Types of DEXA Scanner
DEXA scanners can be broadly categorised into two types;
1. Central Scanners
2. Peripheral Scanners
Central scanners allow measurement of BMD of the hip and spine, which are considered to be the best sites for monitoring overall BMD, and for predicting fracture. Larger whole body scanners can also be used to measure body composition (fat mass, lean body mass, and bone mass).
Central scanners can be subdivided according to the shape of the scanning x-ray beam. These include Pencil Beam, Narrow Angle Fan Beam, Wide Angle Fan Beam, and Flash Beam scanners. Moving from pencil beam to flash beam; in general, the scan time decreases, but the patient dose and cost increases.
Pencil Beam scanners use a fine pencil beam of x-rays combined with a single detector. This scans the patient in a raster fashion back and forth along the patient. Examples include the Norland Excell and XR-46, the Medilink Osteocore & Osteocore 2, and the GE Lunar DPX Series.
Narrow angle fan beam scanners scan the patient in a similar raster fashion, except that the x-ray beam is about 4cm wide and is detected using an array of detectors. The patient is thus scanned in a series of ribbons, which greatly reduces scan time. Examples include the GE Lunar Prodigy Series.
Wide angle fan beam scanners use a much wider beam of x-rays and a longer array of detectors which scans the patient in a single pass (i.e. there is no need for raster scanning back and forth across the patient). Examples include the Hologic range of central scanners.
The Digital Flash Beam scanner consists of a 2 dimension flash for a 1.5 second exposure. This flash eliminates the scanning process and is therefore the fastest type of scanner. An examlpe is the MediLink Osteocore 3.
Peripheral scanners are small portable devices which measure BMD in the heel or forearm. These can be used for screening to estimate overall fracture risk, but they are less accurate than state of the art hip/spine scanners and cannot be used for monitoring therapy. Examples of peripheral scanners include the Norland Apollo heel scanner, and the Osteometer DTX-200 forearm scanner.