Soft Tissue Visualization Using a Highly Efficient Megavoltage Cone ...

The ability of an x-ray imaging system to differentiate soft tissues is affected by the difference in
attenuation coefficient at a given energy between the contrasting objects. For KV CBCT imaging the
contrast changes with energy, because both the photoelectric and Compton effects contributes significantly
to attenuation. For MV energies the Compton effect is by far the dominant interaction process, especially
for material with low atomic number (Z), in which the attenuation coefficient is proportional to electron
density and is nearly independent of Z. Object contrast is therefore constant over a wide range of energies
in the MV regime. Since the attenuation coefficients of KV photons are higher than MV photons in soft
tissue, the percent density contrasts will be easier to detect at KV energies. Thus, while the higher
attenuation of KV photons results in greater transmission losses, it also benefits KV contrast by the squared
ratio of the mass attenuation coefficient for KV and MV energies 1 .
In this paper we present an implementation of a highly efficient MV CBCT imaging system that can
resolve soft tissue contrast with clinically acceptable doses.
2. Methods and Materials
2.1 Timing Interface for Cone Beam Acquisition Mode
The timing interface uses the Radiation-on (Rad-on) signal of linear accelerator (linac) to generate trigger
pulses that externally control the frame readout of flat panel imager. In this mode, a-Si sensors integrate the
signal during the radiation interval of each projection and the data readout is performed at the end of each
projection. The detector is set in the external mode. Five trigger pulses (5 frames scan) are sent to the
external trigger input of the flat panel detector prior to the start of first projection to force sensor discharge
by reading 5 frames in absence of radiation. This eliminates the dark current accumulation prior to the start
of the first projection. No readout is allowed during the projection interval. Avoiding readout during
exposure interval improves the image SNR by reducing the effect of the electronic readout noise, and also
removes the linac pulsing artifacts from the image.
Rad-on
Trigger
2.2 Image Receptor
5 Refresh Pulses
First
Projection
Projection period
Second
Projection
Figure-1: Timing Interface for Cone Beam Mode
Third
Projection
Data Projection 1 Data Projection 2 Data Projection 3