CT Users Group

G Iball¹, D Tout² and H Williams²

¹Leeds Teaching Hospitals NHS Trust,
²Central Manchester Hospitals NHS Foundation Trust

Abstract

Background: Modern PET/CT scanners include 64 slice CT systems which are equivalent to the diagnostic CT scanners offered by the major CT manufacturers. As such, tube current modulation systems are now used for the vast majority of PET/CT scans that are performed on such systems. The quality of CT images produced by the scanner, and therefore the doses delivered vary according to whether the CT images are used for attenuation correction only, attenuation correction and localisation or for full diagnostic purposes.

In order to compare the tube current modulation techniques of the three major PET/CT manufacturers a collaborative study of patient doses and image quality was undertaken.

Method: Routine patient dose audits for standard sized patients (60-80kg) were undertaken for "eyes-thighs" scans on current PET/CT scanners from three manufacturers. The three scanners involved were: GE Discovery 690, Philips Gemini TF and Siemens Biograph mCT. The patient weight range was subsequently extended so that the full range of patient weights was covered and the patient sample size was thus increased up to 70-100 patients per system

In order to compare the effect of the tube current modulation systems on patient doses, dose information was recorded from scanner workstations, PACS and directly from DICOM headers for 70-100 patients of all sizes. The following information was recorded for each patient: weight, age, gender, average CTDI_vol, DLP, average mA (where available) and scan length. In addition to these parameters a measurement of patient size was obtained by measuring the AP and lateral dimensions of the patient on an image at the middle of the liver. In order to compare the effect of the tube current modulation system on image quality the standard deviation of the CT number in the liver was measured on the same image that was used for the patient size measurement. The mA that was used to create this image was also recorded.

An additional, direct, comparison of the AEC systems was made by scanning the same RANDO phantom on all three systems and reading out the mA profile along the patient with the AEC systems activated.

Results: For all three systems patient dose showed a stronger correlation with patient weight than with patient size. The variation in dose with patient weight was very different for the three manufacturers with the GE system showing the largest variation and the Philips system the least - the Siemens system laid between the other two. In terms of image quality the GE system delivered approximately constant image noise for all patient sizes whereas for the Philips and Siemens system the image noise was lowest for small patients and increased with increasing patient size. The combination of these results demonstrates the major differences in the way that the AEC systems function. The mA profiles that were obtained on the RANDO phantom further demonstrated these significant differences.

Conclusions: The CT AEC systems from the three major PET/CT manufacturers yield significantly different tube current modulation patterns and as such deliver different DLPs, organ and effective doses as well as delivering different levels of image quality across the range of patient weights. Users should be aware of how their system works and of steps that could be taken to optimise imaging protocols on all three systems.

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