Elsevier

Physica Medica

Volume 27, Issue 2, April 2011, Pages 69-74
Physica Medica

Technical Note
The impact of overscan on patient dose with first generation multislice CT scanners

https://doi.org/10.1016/j.ejmp.2010.03.001Get rights and content

Abstract

Helical scanning requires the irradiation of larger lengths than those planned. This is referred to as overscan and results to an increase of patient dose. Its impact on patient dose was investigated for three first generation multislice CT scanners; a six-, a quad- and a dual-slice. The amount of overscan was determined using the scanners’ dose-length product (DLP) indications and films positioned on the CT table. With the preset protocol for the chest examination selected in all CT scanners, the overscan length calculated from the DLP indications was 6.3, 3.5 and 2 cm respectively, whereas the corresponding figures derived from the films were 6.6, 4.8 and 2.5 to 3.2 cm. For a 30 cm scan length, the respective contributions of overscan to the DLP values were 17, 10 and 6%, whereas for a scan length of 20 cm the respective values increased to 24, 15 and 9%. For the smallest scan lengths allowed in helical mode, the respective contributions reached 53, 88 and 67% because for the six-slice scanner the smallest scan length was limited to twice the collimation, whereas in the quad and dual scanners no limitation existed. For small scan lengths the presence of overscan cancels out any dose reduction offered by helical scanning with pitch factor values larger than one and therefore the axial mode should be preferred, when this is not prohibited by the diagnostic task in question.

Introduction

Multislice CT scanners (MSCT) are the evolution of helical CT scanners (SSCT), which in turn had succeeded the axial CT scanners. The basic differences of MSCT scanners from their predecessors, is that the MSCT scanners employ a matrix of detector elements (this is why they are alternatively called multidetector CT scanners) and a wider X-ray beam to accommodate the detector array’s larger width [1], [2], [3]. These features allow the acquisition of multiple helixes of data during a single rotation and consequently the fast volume scanning of large regions of human anatomy in a matter of just a few seconds.

The advent of MSCT scanners created serious concerns about the potential of increasing the radiation doses to populations and individual patients. Indeed, with MSCT scanners new diagnostic and interventional CT procedures were introduced into the routine clinical practice and initial surveys exhibited an increase in patient dose [1], [3]. This increase was rather expected considering the design differences and the fact that with a MSCT the scanning of a large volume of the body can be easily carried out using a smaller slice width than that used with a SSCT [1], [3].

In MSCT scanners the collimation of the X-ray beam is increased in order to ensure that the penumbra lies beyond the active detectors and thus all detectors are irradiated uniformly, something that was not required for SSCT scanners. This results to a reduction of the z-axis geometric efficiency (the percentage of the X-ray beam width in the z-direction that is ‘seen’ by the detectors), which becomes more prominent for smaller slice widths [2]. In both SSCT and MSCT scanners smaller slice widths do require an increase in exposure factors to counterbalance the increased image noise. With SSCT smaller slice widths had to be used with caution, since the required increase in exposure factors could not always be accommodated because of the limitations in the anode and tube housing assembly thermal capacity. With MSCT, where multiple images can be acquired with the same tube load used in SSCT to acquire a single image, it is evident that thermal capacity limitations are relaxed and therefore overuse of smaller slice widths is more probable [1], [2].

Fortunately, all modern MSCT scanners present dose saving features, like the automatic exposure control system, that has been proven to reduce substantially the dose without detriment to the diagnostic quality of the CT images [1], [2], [4]. Furthermore, they present dose awareness features, since they offer a direct display of dose information, as the volume weighted CT dose index (CTDIvol) and the dose-length product (DLP) [4]. This feature could potentially contribute to the reduction of patient doses, since the examination protocols can be appropriately adjusted in order to offer the required image quality with the least possible patient dose.

It is well known that patient dose increases with increasing tube potential, tube loading and scanning length. Patient dose also increases with decreasing pitch factor when the exposure factors are kept constant [2], [3]. Thus, the CTDIvol of helical scanning is larger than that of axial scanning when the pitch factor is smaller than 1 and vice versa. It must be noted however, that in some CT scanners the CTDIvol appears to remain constant with varying pitch and equal to that of axial scanning, as the tube current is automatically adapted (proportionally increases with increasing pitch and vice versa) in order to maintain a constant noise level [1], [2].

With the exception of these CT scanners, the helical scanning of a given patient anatomy with a pitch larger than one should result to a smaller DLP value than this resulting from axial scanning of the same anatomy with the same exposure factors. However, in practice this is not true, since due to the interpolation methods used in helical mode, additional partial or full rotations are required at either end of the planned scan length and thus a larger length than that planned is actually scanned, something that is referred to as overscan or over-ranging [1], [2], [3], [5], [6], [7]. The additional dose due to overscan may not be important when large lengths are scanned, but is very important for small lengths [5]. In MSCT the beam width is greater than that in SSCT and therefore it is expected that the contribution of overscan to the patient dose will be more pronounced.

In this study the impact of overscan on patient dose was investigated for three types of first generation MSCT scanners: a six-, a quad- and a dual-slice. The main purpose was to present a simple method for calculating the DLP due to overscan and its relative contribution to the total DLP for different scan lengths and pitch factors selections. Within this context the impact of the pitch factor on patient dose was also reconsidered.

Section snippets

Materials and methods

This study was carried out in three CT facilities, equipped with three different MSCT scanners, namely a Brilliance 6 (Philips Medical Systems, Cleveland, Ohio), an Asteion 4 (Toshiba Medical Systems, Otawara, Japan) and a CT/e Dual Plus (GE Healthcare, Milwaukee WI, USA), henceforth referred to as scanners A, B and C, respectively.

For all scanners the protocol used for chest examinations was selected as the starting point since it is the most often used examination protocol in all three CT

Results

In Fig. 1 the DLP values recorded in the three CT scanners using the chest examination protocol for various scan lengths are shown (data points) along with the best fit lines and the respective equations, in which y stands for the cumulative DLP and x for the planned scan length. The best fit lines of Fig. 1 are straight lines with slope equal to the CTDIvol and intercept that represents the DLP value for a planned scan length equal to zero (DLP(0)). From these equations it is obvious, that in

Discussion

According to the literature, when helical mode is used for scanning, a number of additional rotations are performed, resulting in an increase of patient dose. This was confirmed for the CT scanner models that were included in this study and furthermore it was shown that this additional dose is included in the DLP values displayed on the graphical user interface. Therefore when the accuracy of the DLP indications is checked or when the effective dose or organ doses are calculated using software

Conclusions

In the older MSCT scanners, helical scanning results to an inevitable increase in patient dose due to overscan. The relative contribution of overscan to the patient dose depends on the scanner model, but in general it increases with decreasing planned scan length and increasing pitch factor. Therefore, from the aspect of patient radiation protection, for the CT scanning of small lengths the axial mode should be preferred over the helical mode.

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