Recommendations on Standards for the Design of Medical Diagnostic Equipment for Adults with Disabilities, Advisory Committee Final Report

4.3.2 Transfer Surfaces and Imaging Equipment Functions

The transfer surface (table) of diagnostic imaging equipment serves two purposes: (1) it allows patients to be positioned properly to produce a high-quality image of the anatomical region of interest (with the lowest possible radiation dose for devices using radiation); and (2) imaging is conducted through the transfer surface. Thus, the transfer surface plays an integral role in the exam, is critical to achieving accurate diagnostic results, and influences radiation exposure of the patient. These requirements, combined with the diverse mechanical, electrical, and physics (e.g., functioning of magnets, electrical fields) aspects and needs of different diagnostic imaging equipment, generate a wide variety of equipment designs and configurations. These demands also place some inherent limitations on the design options for transfer surfaces or tables.

Diagnostic imaging equipment groups roughly into the following categories in terms of the functional role of the transfer surfaces, which has design implications:

• Equipment with bores, including CT, PET, PET/CT, NM, and NM/CT. Here the table plays an integral part in achieving the sub-mm dynamic positioning accuracy needed during the scan.

• Magnetic equipment that is open or has a bore. MR shares many similar aspects as equipment with bores, but has special considerations due to the very strong magnetic field.

• DXA. In DXA, the X-ray source is positioned under the patient in a fixed, known geometry; this positioning maximizes diagnostic effectiveness and minimizes radiation doses (see below).

• Conventional XR and fluoroscopy. This equipment has rectangular, radio-translucent tables that may translate in both directions in the horizontal plane.

• Mobile XR. Mobile XR can be moved to the patient and can utilize detachable detectors that often can be placed behind the patient’s anatomy to be imaged without requiring significant movement by patients.

• Interventional XR. This equipment, such as that used in cardiac catheterization/angiography suites, and “surgical” C-arms, virtually always involves patients who have been sedated to some extent prior to transfer onto the surface. After initial diagnostic evaluation (e.g., identification of location and degree of coronary artery blockages), often therapeutic interventions are performed (e.g., insertion of a stent through the catheter).

• Prone breast biopsy tables. These devices are typically used for interventional procedures such as minimally invasive image guided biopsies. Its unique design must accommodate a physician underneath the patient support surface. Since the device is used in interventional procedures, patient sedation and/or local anesthetics are commonly used. These procedures are only performed after screening mammography and/or diagnostic mammography (or another diagnostic imaging assessment) has been performed. If a patient cannot access the prone table, another potential option is an upright stereotactic biopsy system, where patients are either seated or in a side-lying position.^BB

Thus, diagnostic imaging equipment tables fall largely into two main groupings. The first group involves tables used by equipment with bores, such as CT, MR, and NM systems. These tables are generally long and relatively narrow in order to move the patient into the bore space. They are capable of adjusting with the high precision (sub-millimeter accuracy) needed for accurate diagnostic information both vertically and horizontally. These tables are typically rated for patients in excess of 400 lbs.

The second group is those tables used in X-Ray systems. These tables are also often rated for patients in excess of 400 lbs, but are wider than those used with equipment with bores and in many cases are able to move horizontally in two directions. Most current tables are not designed to adjust vertically, but some are capable of rotating to place the patient in a more vertical position needed for specific diagnostic exam.

As suggested by this technology and clinical overview and detailed further in Section 7.2, current structural requirements of specific imaging technologies have implications for the extent to which transfer surfaces on these devices can presently be height adjustable. Many X-Ray systems have imaging components such as X-Ray tubes, high voltage generators, and/or detectors located underneath the table (transfer surface). These components may, with current designs, impede lowering the table to accessible minimum heights.

An instructive example is the DXA scan (dual energy x-ray absorptiometry), used to measure bone density and identify persons with osteoporosis or osteopenia (low bone density but not yet osteoporosis). Bone loss is especially prevalent among persons with mobility disabilities who cannot perform weight-bearing exercise, such as walking or running. Therefore, access to DXA scans is critically important to a substantial subset of individuals with disabilities.

Typically, DXA scans measure bone density in the hip and spine as patients lie on a table and the scanning device – shaped like a large C with one arm passing above and the other passing immediately below the patient – travels from the hip area up to around the patient’s waist. This C-configuration, which brings the lower arm of the scanner close under the patient, minimizes the x-rays required to perform the test. Thus, the radiation exposure from DXA scans is low. However, to allow sufficient room to accommodate the x-ray technology in the lower arm of the C, the table on which patients must lie during DXA scans is relatively high off the ground. Current DXA technology cannot meet the recommended standards for height-adjustable tables with transfer surfaces at low heights.

As described further in Section 5, an issue closely related to transfer surfaces involves the positioning of supports to assist patient transfers. For diagnostic imaging equipment, patient support devices must meet applicable safety factors as delineated in IEC 60601-1. These factors typically range from 4 to 8 times the indicated weight support: for example, a transfer surface labeled to support a 500 lb patient must be designed and tested up to 4,000 lbs. This may have implications for adjustable height table design if, with current engineering methods, mechanical advantages (leverage) diminish as tables lower to lesser distances from the floor.

Notes

BB The Advisory Committee did not compare the relative effectiveness and safety of the prone breast biopsy table and the alternative upright stereotactic biopsy system to determine whether these are equivalent options for breast biopsies.