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Measuring the filtration of mammographic X-ray tubes with molybdenum targets

Regional Medical Physics Service, Royal Victoria Hospital, Belfast BT12 6BA

This excerpt was created in the absence of an abstract.

The method employed in this work, to estimate the filtration of mammographic X-ray sets, makes use of the measurement of the half-value layer (HVL) of the X-ray beam, which is filtered by the air path, the X-ray tube window, an added molybdenum filter and a known thickness of Perspex. Sets of data have been calculated that relate the measured HVL to the added molybdenum filter thickness. The mammographic X-ray sets included in this study were all Siemens Mammomat 2 type, which have molybdenum targets and molybdenum filters and the compression plates employed have a Perspex equivalence of about 1 mm or 2 mm. The kilovoltage is produced by a high-frequency generator and has a nominally constant potential waveform.

It is often necessary to determine the HVL of mammographic X-ray sets, for two different reasons: (a) measurement of mean glandular breast dose (Institute of Physical Sciences in Medicine (IPSM), 1989), and (b) determination of total filtration by the HVL method. The estimation of total filtration of X-ray tubes with tungsten targets has previously been given some attention by the author (Cranley & Fogarty, 1988). In addition, errors in the HVL method have been analysed (Gilmore & Cranley, 1990).

The IPSM method of estimating breast dose has become widely adopted by physicists involved in the NHS Breast Screening Programme. The method involves the determination of both incident air kerma (for correct exposure of a phantom representing the standard breast) and HVL of the incident X-ray beam. The compression device should be in position for these measurements, to satisfy the standard breast conditions.

It is necessary, therefore, to have the compression device in position during the measurement of the HVL. The requirement to estimate the filtration of the mammographic X-ray beam comes from the International Commission on Radiation Protection (ICRP) (1982), which recommends that the total permanent filtration should be equivalent to at least 0.5 mm aluminium or 0.03 mm molybdenum. This implies that the total filtration should be estimated with removable filters, such as the compression plate, excluded.

The current method allows the HVL to be measured

Received for publication October 1, 1990. Revision received December 1, 1990.

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