British Journal of Radiology 75 (2002),763-766 © 2002 The British Institute of Radiology
An alignment method for mammographic X-ray spectroscopy under clinical conditions
S Miyajima, MS
1
K Imagawa, MS
1 and
M Matsumoto, PhD
2
1 Graduate School of Medicine, Course of Health Sciences and 2 School of Allied Health Sciences, Faculty of Medicine, Osaka University, 1-7 Yamadaoka, Suita, Osaka, 565-0871, Japan
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Abstract
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This paper describes an alignment method for mammographic X-ray spectroscopy under clinical conditions. A pinhole, a fluorescent screen, a laser device and the case for a detector are used for alignment of the focal spot, a collimator and a detector. The method determines the line between the focal spot and the point of interest in an X-ray field radiographically. The method allows alignment for both central axis and off-axis directions.
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Introduction
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Mammographic X-ray spectroscopy under clinical conditions is difficult owing to high photon fluence in the X-ray field: a long distance between the focal spot and the detector, and/or a low tube current appropriate for X-ray spectroscopy cannot be realized in clinical mammographic units. Therefore, a collimator of extremely small aperture has to be used to prevent pulse pile-up.
Large detectors such as high purity germanium detectors cannot be utilized in mammographic X-ray spectroscopy on account of geometrical restrictions; the distance between an X-ray port and a cassette holder is typically around 50 cm. Hence, small, room temperature detectors are used for mammographic X-ray spectroscopy. Although the use of CdZnTe detectors with a small aperture collimator (35
50 µm) has been reported [1, 2], no information regarding the alignment procedure is available in these papers.
The alignment methods in published papers for diagnostic X-ray spectroscopy are not applicable to mammographic X-ray spectroscopy under clinical conditions. Most of the methods are based on a pinhole technique [3–7], and the others use a laser beam with two half mirrors [8], a thread [9] and characteristic X-rays from a collimator [10]. All these methods need a wide space between the X-ray tube and the detector. In addition, all of them assume that the focal spot is visible through a tube window. However, the focal spot is invisible from the outside under clinical conditions owing to additional filters and a mirror that are attached to the tube window. Therefore, the methods in previous papers cannot be employed in clinical mammographic units.
A method to facilitate alignment in mammographic X-ray spectroscopy under clinical conditions is described in this paper. The method is based on a pinhole technique and is applicable even if the focal spot cannot be seen directly. The method is also appropriate for diagnostic X-ray spectroscopy.
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Method
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The line between the focal spot and the point of interest (where a detector will be placed) is determined radiographically. A pinhole, a fluorescent screen and a pencil-shaped laser device are employed.
A CdZnTe detector (XR-100T-CZT; Amptek Inc., MA) was employed in this study. A 3 x 3 x 2 mm3 CdZnTe crystal was incorporated in the detector head. A case containing the detector or the pencil-shaped laser device was utilized to facilitate alignment (Figure 1a
).
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Figure 1. (a) The CdZnTe detector (upper left), the pencil-shaped laser device with holder (lower) and the detector case (upper right) used in this study. (b) The laser device with holder inserted in the detector case. A drawing pin is put at the end of the laser device to indicate the direction of the laser beam (arrow). (c) The detector inserted in the detector case.
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First, a fluorescent screen is placed in the X-ray field such that the central mark on the fluorescent screen is at the point of interest. Second, a tube collimator (3 mm diameter) is placed close to the focal spot. When X-rays are emitted, an X-ray image appears on the screen. The tube collimator is adjusted until the X-ray image is centred on the central mark on the screen (Figure 2a). Third, the laser device with the detector case is placed in such a way that the laser beam can pass through the aperture of the tube collimator and the end of the laser device aims at the central mark of the screen (Figures 1b and 2b
). Thus, the axis of the laser device corresponds to the line between the focal spot and the point of interest. To avoid misalignment, the aperture of the collimator should be small and the tube collimator should be located as close to the focal spot as possible. Finally, the laser device is replaced with the detector and the X-ray beam is collimated with a 35 µm aperture at the detector window (Figures 1c and 2c). The experimental set-up is shown in Figure 3.
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Figure 2. Procedure to align the focal spot, the collimator and the detector. (a) Position the central mark of the fluorescent screen at the point of interest in an X-ray field, and adjust the tube collimator near an X-ray port so that the X-ray image on the screen corresponds to the central mark of the screen. (b) Adjust the laser device with the detector case in such a way that the laser beam passes through the aperture of the tube collimator and the end of the laser device aims at the central mark of the screen. (c) Replace the laser device with the detector and collimate the beam at the detector window.
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Results and discussion
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X-ray spectra measured using the alignment method are shown in Figure 4. There are few counts in the channel above the high energy end of the spectra (around 28 keV), meaning that pulse pile-up effects are minimized. The shaping time was 3 µs.
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Figure 4. X-ray spectra measured with the CdZnTe detector in this study. The target was a compound of molybdenum (Mo) and tungsten (W): the target contained 5% W by weight. Additional filters were 0.03 mm Mo and 0.5 mm aluminium (Al). The tube voltage was 28 kV. The small peaks at 8.4 keV and 9.4 keV were attributed to tungsten-L X-rays from the collimator.
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The method described in this paper is appropriate for both mammographic and diagnostic X-ray spectroscopy. It is unique because it is suitable for X-ray spectroscopy under clinical conditions, i.e. where there is not enough space and the target is invisible. Furthermore, the position of the focal spot is usually not specified even if the target is visible through the tube window, i.e. in an X-ray tube without additional filters and a mirror. As the direction of the focal spot is determined radiographically, the method also improves the accuracy of the alignment under experimental conditions.
This method is useful not only for X-ray spectroscopy on the central axis, but also for off-axis directions. Most X-ray spectra in published papers relate to central axis X-rays. However, X-ray spectra are not uniform in an X-ray field [11]. If the X-ray spectra on the central axis are of interest, the tube collimator should be adjusted in such a way that the line between the aperture of the tube collimator and the central mark of the screen is perpendicular to the tube axis. On the other hand, if the off-axis X-ray spectra are of interest, the screen should be placed such that the central mark of the screen corresponds to the off-axis point of interest. Using this method, all points in an X-ray field can be aligned.
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Conclusion
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A method to facilitate alignment in mammographic X-ray spectroscopy under clinical conditions has been described. A pinhole, a fluorescent screen, a pencil-shaped laser device and the case for a detector are used in the alignment procedure. As the focal spot is not usually visible from the outside, the line between the focal spot and the point of interest where the detector is placed is determined radiographically. The method is applicable to X-ray spectroscopy in both mammographic and diagnostic X-ray units. In addition, the method enables alignment of off-axis X-rays.
Received for publication February 12, 2002.
Revision received May 29, 2002.
Accepted for publication June 7, 2002.
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References
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Abstract
Introduction
