technique Chart-Screen Technique with Grid

An important chart to be developed is that one to be used on canine patient over 11 cm and requires use of screen type film and grid. The chart can be based on a series of exposures made of a dog’s abdomen that measures over 11 cm in thickness. A dog with average conformation that is neither obese or unusually thin should be selected for the study. Multiple exposures will be made and a single film may be divided by rubberized lead sheets or by close collimation.

It is difficult to describe how to select the first exposure to be made. Your previous experience with the machine may help you in making this decision. In selection of the kVp setting it is best to stay within the range of 60 to 80 kVp for reasons discussed relative to x-ray tube conservation and film quality. In the example below we have arbitrarily selected 70 kVp and a mAs range of 2.5 to 10.0. The settings for the additional exposures are based on the results of the first trial exposures.

It is important to restress that film processing must be carefully performed since you can base on entire Technique Chart on a single set of films. Control the developing time and temperature as well as possible.

Examination of the first trial film will:

If all of the radiographs are overexposed, as indicated by dark films, the kVp can be reduced by 10 or the mAs can be decreased to one-half. three additional exposures are then made at the same exposure times used in the first trial. If none of the original exposures is satisfactory due to underexposure, an increase in exposure factors is required. If possible, the kVp should be increased by 10 and, again, three trial exposures repeated. If it is not possible to increase the kVp, then the exposure time or the mA can be doubled. This technique of increasing or decreasing factors is continued until a satisfactory exposure is obtained. The acceptable exposure is one that appeals to the individual and that best permits evaluation of both bone and soft tissue shadows.

Example: (see mAs chart for ease in determining mA and time settings)(Table 18-2)

Original trial exposures:

If all exposures are dark (overexposed), decrease the settings to:

or

If all exposures are light (underexposed), increases the settings to:

or

After determination of the best machine settings for the abdomen of the trial dog, it is necessary to expand that portion of the chart so that a technique can be selected for any thickness of abdomen. This is done by taking advantage of the general rule of exposure factors that dictates an increase in kVp settings as the tissue thickness increases. Having expanded the first trial exposure, you now have a technique chart for the abdomen of any dog over 11 cm thickness using your grid.

Next, it is necessary to develop techniques for other parts of the dog’s body, for the cat, for nongrid techniques (table top), and for nonscreen techniques. These additional charts are developed in a similar manner to the first chart for the abdomen. It should be possible for you to estimate rather closely the technique to be used, and it will require fewer trial exposures.  

Table 18-3

GENERAL RULES THAT INFLUENCE EXPOSURE FACTORS

1. Effects of mA

  • a. Film density is directly proportional to mA.
  • b. mA does not appreciably alter contrast. This is true over a limited density range.

2. Effects of exposure time (length of exposure)

  • a. Film density is directly proportional to exposure time.
  • b. Time does not appreciably alter contrast. This is true over a limited density range.

3. mAs concept (mAs = milliampere seconds) (see Table 18-2)

  • a. The product of mA and time in seconds is commonly considered as a single factor.
  • b. This is a useful factor because film density is directly proportional to mAs.

4. Effects of distance (inverse-square law) (see Fig. 18-1)

  • a. Film density is inversely proportional to the focal-film distance squared
  • b. Focal-film distance does not appreciably alter contrast. This is true over a limited film density range.

new FFD2 = new mAs

old FFD2 = old mAs

Problem: If the mAs at 75 cm is 10, what is the mAs at 100 cm?

 

5625X = 10,000x 10

(100 cm)2 = X X = 100,000

(75 cm)2 = 10 mAs 5625

Answer: X = 17.8 mAs

5. Effects of kVp

  • a. Low kVp produces a short scale of contrast with large differences between each step. High kVp produces longer scale of contrast with small differences between each step.

6. cm thickness vs kVp

  • a. Change in cm thickness of a patient requires a change in kVp if other factors remain constant.
  • b. For each additional cm thickness, add 2 kVp up to 80 kVp; add 3 kVp between 80 and 100 kVp; and add 4 kVp over 100 kVp (example: if 14 cm = 76 kVp, then 17 cm = 83 kVp).

7. a. There is a general relationship between mAs and kVp settings.

  • b. If you double the mAs, subtract 10 kVp to maintain the same film density. (70-80 kVp range)
  • c. If you halve the mAs, add 10 kVp to maintain the same film density. (70-80 kVp range)
  • d. Outside the 70-80 kVp range, different factors must be used to maintain the same film density. If mAs is doubled or halved in the kVp ranges listed, there is an appropriate change in kVp settings required to maintain a similar film density.

 

kVp setting

kVp range

40-50

+4

50-60

+6

60-70

+8

70-80

+10

80-90

+12

90-100

+14

100-110

+16

e. If a radiographic technique is too dark or too light, the table above can be used to correct the exposure. If a technique is considered unsatisfactory, the change in technique must be equivalent to halving or doubling the mAs. If the change in technique is to be in the kVp, the appropriate change can be selected from the table above, considering the kVp range of the original exposure.

8. Exposure latitude

  • a. There is a range of kVp settings over which film densities produced remain within an acceptable range. In other words, a certain degree of error can be made in a technique setting and still produce an acceptable film.

kVp range

Exposure latitude (kVp)

40-50

+2

70-80

+4

90-100

+6

9. The following rules are recommended under certain conditions to modify the machine settings taken from a technique chart for the abdomen.

  • a. Thorax: Halve mAs
  • b. Immature dog:Halve mAs
  • c. Cat:Halve mAs
  • d. Heavy-muscled or obese animal:Double mAs
  • e. Plaster cast:Double mAs
  • f. GI tract using barium sulfate:Increase kVp for contrast studies(5-10 kVp)
  • g. Skull, vertebral column, or pelvis: Increase kVp(5-10kVp)
  • h. Soft tissues in the cervical region: Decrease kVp (5-10 kVp)
  • I. Adjust the radiographic technique by decreasing the kVp and increasing the mAs settings in certain contrast studies using iodinated substances to produce a radiograph with a short scale of contrast (IVP, myelogram, tract injection)

A table is provided that permits comparison of mA settings and time settings with the resulting mAs value. This chart can be used to determine new mA and time settings that provide a similar mAs value or can be used to determine the best mA and time settings for a given mAs value (Table 18-2).

Table 18-4

FLOW CHART FOR DEVELOPMENT OF A TECHNIQUE CHART

A. Technique chart for studies of a dog over 11 cm in thickness requiring use of a grid

1. Selection of model dog

  • a. Mature
  • b. Moderate muscling
  • c. Not obese
  • d. Not thin
  • e. Hair coat clean and not unduly heavy or long

2. X-ray machine

  • a. Evaluate incoming line voltage (if not possible, be certain that there is no heavy voltage drain on the circuit)
  • b. Set tube-film distance at 90 cm to 100 cm (36" to 40") - if unit 100 mA or higher
  • OR Set tube-film distance at 60 cm to 75 cm (24" to 30") - if unit 30 mA or less
  • c. Position beam filter (2 mm al. equivalent)
  • d. Position collimation device commonly used

3. Cassette and film

  • a. Select cassette with intensifying screens that represent the speed and age of other screen pairs (speed recommended is dependent on mA capability of x-ray machine)
  • b. Select film that is not out of date or damaged (speed recommended is dependent on mA capability of x-ray machine)

4. Grid

  • a. Position grid over cassette if exposure is to be on the table top OR Check position of grid if under table and used with a bucky tray or grid cabinet or holder
  • b. Ensure grid is clean and without artifacts.

5. Dark room

  • a. Manual wet tank processing
    • 1) Standardize developer solution temperature (preferably 200 C(680F)
    • 2) Ensure solutions are of average strength (not contaminated or exhausted but not necessarily freshly mixed)
    • 3) Check timer clock for accuracy and establish developer time (preferably 5 minutes)
  • b. Automatic processor
    • 1) Check developer replenisher rate with recommendations on unit
    • 2) Check solution temperature with recommendations on unit

6. First test exposures of abdomen of a dog measuring over 11 cm thickness

  • a. Divide cassette by lead sheets or use collimator shutter position
  • b. Position dog on side for lateral abdomen study
  • c. Measure carefully the width of the portion of the abdomen to be radiographed-record this measurement
  • d. Make three test exposures on one film of 70 kVp and 2.5, 5.0 and mAs (use mAs as near as possible to these recommendations)
  • e. Process film-if underexposed, go to #7 below -if overexposed, go to #8 below -if one of the exposures is correct, go to #9 below

7. Subsequent test exposures if film is underexposed

  • a. Repeat all steps in #1 through #5 as before
  • b. Make three test exposures of 70 kVp and 10,20 and 40 mAs OR Make three test exposures of 80 kVp and 5,10 and 20 mAs
  • c. Process film - if underexposed, reevaluate speed of intensifying screens and film and consider a faster combination OR Revaluate the character of the grid and select one that permits use without as great an increase in exposure factors OR Decrease the focal-film distance -if one of the exposures is correct, go to #9 below

8. Subsequent test exposures if film is overexposed

  • a. Repeat all steps in #1 through #5 as before
  • b. Make three test exposures of 70 kVp and 0.6, 1.2 and 2.5 mAs
  • c. Process film-if overexposed, reevaluate speed of intensifying screens and film and consider a slower combination OR Reevaluate the character of the grid and select one that permits use with a greater increase in exposure factors OR Increase the focal-film distance to 100 cm(40") if one of the exposures is correct, go to #9 below

9. Correct exposure (correct film density)

  • a. Note the kVp and mAs of the best exposure
  • b. Consider altering the exposure to achieve the best kVp and mAs combination for your machine using rules given in the previous discussion
    • 1) If a 100 mA maximum unit, try to use the shortest exposure time and highest mA settings possible for your machine
    • 2) If unit has capability of over 100mA, try to use additional latitude available in selection of kVp and mA ranges in which you can work
  • c. Go to the technique chart on the following page
    • 1) Complete the chart in the upper right corner concerning the grid and focal-film distance used
    • 2) Find the cm thickness of the trial dog on the left side of the chart
  • 3) Move across the chart on that line until you reach the kVp value closest to the one used for the trial exposure
  • 4) Mark that column at the bottom of the chart "abdomen" and record the mAs value used. This is a constant mAs-variable kVp technique for the abdomen!

References:

 

CHANGE IN FILM DENSITY