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

Two image receptors that were used for this study contained two components; a metal build up
plate/phosphor screen and a flat panel imager. The flat panel imager had an active area of 41 x 41 cm 2 , and
contained 1024 x1024 elements of 0.4 mm pitch. The fastest frame rate was 7 frames per second and the
output data was 16-bit. Flat panel detectors generate frames either using “Free Running” or “External
Triggered”. Free running means that the detector sends out continuous frames according to the selected
frame time. External triggered means that there is an external trigger which allows the generation of the
maskable interrupt request.
The phosphor screen of one receptor (receptor 1) was a 133 mg/cm 2 Gd 2O 2S screen (Lanex Fast) and for
the receptor 2 was an 800 µm CsI (TI) scintillator screen. Both receptors were using a 1mm copper buildup
plate. The charge amplifier capacitance of receptors 1 and 2 were 5pF and 1pF, respectively.
2.3 Cone Beam Image Acquisitions and Corrections
Cone beam CT starts by acquiring a large number of projection images acquired as the X-ray source
follows an orbital path. At every angular increment, i.e., every degree, an image is acquired. 201
projections were acquired over 201 degrees (1° rotation between projections) around the subject. The
gantry speed was set to 109° per minute.
The Siemens PRIMUS® linear accelerator parameters were adjusted to allow the delivery of a small
fraction of a Monitor Unit (MU) per image. A very precise dose control was achieved by loading the
accelerator by very small amount of electrons per pulse. In the current design, the injection is phased with
the RF power at startup or during re-phasing of the injector at the programmed angular increment. With
normal beam parameters, the phasing or re-phasing of the injection generates a shift in the resonant
frequency of the accelerator. Because of the lag of the automatic frequency control (AFC), the beam
formation can be significant (hundreds of milliseconds). In order to improve the beam formation, the beam
parameters were adjusted such that the dose per pulse was reduced to approximately 10% of normal, and
the pulse repetition frequency (PRF) was adjusted to 250 pps. The reduced loading resulted in an
insignificant shift in the resonant frequency of the accelerator, resulting in significant improvement of the
beam formation (less than a few ms). The increased PRF improved the granularity of the control system,
the significance of every dose pulse was reduced, the predictability of the system increased, and the beam
formation time decreased to only several milliseconds.
Three corrections were performed; Offset, Gain, and Defected pixel correction. Offset correction is used to
correct the dark current of each pixel for a specified frame or integration time. The integration time for each
projection is equal to the projection period (figure-1), and therefore, the Offset correction images were
acquired with the frame time that is equal to the projection period.
The Gain correction is used to homogenize the flat field image intensity. The Defected pixel correction
allows a software repair of defected pixels to enhance image quality. Improper pixel values are replaced
with the averaged value of the adjacent pixels, while defected pixels are not averaged.
2.4 Preprocessing by Resolution/Dose Adaptive Filter
A unique adaptive noise removal filter was used for image enhancement 2 . The filter utilizes pixel-wise
nonlinear and adaptive techniques to estimate the additive noise power before filtering. The characteristics
of the filter are adapted to the input X-ray flux and image spatial resolution. Filtering is minimal to the
images that receives high X-ray flux, and increases with the decreasing flux. To minimize the loss of
resolution, minimum filtering is applied to the area of an image with large variance to preserve the edges
and other high frequency components of an image 2 .
Before data normalization and reconstruction, the corrected projection images (Offset, Gain, and Defected
pixel) were processed by the resolution/dose adaptive filter using a local matrix size of 8x8.
2.5 Data Normalization and Reconstruction