British Journal of Radiology 75 (2002),831-836 © 2002 The British Institute of Radiology
Macroradiography using conventional radiographic X-ray equipment
R A Davidson, BBus, MIR, GAIP
1 and
S Bowman, PhD, MA, SRR
2
1 School of Clinical Sciences, Charles Sturt University, Booroma Road, Wagga Wagga NSW 2678 and 2 Whyalla Campus, University of South Australia, Whyalla SA 5600, Australia
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Abstract
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Macroradiography is a radiographic imaging technique used to increase the size of the image relative to that of the object. Macroradiographic images suffer degradation due mainly to geometric unsharpness (Ug). Ug results from the finite size of the focal spot of the X-ray tube. Specialized equipment with a small effective focal spot size (Eff FSS) is generally used to perform macroradiography. The specialized nature of the equipment means macroradiographic examinations are not commonly undertaken. Macroradiographic examinations can also be performed on conventional radiographic equipment because the Eff FSS varies in the X-ray beam along the cathode–anode axis. Calculations and measurements of FSSs at different positions along the cathode–anode direction of the X-ray beam have been made. A simple technique of angling the X-ray tube 10° towards the cathode reduces the Eff FSS in one dimension while still maintaining a vertical central ray. Reduced beam coverage results from this technique and an increase in radiographic exposure is required to compensate for the anode heel effect. Macroradiographic images of line pair phantoms and a hand-wrist phantom, at various tube angles, have been obtained to compare image detail.
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Introduction
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Macroradiography is a technique that uses geometric factors to enlarge the radiographic image [1–3] in order to enhance visualization of parts of that image [4–6]. Macroradiography allows small detail to become more obvious in the image [2–6].
Clark et al [2] identify a number of applications for macroradiography including carpal bones, petrous bones, the lacrimal system and nodular patterns in chest radiographs. Many authors [7–12] describe the use and value of macroradiography in orthopaedic radiological examinations when compared with general radiological examinations. Other authors [13–17] describe uses of macroradiography for radiological examinations other than orthopaedic.
Some of the traditional applications of macroradiography are being questioned [18, 19]. The new imaging modalities of MRI [18, 19] and digital radiography [20] are taking over from macroradiography. Even with the advent of these modalities, the main reason that macroradiographic examinations are not commonly undertaken is the requirement for specialized equipment [2–6]. The objective of this paper is to describe a technique that can be undertaken using conventional general radiographic equipment. This could provide a simple, plain radiographic technique that obviates the need for further imaging using other modalities such as MRI or nuclear medicine.
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Macroradiographic technique
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All radiographic images are enlarged relative to the size of the object, to some extent owing to the physical requirement that the object must be at some distance from the imaging plane. Radiographers usually endeavour to reduce magnification by maximizing focal–film distance (FFD) and minimizing the object–film distance (OFD) [2, 21]. When undertaking macroradiography, purposely increasing the OFD in relation to a fixed FFD increases magnification of the size of the image relative to the size of the object [1, 2, 21].
Geometric unsharpness (Ug) is the main disadvantage of macroradiography. It is present because radiation from an X-ray tube is not produced from a point source [21]. The degree of Ug is dependent upon focal spot size (FSS), FFD and OFD [1, 21]. The prime means of reducing Ug in macroradiography is to minimize FSS. Clark et al [2] recommend that, for a magnification factor of 2 or greater, a FSS of 0.1–0.3 mm should be used. Most general X-ray sets have FSSs in the order of 0.6 mm to 1.5 mm and hence are not considered to be practical for use in macroradiography.
To undertake macroradiography with acceptable levels of Ug using conventional X-ray equipment, a modification of standard radiographic technique is required.
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Focal spot size measurement
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Effective FSS (Eff FSS) is the main determinant of spatial resolution or radiographic detail in a radiograph [1, 2, 21]. The Eff FSS results from a relationship between actual FSS, that is the area where the electrons strike the anode or target (CB in Figure 1
), and the anode angle. Eff FSS is measured to National Electronic Manufacturers Association standard [22] and is measured at the point on the central axis of the primary X-ray beam. [1, 21].
Figure 1
shows diagrammatically how the Eff FSS changes with respect to the position of measurement along the cathode–anode axis of the primary X-ray beam. At the cathode end of the beam, Eff FSS is larger than at the anode end. Figure 2 shows pinhole images of the focal spot. Figure 2A is taken at an angle of -10° (an angle or tilt of the X-ray tube towards the anode end of the tube) and Figure 2B is taken at the position of the central ray (CR).
Calculation of the positional-dependent Eff FSS, with respect to the angle 
(Figure 1), requires knowledge of the X-ray tube factors, supplied by the manufacturer, tube angle 
(Figure 1) and Eff FSS at the central beam.
Using Equation 1 below, calculations of the positional-dependent Eff FSS can be made.
where DB (when is positive, or D'B when is negative) is the calculated positional-dependent Eff FSS, AB is the Eff FSS given by the manufacturer (effective focal spot as measured at the point of the CR), is the target angle given by the manufacturer and is the angle of the positional-dependent effective focal spot.
Measurements of the positional-dependent Eff FSSs were made using a 2° star resolution phantom (Model No. 07-510; Nuclear Associates, Carle Place, NY). Owing to the physical size of the star resolution phantom and the requirement that the phantom is placed in close proximity to the X-ray tube for magnification of the phantom image, only three positional-dependent Eff FSS measurements could be made along the cathode–anode axis of the beam. Three measurements were made at each angle of being 10°, 0° and -10° within the X-ray beam. For the purpose of this paper, a negative angle will indicate an angle of the X-ray tube towards the cathode end of the cathode–anode axis of the primary X-ray beam, or alternatively, a measurement within the beam at the anode end of the cathode–anode axis (see Figure 1).
Measurements of positional-dependent effective focal spots were made on three general X-ray tubes with manufacturer specified focal spots and target angles of:
- 0.6 mm Eff FSS/12° target angle (Model A-192; Varian, Salt Lake City, UT),
- 0.6 mm Eff FSS/19.5° target angle (Model DRX-3724 HD; Toshiba Corporation, Tochigi-Ken, Japan) and
- 1.0 mm Eff FSS/16° target angle (Model DRX-1603B; Toshiba Corporation, Tochigi-Ken, Japan).
Table 1 shows both the calculated positional-dependent Eff FSS and the measured FSSs. Mean measurements of the Eff FSS are within NEMA standards [22] of the calculated positional Eff FSS based on manufacturer stated FSSs. The measurements show an increased size of focal spots compared with the manufacturers' stated size. This is consistent with blooming of the focal spot given that each of the three X-ray tubes is more than 5 years old.
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Table 1. Positional-dependent effective focal spot sizes (Eff FSSs) (mm) at various positions in the beam of typical X-ray tubes. (negative angles are towards the anode)
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It must be noted that the FSS is only reduced in one dimension, the anode–cathode direction, not at 90° to this direction.
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Spatial resolution
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Whilst measurement of Eff FSS can provide an indication of spatial resolution of an image, visualization of the image and measurement of the resolution give a more accurate assessment of the value of focal spot size changes.
Spatial resolution of the macroradiographic techniques was tested by capturing images of a line pair phantom (Model No. 07-521; Nuclear Associates, Carle Place, NY) and of a hand-wrist phantom on a typical general use X-ray unit.
Macroradiographic images at a magnification factor of 2 were produced using a combination of Kodak Lanex fine screens (Kodak Lanex, Coburg, Australia) and Konica medical film MGH-SR, (Shimadzu Medical Systems, Rydalmere, Australia), and were obtained using a Toshiba medium frequency X-ray unit, model KXO-30R (Toshiba Corporation, Tochigi-Ken, Japan) and a 0.6 mm Eff FSS/12° target angle X-ray tube (Model A-192; Varian, Salt Lake City, UT).
The images obtained were:
- line pair phantom: angle of 0° in X-ray beam; measured positional-dependent Eff FSS 0.76 mm; magnification factor 2; 60 kVp, 100 mA, 0.1 s (Figure 3a),
- line pair phantom: angle of -10° in X-ray beam; measured positional-dependent Eff FSS 0.14 mm; magnification factor 2; 60 kVp, 100 mA, 0.1 s (Figure 3b),
- hand-wrist phantom: angle of 0° in X-ray beam; measured positional-dependent Eff FSS 0.76 mm; magnification factor 2; 60 kVp, 100 mA, 0.35 s (Figure 4),
- hand-wrist phantom: angle of -10° in X-ray beam; fingers of phantom in cathode–anode direction; measured positional-dependent Eff FSS 0.14 mm; magnification factor 2; 60 kVp, 100 mA, 0.35 s (Figure 5), and
- hand-wrist phantom: angle of -10° in X-ray beam; fingers of phantom at 90° to the cathode–anode direction; measured positional-dependent Eff FSS 0.14 mm; magnification factor 2; 60 kVp, 100 mA, 0.35 s (Figure 6).
A reduction in optical density of the order of 20% in the original images taken at 0° and -10° positions, due to the anode heel effect, was noted. (NB. the figures have undergone contrast adjustment for display purpose). Anode heel effects are still seen in Figure 5 as a change of density from top to bottom and in Figure 6 from left to right.
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Figure 3. Macroradiograph of line pair phantoms at magnification factor of 2. (A) Angle of 0°; measured positional-dependent effective focal spot size (Eff FSS) of 0.76 mm. (B) Angle of -10°; measured positional-dependent Eff FSS of 0.14 mm.
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Figure 4. Macroradiograph of hand-wrist phantom: magnification factor of 2; angle of 0°; measured positional-dependent effective focal spot size of 0.76 mm.
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Figure 5. Macroradiograph of hand-wrist phantom: magnification factor of 2; angle of -10°; long axis of trabecular pattern parallel to cathode–anode axis (fingers pointing parallel to cathode–anode axis); measured positional-dependent effective focal spot size of 0.14 mm.
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Figure 6. Macroradiograph of hand-wrist phantom: magnification factor of 2; angle of -10°; long axis of trabecular pattern at 90° to cathode–anode axis (fingers pointing at 90° to cathode–anode axis); measured positional-dependent effective focal spot size of 0.14 mm.
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Discussion
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Measurements show variations of FSS, dependent upon the position of measurement/calculation in the X-ray beam. These are consistent with the expected, calculated values of the focal spot sizes (Table 1). The prime factor for determining FSS is the position of measurement along the cathode–anode axis of the beam. When the position along the cathode–anode axis is towards the anode end of the beam, the focal spot's dimension is reduced in that direction, but it is not reduced in the dimension at 90° to the cathode–anode axis.
To overcome distortion of the image of the object that can potentially occur when the object is offset in the X-ray beam, a simple technique of angling the X-ray tube can be used. The CR of the beam is angled and the offset beam, with the desired positional-dependent Eff FSS, becomes vertical. The now vertical beam will have a FSS dependent upon the angle between the vertical beam and CR. When the CR is angled 10° towards the cathode end of the X-ray tube, advantages of the reduced FSS in one dimension can be gained. Trade-offs must be considered when using this section of the beam. These trade-offs are reduced beam size and hence anatomical coverage and increased anode heel effect.
A macroradiographic image of the line pair phantom taken at the point of the CR (angle of 0°), with a magnification factor of 2 (Figure 3a), shows a maximum resolution of 2.0 line pairs mm-1 (note the aliasing below 2 line pairs mm-1). The Eff FFS at this location in the X-ray beam was given as 0.6 mm and measured at 0.76 mm. The comparison macroradiographic image (Figure 3b) of the same phantom was taken at an angle of -10°. This shows a resolution of 4 line pairs mm-1, twice the measured resolution at 0°. Such an improvement in spatial resolution is consistent with the reduction of FSS.
The improved resolution gained from this technique is only in one dimension. This is a potential concern for imaging anatomy. Figures 5 and 6 both show improved spatial detail over Figure 4. Aligning the anatomy of interest, in this case the trabecular pattern of the carpus bones at 90°, to the cathode–anode axis of X-ray tube, the reduced FSS and improved spatial detail in this direction can be utilized. Figure 5 was exposed with the fingers and trabecular pattern of the carpus pointing in the direction of the cathode–anode axis. Figure 6 was exposed with the fingers and trabecular pattern of the carpus at 90° to the cathode–anode axis. Figure 6 shows improved resolution of vertical edges and the trabecular pattern compared with Figure 5.
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Conclusion
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Macroradiographic examinations have not been commonly used in the past owing to the specialized equipment required to undertake such examinations. It is possible to show improved spatial resolution and radiographic detail when undertaking macroradiographic examinations using conventional X-ray equipment. Angling the X-ray tube in the cathode–anode direction alters the Eff FSS at the point of the CR. Angling the X-ray tube towards the cathode end of the tube reduces the Eff FSS in that dimension.
The reduction of the Eff FSS using this technique has shown improved radiographic detail on both line pair resolution phantom images and on carpal macroradiographic images. Image detail is only improved in one direction. The anatomy of interest, such as the trabecular pattern of the carpus, must be orientated to take advantage of this improved detail.
Negative effects of the use of this technique are reduced radiographic beam coverage and the fact that, to maintain image optical density, radiographic exposure must be increased by approximately 20% owing to the anode heel effect.
To determine the clinical viability of this macroradiographic technique, a controlled study is planned. If improved anatomical detail is consistently shown with improved diagnostic confidence, this technique could provide a viable imaging option where initial diagnosis using conventional radiography is inconclusive. Such a simple technique, available on all conventional X-ray equipment, could possibly decrease patient referrals to other imaging modalities such as nuclear medicine and MRI.
Received for publication December 11, 2001.
Revision received May 15, 2002.
Accepted for publication May 17, 2002.
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Abstract
Introduction
