Technique Charts Radtech

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:

1) speed of intensifying screens, 2) age of intensifying screens, 3) speed of film, 4) focal-film distance, 5) amount of beam filtration, 6) temperature and time of film processing, 7) type of grid, and 8) inherent differences in the settings of x-ray machines. It is because of

these things that a technique chart must be developed for each particular x-ray machine and processing facility.

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:

1) Value of incoming line voltage, 2) Focal-film distance should be standardized to provide a satisfactory

beam intensity without image distortion, 90 cm to 100 cm (36" to 40"). 3) Aluminum filter (1-2 mm equivalent)a and coning device to be

used should be in position, 4) Type of intensifying screens and film speed should be preselected, 5) Grid should be selected and exposures made using this grid, 6) Darkroom operation should be determined and darkroom chemistry,

time of developing, and temperature of developing standardized(Table 18-1).

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

1) use of screen or non-screen technique, 2) use or non-use of a grid and 3) use of several film/screen combinations. It is also possible to make a

technique chart dependent on species: i.e. dog, cat, avian, etc.

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:

1) include an exposure of satisfactory density, 2) show all exposures to be dark (overexposed) or 3) show all exposures to be light (underexposed).

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:

1) 70 kVp and 2.5 mAs 2) 70 kVp and 5.0 mAs 3) 70 kVp and 10.0 mAs

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 FFD 2 = 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 range40-50 +450-60 +660-70 +870-80 +1080-90 +12

90-100 +14100-110 +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 +270-80 +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 processingo 1) Standardize developer solution temperature (preferably

200 C(680F)o 2) Ensure solutions are of average strength (not

contaminated or exhausted but not necessarily freshly mixed)

o 3) Check timer clock for accuracy and establish developer time (preferably 5 minutes)

b. Automatic processoro 1) Check developer replenisher rate with recommendations

on unito 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 ORRevaluate the character of the grid and select one that permits use without as great an increase in exposure factors ORDecrease 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 ORReevaluate 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

o 1) If a 100 mA maximum unit, try to use the shortest exposure time and highest mA settings possible for your machine

o 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 pageo 1) Complete the chart in the upper right corner concerning

the grid and focal-film distance usedo 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!

d. Test exposures of other parts of the dog over 10 cm in thicknesso 1) Repeat the procedure for the thorax, extremities, pelvis,

vertebral column, and skull ORo 2) Use the following estimated values and complete the technique

chart. Assume for the purposes of this example that column #9 is the column you have selected for the abdominal studies.

a) Select the next higher column (#10) and this is the technique chart for "extremities"

b) Select the column that is two columns higher (#11) and this is the technique chart for the "vertebral column" and the "skull"

c) Select the column that is three columns higher (#12) and this is the technique chart for the "pelvis" and "hip joints"

d) Select the column that is five columns lower (#4) and this is the technique chart for the "thorax"

e) Remember that the mAs is the same for all columns

B. Test exposures to establish a technique chart for parts of the dog that measure less than 11 cm thickness

1. Follow the same procedure as described above except

o a. Remove the grid from use

o b. Select a different dog that has an abdomen that measures 10 cm or less

2. Evaluate the film used to establish the technique chart to be used with a grid and estimate the exposure range to be used to produce test films for the chart to be used without a grid.

3. Make a new technique chart for use without a grid by determining the correct columns for all anatomical parts. This chart is also appropriate for the anatomical parts of a larger dog that measure 10 cm or less

C. Test exposures to establish a technique for a cat


o a. Remove the grid from useo b. Select a cat that has an abdomen that measures less than 10

cmo 2. Evaluate the film used to establish the technique chart to be

used without a grid to estimate the exposure range to be used to produce the test films for the chart for the cat.

o 3. Make a new technique chart for the cat for use without a grid. Determine the correct columns for all anatomical parts. This chart is appropriate for all parts of the cat that measure 10 cm or less. In cats in which the area to be radiographed measures over 11 cm in thickness, use the dog chart.

D. Test exposures for non-screen technique chart


o a. Limit tissue thickness to 1 cm to 5 cmo b. Use an extremity such as the paw as a model exposure

2. Make a new technique chart for extremities of 1 cm to 5 cm thickness, intra-oral studies, and avian studies. this chart is less exact than the other and requires correction as you use the chart.

E. Correct the technique charts to suit your individual desires and to permit you to obtain the most from your x-ray machine

o 1. Shift columns as you discover that a particular examination is consistently over or underexposed

o 2. Shift columns to lower kVp settings to permit you to utilize your machine to the best advantage. (Remember that adding 10 kVp to a technique and dividing the mAs in half produces a comparable radiograph.) If you shift columns to utilize a different kVp range, be sure that you alter the mAs setting

Table 18-5

WHEN TECHNIQUE CHARTS FAIL TO WORK

Under certain circumstances, technique charts fail to provide the machine settings that produce satisfactory radiograph. These situations fall into two basic categories, one in which the radiographic density is less than suspected and the other in which the radiographic density is greater than expected. Some of the conditions that cause an increase in radiographic density are a decrease in tissue density and therefore a decrease in photon attenuation. Knowledge of these conditions prior to radiography enables you to make downward adjustments in technique. You may either decrease kVp settings by 5 to 10 or decrease the mAs setting by halving it.

o 1. Focal destructive bone disease (primary bone tumor)o 2. Generalized bone disease (nutritional secondary

hyperparathyroidism)o 3. Emphysematous soft tissueso 4. Pneumothoraxo 5. Megaesophaguso 6. Massive pulmonary air trappingo 7. Pulmonary cystso 8. Aerophagia with dilated air filler stomacho 9. Pneumocolon following enemao 10.Pneumocystogram with airfilled urinary bladder

o 11. Soft tissue atrophy (if using a technique that was adequate on earlier study)

o 12. Weight loss that decreases thoracic wall to lung ratioo 13. Pneumoperitoneum

Some of the conditions that cause a decrease in radiographic density are due to an increase in tissue density and resulting increase in photon attenuation. Knowledge of these conditions prior to radiography enable you to make upward adjustments in technique. You may either increase kVp settings by 5 to 10 or increase the mAs setting by doubling it

o 1. Pleural effusiono 2. Pulmonary fluid (pneumonia, edema, or hemorrhage)o 3. Pneumoconiosiso 4. Bronchiectasiso 5. Post-thoracotomyo 6. Cardiomegalyo 7. Pericardial fluido 8. Asciteso 9. Large, fluid density, abdominal masso 10. Ingesta filled gastro-intestinal tracto 11. Weight loss that decreases the fat to abdominal organ ratioo 12. Soft tissue edemao 13. Soft tissue calcificationo 14. Soft tissue atrophy - increases the bone to soft tissue ratioo 15. Positive contrast mediumo 17. Fungal skeletal disease

Inverse square law

CHANGE IN FILM DENSITY

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