Overranging and overbeaming measurement in area detector computed tomography: A method for simultaneous measurement in volume helical acquisition

Abstract Purpose We propose a novel method to assess overbeaming and overranging, as well as the effect of reducing longitudinal exposure range, by using a dynamic z‐collimator in area detector computed tomography. Methods and materials A 500‐mm diameter cylindrical imaging plate was exposed by helical scanning in a dark room. The beam collimation of the helical acquisitions was set at 32 and 80 mm. Overbeaming and overranging with the dynamic z‐collimator were measured. Results The actual beam widths were approximately 39 and 88 mm at 32 and 80 mm collimation, respectively, and were relatively reduced owing to increased beam collimation. Overranging was 27.0 and 48.2 mm with a pitch of 0.83 and 1.49 at 32 mm collimation and 72.5 and 83.1 mm with a pitch of 0.87 and 0.99 at 80 mm collimation. The dynamic z‐collimator relatively reduced the overranging by 17.3% and 17.1% for the 32 and 80 mm collimation, respectively. Conclusion We devised a method to simultaneously measure overbeaming and overranging with only one helical acquisition. Although the dynamic z‐collimator reduced the overranging by approximately 17%, wider collimation widths and higher pitch settings would increase the exposure dose outside the scan range.

MDCT varies depending on the number of detector configurations, BW, and scanning geometry, it is scanner specific. Although overbeaming relatively decreases owing to the increased detector row, 3,4 overbeaming in recent volume helical scanning has not necessarily been clarified.
X-ray exposure in helical scanning is extended outside of the set scan range along the z-direction. The exposure length extension of helical scanning, which is called overranging, tends to increase because of the larger detector coverage and pitch selection. [5][6][7] Increased overranging in helical scanning could not be ignored in children with a short scan range or in the case of radiosensitive organs located near the scan range. 8 Radiation exposure to radiosensitive organs outside the scan range extending in the z-direction should be minimized. To reduce the increased overranging in MDCT, a dynamic z-collimator was installed in advanced CT scanners. 5,9,10 Shirasaka et al. 11 measured overranging in a 128-detector row CT scanner (Brilliance iCT; Philips Healthcare, Cleveland, OH, USA) and concluded that the spiral dynamic z-collimator is important for unnecessary overrange dose reduction.
Measurement methods have already been established for overranging and overbeaming. Generally, individual measurements are performed to quantify overranging and overbeaming. A film method is used to measure overbeaming, which is evaluated using the full width at half maximum (FWHM) of the dose profile. 12 A computed radiography imaging plate data has been reported to be used for the measurement of CT collimation width, including overbeaming for quality control scripts. 13 To measure overranging, a method using a film or dosimeter is performed. 5,6 Although the measurement of overranging and overbeaming for volume helical scanning is important for exposure dose management of patients, measuring overranging and overbeaming with several scan parameters is laborious.
We proposed a novel method to quantify overranging and overbeaming by using only one helical acquisition. To our knowledge, no report has evaluated overranging and overbeaming measurements by using one helical acquisition. We proposed a novel method to assess overbeaming and overranging, as well as the effect of reducing longitudinal exposure range, by using a dynamic z-collimator. converted into an effective exposure (E) using the following equation in a manner similar to the previous work for the digital radiography system 14 : where G is the gray level, and the G of this CR system was 1024.

2.B | Overbeaming measurement
For overbeaming measurements, the profile in the perpendicular direction to a single stripe was plotted (bottom left graph, Fig. 2).
We obtained three perpendicular profiles from arbitrary stripes in each stripe image. The tilt angle of the stripe was determined using ImageJ software. The actual BW at the rotation center was inversesquare corrected using the measured FWHM of the profile curve and was calculated using the following equation: where FWHM is the measured FWHM at the stripe image, and FSD and FID are the respective focus surface and focus isocenter distances corresponding to the geometric arrangement of the CT scanner. 15 The actual BW was the estimated actual BW at the rotation center. The dose efficiency (DE) 2 was calculated by dividing the nominal BW by the actual BW.

2.C | Overranging measurement
To measure overranging, a rectangular region of interest (ROI) containing the entire stripe image was placed (dashed line, Fig. 2). The mean values along the slice direction were acquired, and profiles along the z-direction were plotted (bottom right graph, Fig. 2). The overranging was calculated by the following equation: where FWTM is the measured full width at tenth maximum at the profile, and d is the scan range. The FWTM of the profile was defined as the actual exposure length along the z-direction.
Furthermore, overranging was compared with and without the use of an active collimation at the same scan parameters. The relative reduction rate (%) of overranging with and without an active collimation was measured, and the dose length products (mGy cm) displayed at the CT console were recorded. With and without active collimation measurements, a high-pitch setting (1.49 and 0.99) was used for 32 and 80 mm BWs.

2.D | Accuracy of the stripe image measurement
To verify the accuracy of the calculated overbeaming and overranging, we compared the values measured by our method and a conventional method. In the conventional method, 12        In our study the spiral activation of the x-ray fluence must be irradiated as an independent stripe to measure overbeaming. At a pitch of ≤ 1.0, the stripes overlapped each other at the rotation center. To solve this problem, we used a large diameter measuring instrument, thus making it possible to display each stripe separately even at a low pitch (Fig. 5) Our study involved several potential limitations. First, measurements were performed using the CR plates of a computed radiography system. Given that the CR plate can erase the exposure information, it is effective for repeated measurements. However, many radiology departments are getting rid of their CR systems in favor of digital radiography systems. This problem might be solved using radiochromic film 21 instead of CR plates. Second, our study showed results of limited scan parameter combinations. Other beam collimations, pitches, and rotational times were not evaluated. Third, although the degree of overranging reduction by the active collimation might vary depending on the CT scanner generations, our study did not disclose it. Forth, the accuracy of the repeated measurement of overranging for our method did not disclose it, would be future research subjects.

| DISCUSSION
In conclusion, our novel measurement method allowed us to simultaneously measure overbeaming and overranging with only one helical acquisition. Overbeaming was found to relatively decrease as the BW increased. Although the active collimator reduced overranging by up to 17%, wider beam collimation and higher pitch settings increased the exposure dose outside of the scan range.

CONF LICT OF I NTEREST
No conflict of interest.