2 COMPENSATING FILTERS PETER J. BARGER

Transcription

1 2 COMPENSTING FILTERS PETER J. RGER, xiolateral projection of the hip (Danelius-Miller method) without compensating fi lter., Same projection with Ferlic swimmer s fi lter. OUTLINE Introduction, 46 Physical principles, 49 Specifi c applications, 51 Compensating fi lters in this atlas, 55

2 Compensating filters Introduction In most cases, radiography is accomplished using a single exposure technique for a given body structure. However, some structures contain areas of significantly varied tissue density that must be shown on one image. These structures present special challenges in demonstrating the anatomic structures with an acceptable range of densities. Often, two exposures must be made on these body structures, doubling the radiation exposure to the patient. Typically, if one exposure is used, a technique is selected to adequately penetrate the most dense area of anatomy. In this case, the radiologist will highlight the dark anatomic area on the image with a hot-light. However, these images often have to be viewed by other physicians without having such a light available. With digital radiography systems, the image can be adjusted with the computer to lighten the dark area of anatomy; however, the large difference in transmitted x-rays often exceeds the dynamic range of the software. This can result in images that appear low in contrast, contain high noise, or show processing artifacts. Examples of x-ray projections that have to demonstrate significantly varied tissue density include the P projection of the thoracic spine, the axiolateral projection (Danelius-Miller method) of the hip, and the lateral cervicothoracic region (swimmer s technique) (Fig. 2-1). Exposure of these structures with a uniformly intense C 10 cm, C7 28 cm, T1 20 cm 10 cm 8 cm, T1 26 cm, T12 Fig. 2-1 ody structures with signifi cantly varied tissue density include thoracic spine (P) (), hip (lateral) (), and cervicothoracic region (lateral) (C). Note the different thicknesses in these areas. Use of compensating fi lters will allow these structures to be demonstrated with one exposure. 46

3 x-ray beam results in the production of an image with areas of underexposed or overexposed anatomy. To compensate for these variations in tissue density, specially designed attenuating devices called compensating filters can be placed between the radiographic tube and the image receptor (IR). The resulting attenuated beam more appropriately exposes the various tissue densities of the anatomy and reveals more anatomic detail. Equally important, the filter will somewhat reduce the entrance skin exposure (ESE) and therefore the dose to some of the organs in the body (Fig. 2-2). The technique of compensatory filtration was first applied in 1905 by George Pfahler, 1 not long after x-rays were first discovered. Pfahler used wet shoe leather as the filter by wrapping it around a patient s arm. Compensating filters of one type or another have been in use since that time. Some of the most common filters in use today are shown in Fig These filters can be used with both screen-film 1 Pfahler GE: roentgen filter and a universal diaphragm and protecting screen, Transcripts of the merican Roentgen Ray Society 217, and digital imaging systems to improve the image quality of a variety of anatomic areas. With most digital systems, filters are necessary to obtain a diagnostic image of an extreme density-different body part. In addition, radiation exposure to the patient is lowered through elimination of extra exposures needed to demonstrate all of the anatomy and through the beam hardening effect of the attenuating filter. The increasing thickness of the filter over the thinner body part also acts to reduce exposure. Introduction Primary x-rays Primary x-rays Wedge filter Trough filter Filtered x-rays Filtered x-rays Fig. 2-2, Wedge fi lter in position for an P projection of the thoracic spine. Note how the thick portion of the wedge partially attenuates the x-ray beam over the upper thoracic area while the nonfi lter area receives the full exposure to penetrate the thick portion of the spine. n even image density results., Trough fi lter in position for an P projection of the chest. Note how the two side wedges partially attenuate the x-ray beam over the lung areas while the mediastinum receives the full exposure. better-quality image of the chest and mediastinal structures results. 47

4 Compensating filters C D E Fig. 2-3 Examples of compensating fi lters in use today., Supertech wedge, collimator-mounted ClearPb fi lter used for the P projection of the hips, knees, and ankles on long (51-inch) fi lm., Trough, collimator-mounted aluminum fi lter with a double-wedge used for P projections of the thoracic spine. C, oomerang contact fi lter used for P projections of the shoulder and facial bones. D, Ferlic, collimator-mounted fi lter used for P and P oblique (scapular Y) projections of the shoulder. E, Ferlic, collimator-mounted fi lter used for lateral projections of the cervicothoracic region (swimmer s technique) and axiolateral projections (Danelius-Miller method) of the hip. 48

5 The appropriate use of radiographic compensating filters is an important addition to the radiographer s skill set. The radiographer determines whether or not to use a filter based on an assessment of the patient and then determines the type and exact position of the filter. This is accomplished while positioning the patient. Radiographic projections of the lateral hip and the lateral C7-T1 cervicothoracic region in most instances will require a filter to demonstrate all the anatomy on one image. Projections such as the P shoulder and P thoracic spine may not need a filter on hyposthenic-shaped patients; however, on hypersthenic-shaped patients and patients who are barrel-chested or obese, a filter is necessary. Pediatric patients seldom require a filter, except when P lateral projections of the full spine are done in cases of spinal curvatures such as scoliosis. Compensating filters for full-spine radiography not only allow the entire spine to be imaged with one exposure, they also significantly reduce the radiation exposure to the young age group that requires these images. 1-3 Physical Principles Compensating filters are manufactured in a variety of shapes and are composed of several materials. The shape or material chosen is based on the particular body part to be imaged. The exact placement of the filter also varies, with most being placed between the x-ray tube and the skin surface, although some are placed under the patient. Filters placed under the patient often produce distinct outlines of the filter, which can be objectionable to the radiologist. SHPE The wedge is the simplest and most common of the compensating filter shapes. It is used to improve the image quality of a wide variety of body parts. n array of more complex-shaped filters have been developed for technically challenging anatomic areas, including the trough, scoliosis, Ferlic, 1 and oomerang. 2 Some filters have multiple uses. filter that is shaped for one area of the body can also be adapted for other body structures. Filters such as the Ferlic cervicothoracic lateral projection (swimmer s technique) filter can be adapted for the axiolateral projection (Danelius-Miller technique) of the hip with excellent results. COMPOSITION Compensating filters are composed of a substance of sufficiently high atomic number to attenuate the x-ray beam. The most common filter materials are aluminum and high-density plastics. They are manufactured with different thickness of the material and generally distributed in a smoothly graduated way that corresponds with the distribution of the different tissue densities of the anatomy (see Fig. 2-2). luminum is an efficient attenuator and thus a common filter material; however, when placed between the x-ray tube and the patient, it blocks the field light. This makes positioning of the patient and the central ray more challenging. Many aluminum filters have a 100% x-ray transmission zone (see Fig. 2-3,, D, and E), and positioning is made slightly easier. Radiographers who use aluminum filters must first complete the positioning of the patient and alignment of the central ray before mounting the filter to the collimator. Some manufacturers offer compensating filters made from clear leaded plastic, known as Clear Pb, 1 which allows the field light to shine through to the patient but still attenuates the x-ray beam. However, this leaded plastic is not appropriate for all filter uses, such as in the extremely dense area of the shoulder during lateral spine radiography, because the thickness required to sufficiently attenuate the beam would result in a prohibitively heavy device. In these cases, aluminum is generally used. The oomerang (Fig. 2-3, C) filter is composed of an attenuating silicon rubber compound, and some models of this filter have an imbedded metal bead chain to mark the filter edge. Physical principles 1 Gray JE, Stears JG, Frank ED: Shaped, lead-loaded acrylic filters for patient exposure reduction and image quality improvement, Radiology 146:3, 825, Frank ED et al: Use of the posterior-anterior projection as a method of reducing x-ray exposure to specific radiosensitive organs, Radiol Technol 54:343, Nash CL Jr et al: Risks of exposure to x-rays in patients undergoing long-term treatment for scoliosis, J one Joint Surg m 61:371, Ferlic. Ferlic Filter Company LLC, White ear Lake, Minn. 2 oomerang. Octostop, Inc., Laval, Canada. 1 ClearPb. Nuclear ssociates, Hicksville, NY. 49

6 Compensating filters PLCEMENT Compensating filters are most often placed in the x-ray beam between the x-ray tube and patient. roadly, filters fall into two categories based on their location during use: collimator-mounted filters and contact filters. The collimator-mounted filters are mounted on the collimator, either using rails installed on both sides of the window on the collimator housing (Fig. 2-4) or using magnets. Contact compensating filters are placed either directly on the patient or between the anatomy and the IR (Fig. 2-5). Generally, those filters placed between the primary beam and the body will have the added benefit of a reduction in radiation exposure to the patient due to the beam hardening effect of the filter, whereas those placed between anatomy and the IR will have no effect on patient exposure. Measurements provided with the Ferlic filters show radiation exposure reductions of between 50% and 80%, depending on the kvp, in the anatomic area covered by the filter. Measurements by Frank et al. 1 show exposure reductions 1 Frank ED et al: Use of the posterior-anterior projection as a method of reducing x-ray exposure to specific radiosensitive organs, Radiol Technol 54:343, of between 20% and 69% to the thyroid, sternum, and breasts. oth types have the same effect upon the finished image, which is a more uniform radiographic density even though the tissue density varied greatly. Filters can be improvised as well, with radiographers creating their own version of attenuation control devices such as filled bags of saline solution. ags of solution, however, will increase scattered radiation. Use of improvised filters is not recommended because there is potential for creating unknown artifacts in the image. Fig. 2-4 The Ferlic collimator-mounted fi lter positioned on the collimator for an P projection of the shoulder. Fig. 2-5 The oomerang contact fi lter in position for an P projection of the shoulder. 50

7 Specific pplications The choice of compensating filter to be used depends on the distribution of tissue densities of the anatomy to be radiographed. However, as illustrated in Table 2-1, most of these imaging challenges can be solved with only a few filter shapes. The following are examples of the most common compensating filter applications. The wedge fi lter is used for areas of the body where tissue density varies significantly from one end to the other along the long axis of the body. For example, the wedge filter can be used to improve image quality of P projections of the thoracic spine (Fig. 2-6). The trough fi lter is best used for areas of the body where the subject density in the center is much greater than at the edges. This filter has been successfully TLE 2-1 Common x-ray projections for which fi lters improve image quality* applied to improving P projections of the chest (Fig. 2-7). The Ferlic swimmer s fi lter is a collimator-mounted filter created to improve imaging of the lateral projection of the cervicothoracic region (swimmer s technique) (Fig. 2-8), but it is also used for the axiolateral projection of the hip (Danelius-Miller method) (Fig. 2-9). The Ferlic shoulder filter, also a collimator-mounted filter, is designed specifically to image natomy/projection Filter Type Thick portion oriented to Improved demonstration of Mandible xiolateral Oblique Ferlic swimmer s Collimator nterior of mandible Mandibular symphysis Nasal ones Lateral Facial ones Lateral Cervicothoracic Lateral Thoracic Spine P Shoulder P Shoulder xial Shoulder Oblique Chest P bdomen P Upright bdomen P Decubitus Lateral Hip xiolateral Hip P (Emaciated Patient) Foot P Calcaneus xial Hip-Knee-nkle 51 P Wedge Collimator nterior Nasal bones/cartilage oomerang Contact nterior nterior facial structures Ferlic swimmer s Collimator Upper cervical C6-T2 Wedge Collimator Upper thoracic Upper thoracic oomerang Contact and collimator C joint Ferlic shoulder C joint C joint C joint Ferlic swimmer s Collimator Humerus Humerus oomerang Contact and collimator Humeral head Ferlic shoulder Humeral head Glenoid fossa Glenoid fossa Supertech /trough Collimator Sides of chest Mediastinum Wedge Collimator Upper abdomen Diaphragm Wedge Collimator Side furthest from table bdomen side up Ferlic swimmer s Collimator Distal femur Proximal femur Wedge Collimator Greater trochanter Femoral head Wedge/gentle slope Contact and collimator Toes Forefoot Ferlic swimmer s Collimator Calcaneus Posterior calcaneus Supertech / full-length leg Ferlic swimmer s Collimator Tibia/fi bula Distal tibia-fi bula Specific applications *Not inclusive. Other body structures can be imaged. Other fi lters are available in the market. Ferlic. Ferlic Filter Company, LLC, White ear Lake, Minn. oomerang. Octostop, Inc., Laval, Canada. Supertech. Elkhart, Ind. 51

8 Compensating filters Fig. 2-6, P projection of the thoracic spine without compensating fi lter., Same projection with wedge fi lter. Note more even density of the spine, and all vertebrae are shown. 52 Fig. 2-7, P projection of the chest without compensating fi lter., Same projection with Supertech trough fi lter. Note lower lung fi elds and mediastinum are better demonstrated.

9 Specific applications Fig. 2-9, xiolateral projection of the hip (Danelius-Miller method) without compensating fi lter., Same projection with Ferlic swimmer s fi lter. Note how acetabulum and end of metal shaft are seen on one image. Fig. 2-8, Lateral projection of the cervicothoracic region (swimmer s technique) without compensating fi lter., Same projection with Ferlic swimmer s fi lter. Note how C7 and T1 area is penetrated and shown. 53

10 Compensating filters the shoulder in both the supine and upright positions. The oomerang filter was designed to conform to the shape of the shoulder and create images of more uniform radiographic density at the superior margins (Fig. 2-10). This is a contact filter placed between the anatomy and the IR (see Fig. 2-5). It can also be used effectively for lateral facial bone images. lthough effective in compensating for differences in anatomic density, it does not reduce radiation exposure because it is located behind the patient. The Ferlic shoulder filter is a collimator-mounted filter also designed specifically to image the shoulder (Fig. 2-11). ecause this filter is placed in the primary x-ray Fig. 2-10, P projection of the shoulder without compensating fi lter., Same projection using the oomerang contact fi lter. 54 Fig. 2-11, P projection of the shoulder without compensating fi lter., Same projection using the Ferlic shoulder fi lter, collimator mounted. Note greater visualization of the acromion, C joint, and humeral head.

11 beam, it also acts to reduce radiation exposure to the patient. The scoliosis fi lters are used with two of the most challenging projections to obtain, the P (Frank et al. method) and lateral full-spine projections for evaluation of spinal curvatures. These are challenging because the cervical, thoracic, and lumbosacral spines have to be demonstrated on one image. One exposure technique has to be set for what normally would be three separate exposures. With the use of compensating filters, the P projection can be made with a wedge filter positioned over the cervical and thoracic spines R (Fig. 2-12, ). For the lateral projection, two double-wedge filters are positioned over the midthoracic area and the cervical spine (see Fig. 2-12, ). The exposure technique for both the P and lateral projections is set to penetrate the most dense area the lumbar spine. The filters attenuate enough of the exposure over the cervical and thoracic areas to adequately demonstrate the thoracic and cervical spines. Highly specialized compensating filters are also used in other areas of the radiology department. During digital fluoroscopy, convex and concave conical-shaped filters are used to compensate for the round R image intensifier. In computed tomography (CT), bow-tie shaped filters are used to compensate for the rounded shape of the head. Radiographers must use caution when mounting and removing compensating filters on the collimator while the x-ray tube is over the patient. There have been instances when filters did not attach properly, did not get positioned into the filter track, or were forgotten and fell onto the patient when the tube was moved. ll compensating filters, especially the aluminum ones, are moderately heavy with sharp edges; therefore they can cause injury to the patient if dropped. When positioning the filter to the underside of the collimator, and when removing it, two hands must be used (Fig. 2-13). One hand should attach the filter while the other is positioned to catch the filter if it does not attach properly. Compensating filters in this atlas Compensating Filters in This tlas ody structures whose radiographic images can be improved through the use of compensating filters are identified throughout the atlas directly on the projection page. special icon,, will identify the use of a filter. Fig. 2-12, P projection (Frank et al. method) of the cervical, thoracic, and lumbar spine using a wedge fi lter., Lateral projection of the cervical, thoracic, and lumbar spine using two bilateral wedge fi lters. Note that the entire spine can be imaged on both projections with the use of a compensating fi lter. Fig Two hands must be used to attach and remove collimator-mounted fi lters. Note that one hand is used to catch the fi lter in case it is dropped. 55

12