Radiographic Absorptiometry
In 1939 Mack et al [MACK1939] described a reproducible method of measuring the density of bones and teeth from radiographs. Radiographic absorptiometry is a technique based on a comparison of the difference in attenuation of a beam of ionising radiation by bone and an aluminum step wedge when irradiated simultaneously. The aluminium step wedge, calibrated to known density values, allows the observer to calculate the relative phalangeal bone mineral density [MORG1967]. Although the technique is relatively inexpensive and easy to use it does rely on the subjective placement of a densitometer over the region of interest which limits its diagnostic reproducibility. It could also be argued that the assessment of phalangeal bone is not adequately representative of the bone mineral density at skeletal locations which are composed of predominantly trabecular bone, for example the lumbar spine. The vertebrae are reported by some to contain between 60% and 70% trabecular bone [VOGE1987], whilst others are more generous suggesting that they contain between 66% and 90% trabecular bone [EINH1996]. This is an important consideration when the aim of the radiological examination is to assess the fracture risk of a bone high in trabeculae.
Single Photon Absorptiometry (SPA)
Initially described in 1963 [CAME1963], SPA depends on the attenuation by bone of a beam of gamma rays emitted by a radioisotope source. The most common source of the gamma rays is lodine^^^ which has a photon energy of 27.5 kilo electron Volts (keV) and a half life (T^ ) of 60 days [GUGL1995]. As the time taken for half of the nuclei present in the I o d i n e s o u r c e to disintegrate is only 60 days it is clear that maintaining the lodine^^^ source incurs considerable continuous costs as it will need to be replaced several times each year if the efficiency of the SPA examination is to be regulated. The pencil beam of gamma photons produced by the radioisotope and attenuated by bone is detected by a sodium iodide detector with a narrow beam collimator. SPA provides a measure of bone mineral content (BMC) in grammes.centimetre'^ length unit at appendicular skeletal sites such as the distal radius and ulna [LAND1981] and the calcaneum [KLEM1976]. Only appendicular skeletal sites can be examined with SPA as the area under investigation needs to be
C hapter 4 Diagnosing osteoporosis
immersed in water or wrapped in an alternative tissue equivalent substance to obtain a constant soft tissue thickness overlying the bone. Water may be used as a tissue equivalent material since its density, average atomic number and electron density are almost identical to those of muscle tissue and blood and close to those of fat [MERE1974]. Differential photon absorption of the bone mineral in the path of the beam can then be calculated. The effective dose equivalent (EDE) of an SPA scan is estimated at approximately 1 microSievert (fiSw).
Single X-Ray Absorptiometry (SXA)
The physical principles behind the SXA technique are the same as those for SPA, however the use of an X-ray tube to produce the ionising radiation allows faster scanning times. In SXA an X-ray tube with a low voltage generator (40kV) produces an X-ray beam which is filtered using tin to give a beam with a lower energy spectrum comparable to SPA [BORG 1995]. The improvement in scanning speed in turn improves the precision of the technique as it alleviates the problem of patient movement during a scan. The technique is also more cost effective as the permanent source of ionising radiation obviates the need to replace a radioisotope source throughout the year.
Dual Photon Absorptiometry (DPA)
The low energy levels of the ionising radiation used in the SPA and SXA techniques limits their ability to examine anything other than superficial skeletal sites such as the appendicular skeleton. In addition the techniques necessitate the use of a water bath or other tissue equivalent material. The development of a dual energy method of assessing bone mineral content obviates the need for a water bath and allows measurement of axial skeletal sites, such as the lumbar spine and proximal femur. A radioisotope with a dual energy, such as Gadolinium^^^ which has two energy peaks of 44 and lOOkeV is typically used [GUGL1995]. The lower energy peak of 44keV is attenuated more by soft tissue than the higher energy peak of lOOkeV, but both are attenuated more by mineral in bone than by soft tissue. Thus a calculation of the relative difference in attenuation values between soft tissue and mineral and between the low and the high energy peaks allows the mineral content of the bone under examination to be assessed.
Dual Energy X-ray A bsorptiom etry (DXA)
In 1987 the first commercial X-ray system was developed which could produce X- ray beams of two energy peaks able to replace DPA [STEI1987]. The method of production of the two energy peaks of the X-ray beam depends on the manufacturer of the equipment but values of 70kVp and 140kVp may be produced with alternating pulses or filtered from an X-ray spectrum.
The effective dose equivalent often quoted for a DXA scan is between 1 and 3 microSieverts, which is considerably lower than the 60 microSieverts cited [GUGL1997] for a typical quantitative computed tomography (QCT) spinal scan. Recently however, as a result of the development of new generation fan beam DXA devices, the inherent radiation dose of a DXA scan has increased and values similar to those received during a QCT scan are now being quoted. In a recent study [STEE1998] the effective dose equivalent for an AP lumbar spine scan on a Lunar Expert-XL fan beam densitometer was reported as 59 microSieverts, whilst for a total body scan it was higher at 75 microSieverts. In the recent past to a certain extent advocates of the DXA technique used the lower radiation dose as a marketing tool. This met with considerable success, as dual energy X-ray absorptiometry is the most common osteoporosis assessment device used in the United Kingdom with approximately four available per million population [EURO 1998]. The risk of increased exposure by the fan beam is balanced against the benefit of the improved resolution it allows.
The bone mineral content is expressed in grammes but DXA software programmes also calculate a measurement of bone density described as areal bone mineral density in grammes.centrimetre'^. As true density is based on a volumetric measurement the value of the two dimensional density measurement calculated by DXA needs to be assessed carefully. The two dimensional measurement is calculated because the true volume of bone cannot be accurately measured using the DXA technique. A volume based on the bone’s area is therefore predicted and the density calculated using this value.
Chapter 4 Diagnosing osteoporosis
The technique of DPA is now less common having been superceded by DXA. As with SXA and SPA the physical principles behind the X-ray technique of DXA are based on those of the radioisotope technique.
Quantitative Computed Tomography (QCT)
QCT can be performed in single energy or dual energy mode. Each QCT scan examines both the patient and a reference phantom concurrently. The reference phantom is composed of various concentrations of solid hydroxyapatite providing a series of linear attenuation coefficients comparable to a range of values between normal and osteoporotic bone. The region of interest (ROI) in the central portion of the trabecular rich vertebral body is selected and a CT number is allocated to it. CT numbers are measured in Hounsfield units (HU) where water represents zero HU and air -1000 HU [GUGL1997]. A comparison between the HU of the phantom and bone enables bone mineral density in grammes.centimetre'^ to be calculated. This simultaneous scanning of phantom and body tissue allows for the instabilities of the scanner and the effect of variable beam hardening and patient body habitus [GUGL1997].
Since QCT evolved in the late 1970s [GENA 1977] it has been used in many centres in preference to other densitometry techniques. This is for three key reasons, the first and possibly most pragmatic reason is logistical, namely that many centres already possess a Computed Tomography (CT) scanner for imaging and diagnostic purposes and so the technique is deemed more cost effective. The second and clinically more important reason is the inherent ability of the CT technique to be able to differentiate between trabecular and cortical bone. This is an important consideration as trabecular bone is metabolically more active and is therefore most likely to show the earliest signs of the impact of osteoporosis. The third reason is also clinical in that QCT is reported to be the only technique that can measure a true bone mineral density as it is capable of accurately measuring, rather than merely estimating bone volume. One of the disadvantages of QCT is that it carries a moderately higher radiation burden than most of the other ionising radiation methods of assessing bone mineral density. An EDE of approximately 60 /xSv including 30 jLtSv for the location image is typical [GUGL1997].
Peripheral QCT (pQCT)
Ruegsegger et al [RUEG1976] developed the first computed tomography system to determine the bone mineral density of parts of the peripheral skeleton specifically the radius and ulna and from this development the term pQCT was derived. The original system used a radioisotope source of lodine^^^ and a sodium iodide crystal detector. Subsequent developments included replacing the radioisotope source with an X-ray tube and using a fanbeam of radiation, allowing the procedure time to be reduced minimising movement artifact. The EDE of pQCT is 0.03 /iSv.