Ludwig Ritschl, Presenter: Nothing to Disclose

Jan Kuntz, Abstract Co-Author: Nothing to Disclose

Michael Knaup PhD, Abstract Co-Author: Nothing to Disclose

Marc Kachelriess PhD, Abstract Co-Author: Nothing to Disclose

To increase the dynamic range of flat detectors in CT without increasing dose or scan time.

The dynamic range R of x-ray detectors is the ratio between the highest detectable signal (just before overexposure) and the lowest detectable signal (where x-ray quantum noise = electronic noise). Achieving low contrast resolution (e.g. 5 HU contrast of 5 mm objects) in human beings requires R ≥ 106 which includes two factors: the accuracy of attenuation measurements in each ray and the capturing of significant attenuation differences between different rays due to differences of ray position (peripheral vs. central rays). Flat detectors, however, operate at R ≈ 103 and avoiding underexposure for central rays typically means accepting overexposure for peripheral rays and thus truncation artifacts. Dual or multi-exposure techniques could be a remedy if dose and scan time did not increase. We propose a new multi-exposure technique that performs dense sampling with high exposure levels interrupted from time to time by a sparse low exposure sampling (e.g. every 16th projection). We generalized the compressed sensing-based iTV algorithm [Phys. Med. Biol. 56:1545] to optimally combine the highly sampled high exposure data with the interleaved sparsely sampled low exposure data. The generalized iTV method was verified using simulated as well as measured data, acquired with a Varian flat detector, and was compared to a situation where two exposures were made in a conventional way and with the standard situation of having only one exposure while accepting overexposure in the peripheral patient areas (e.g. in the skin).

The images with extended dynamic range and generalized iTV reconstruction are nearly undistinguishable from those with double exposure. Minor differences are visible only in the peripheral areas where only very sparse information was available for iTV. Dose and scan time remain the same as with today’s single exposure scans.

Sparsely sampling the low exposure CT scan and interleaving many high exposure projections combined with compressed sensing reconstruction is sufficient to provide images nearly equivalent to a CT scan with a high dynamic range detector.

Flat detector CT, in particular images in interventional CT and in image-guided radiation therapy, can significantly benefit from the dynamic range extension and the improved low contrast resolution.

Ritschl, L, Kuntz, J, Knaup, M, Kachelriess, M, Dynamic Range Extension in Flat Detector CT Using a Compressed Sensing-based Multi-exposure Technique.  Radiological Society of North America 2013 Scientific Assembly and Annual Meeting, December 1 - December 6, 2013 ,Chicago IL. http://archive.rsna.org/2013/13020611.html