Formulating X-ray Techniques

Formulating X-ray Techniques

Most radiography departments have a functional technique chart that provides appropriate exposures for most circumstances. This chapter explores the components of technique charts and explains their limitations. When there is no chart or when the existing chart is inadequate, the limited operator must obtain or create a suitable chart. It is important to remember that no chart meets the requirements for every circumstance. When the planned procedure differs from the chart, the operator must make appropriate changes in the exposure settings. These radiographic examinations might include those involving patients who are larger or smaller than the measurements provided on the chart or patients whose conditions affect the amount of exposure required. Permanent changes in the type of grid, kilovoltage (kVp), milliamperage (mA), or source–image receptor distance (SID) will require a change in technique.

There is no one perfect set of factors that must be used for each exposure. A number of possible combinations may produce a diagnostic radiograph. The limited operator is responsible for ensuring that the technique chart is available, complete, and consistent. It must meet the requirements of the equipment and the preferences of the physician who will interpret the radiographs.

This chapter contains some formulas and mathematical calculations that all limited operators must master. The basic mathematical skills needed to perform these calculations are reviewed in Chapter 3, which also includes additional examples and practice problems to increase your proficiency.

Technique Charts

A technique chart is a listing of the various radiographic examinations performed in the facility. It provides exposure factors for each body part according to its thickness. Table 10-1 is an example of a portion of a manual exposure technique chart. It is called manual because each technical factor is set “manually” by the x-ray operator. It includes the following information: type of examination, projection, SID, patient/part measurement (in centimeters), kVp, mA, exposure time, and a grid (Bucky) notation.

X-ray machines that use automatic exposure control (AEC) or anatomically programmed radiography (APR) systems must also have a technique chart. The chart should indicate all the items that a manual technique chart does except the exposure time (which is automatic). The AEC chart will also have to indicate which of the three detectors to use for each projection. Most of these charts will have a combination of both AEC and manual techniques (Table 10-2).

Some limited operators become so familiar with the operation of their equipment over time that they tend to memorize or estimate exposures and do not feel the need for a technique chart. This practice may result in outdated or unavailable charts and may cause unnecessary exposure errors. Radiation control regulations may require posting of a current technique chart and may also specify the information to be included. In Oregon, for example, regulations require that technique charts include a notation that gonad shielding is required for specific examinations. The Joint Commission, the official organization that accredits hospitals and clinics, establishes standards for institutions that receive Medicare payments, and these standards include requirements for x-ray technique charts in radiography departments. Limited operators must be aware of requirements for technique charts and ensure that their charts conform to the regulatory standards.

Technique charts are unique to each x-ray machine and each facility. The x-ray machine manufacturer cannot supply a definitive chart with the machine because the exposures will vary considerably depending on the types of grids, tabletops, and SID. When a new chart is necessary, there are several possible sources.

Some of the major x-ray vendors will supply computer-generated charts for their customers. The local technical representative of your x-ray supply company may come to your department on request and do some testing to obtain the necessary data. These data are submitted to the company, and the chart is sent to your facility.

In some communities, experienced radiologic technologists will prepare a technique chart for a fee. This option is often advantageous because the chart can be made specifically for your facility and equipment, using only settings available on your control panel. Exposures may be provided for any procedures unique to your facility. Such a chart is also more likely to conform to local radiation control regulations.

A chart that needs to be changed because all of the exposures are too light or too dark can probably be modified easily. When the chart is consistent throughout—that is, all of the exposures are too light or too dark to about the same degree—it is a simple matter of increasing or decreasing all of the exposure times by a specific percentage to correct the radiographic density for all exposures. If your existing chart is not consistent, the use of a consistent chart borrowed from another facility with the identical equipment or from Appendix D may provide a starting point.

An exposure technique chart should be prepared with a calibrated x-ray machine and the specific type of image receptor (IR) used. After this, a technique chart that is not working will require checking the calibration of the x-ray machine, the digital processor system, or both. Sometimes the problem is an x-ray operator who is not following the techniques posted on the chart. Technically, a well-prepared exposure technique chart should never be changed. Also a chart should not be changed because of temporary changes in the calibration of the x-ray machine. Before a chart that has been working well is changed, all potential factors that could affect the techniques should be evaluated. A radiologic technologist with experience in using quality control test tools can easily determine if the x-ray machine is calibrated before calling the service personnel to invasively check the machine.

Finally, if you decide to prepare a chart yourself, products are available to assist you. Supertech Inc.* offers both computer software and a handheld slide rule to calculate exposures. Both products are supplied with a small penetrometer and a master density chart (Fig. 10-1, A). These are used to test your system and gather the basic data needed to tailor the tool to your x-ray department. Complete instructions are included. The Supertech computer software generates technique charts that conform to The Joint Commission standards.

Regardless of whether you prepare the chart yourself or arrange for its preparation by someone else, some testing is needed to establish baseline data for your system. This testing should be done when the processor is functioning at optimal levels. The same IRs that will be used with the chart should be used for testing. It is helpful to have a record of exposures kept over a period of time that lists the examination, measurement, exposure factors, and an assessment of the images produced.

Before using your technique chart to take radiographs of patients, it is helpful to test some of your exposures using a radiographic phantom. A radiographic phantom is a human skeleton, or portion of a skeleton, encased in a plastic material that is similar in density to human tissue. You may already be familiar with phantoms through experience in your radiography education program. A good phantom provides an excellent simulation of radiography of a human patient. It may be possible to borrow one from your film company’s technical representative, from your x-ray supply dealer, or from a radiography education program.

Computerized control units with programmed exposure settings or “anatomic programming” such as APR will automatically select the kVp, milliampere-seconds (mAs), and AEC detectors for an examination when the radiographer selects the body part and enters the measurement. To function accurately, these units must first be programmed with exposures that meet the requirements of the facility. In other words, a technique chart must be created and entered into the control’s computer. Even with this type of equipment, radiation control agencies may require posting of a printed technique chart.

Patient Measurement

Technique charts are based on the measurement of the body part to be radiographed. The radiographer must measure the body part accurately to select the correct exposure from the technique chart or to obtain the correct exposure with a programmable computerized control.

The tool for body part measurement is called a caliper (Fig. 10-1, B). The main shaft of the caliper is a flat strip of metal, calibrated in both inches and centimeters. There are two perpendicular extensions from the shaft: one is permanently affixed to one end of the shaft and the other slides up and down the shaft. These two extensions form “jaws” between which the body part is measured.

It is usual for technique charts and computerized controls to specify the part measurement in centimeters. The dimension to be measured is the thickness through which the x-ray beam will pass. Measurements of the trunk of the body should always be made in the same general position as for radiography because measurements may change significantly when the patient changes position. For example, upright measurements of the abdomen are often 4 to 6 cm greater than measurements of the same region when the patient is lying down.

When a body part is measured, the fixed jaw of the caliper is placed under or against the part and the movable jaw is brought snugly and firmly against the patient on the opposite side (Fig. 10-2). You must take care that the jaws of the caliper remain parallel to each other. Pressing the jaws too tightly against the patient may cause them to spread apart at the open end, resulting in an inaccurate measurement. You must also take care that you do not measure air space. For instance, if the patient is lying on the table and you are measuring the thickness of the patient at the waist, the arch of the patient’s back may leave a space between the back and the tabletop. Measuring from the tabletop to the surface of the patient’s abdomen will give an inaccurate measurement. Both jaws of the caliper must be firmly in contact with the body part.

Body parts are usually measured through the path of the central ray. Some parts, however, may be measured through their thickest portion, or another method may be used. For example, exposures for the anteroposterior (AP) open-mouth projection of the upper cervical spine are usually based on the cervical spine measurement taken in the midcervical region. When there is variation in measurement method, it should be stated in the technique chart. The technique chart is designed for a specific measurement method and will not produce accurate results unless the measurement is consistent with the method intended by the chart.

Fixed Kilovoltage Vs. Variable Kilovoltage

Years ago, it was common to construct technique charts based on a specific mAs value for each projection and to vary the kVp by 2 to 3 kVp/cm for changes in patient/part size. This type of chart is called a variable kVp chart. One of the advantages of a variable kVp chart is that overall image contrast is higher, which may provide greater visibility of detail. Also, variable kVp enables small incremental changes in exposure techniques that mA and exposure time cannot. Another type of chart is a fixed kVp chart. For this method, an optimum kVp value is established for each projection and the mAs is varied according to the patient/part thickness. The advantages of a fixed kVp chart are as follows. When the kVp levels are kept to the high end of the optimum range, exposures will have more latitude for exposure error. Latitude means that a wider range of densities, especially grays, are shown on the image. Exposures may be designated for small, medium, large, and extra-large patients, rather than having a separate listing for each centimeter measurement. When this is the case, each size category should state the size range in centimeters. Radiation exposure to patients may be somewhat lower with fixed kVp technique charts. In most departments, there are some charts that are set up as variable kVp and some that are fixed kVp.

Mar 7, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on Formulating X-ray Techniques
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