Ing-Ming Hwang

A novel image quality index using Moran I statistics

Measurement of image quality is very important for various applications such as image compression, restoration and enhancement. Conventional methods (e.g., mean squared error; MSE) use error summation to measure quality change pixel by pixel and do not correlate well with subjective quality measurement. This is due to the fact that human eyes extract structural information from the viewing field. In this study a new quality index using a Moran I statistics is proposed. The Moran statistic that measures the sharpness from a local area is a good index of quality as most image processing techniques alter the smoothness of the image. Preliminary results show that the new quality index outperforms the MSE significantly under various types of image distortions.

Calibration of EBT2 film by the PDD method with scanner non-uniformity correction.

The EBT2 film together with a flatbed scanner is a convenient dosimetry QA tool for verification of clinical radiotherapy treatments. However, it suffers from a relatively high degree of uncertainty and a tedious film calibration process for every new lot of films, including cutting the films into several small pieces, exposing with different doses, restoring them back and selecting the proper region of interest (ROI) for each piece for curve fitting. In this work, we present a percentage depth dose (PDD) method that can accurately calibrate the EBT2 film together with the scanner non-uniformity correction and provide an easy way to perform film dosimetry. All films were scanned before and after the irradiation in one of the two homemade 2 mm thick acrylic frames (one portrait and the other landscape), which was located at a fixed position on the scan bed of an Epson 10 000XL scanner. After the pre-irradiated scan, the film was placed parallel to the beam central axis and sandwiched between six polystyrene plates (5 cm thick each), followed by irradiation of a 20 × 20 cm² 6 MV photon beam. Two different beams on times were used on two different films to deliver a dose to the film ranging from 32 to 320 cGy. After the post-irradiated scan, the net optical densities for a total of 235 points on the beam central axis on the films were auto-extracted and compared with the corresponding depth doses that were calculated through the measurement of a 0.6 cc farmer chamber and the related PDD table to perform the curve fitting. The portrait film location was selected for routine calibration, since the central beam axis on the film is parallel to the scanning direction, where non-uniformity correction is not needed (Ferreira et al 2009 Phys. Med. Biol. 54 1073-85). To perform the scanner non-uniformity calibration, the cross-beam profiles of the film were analysed by referencing the measured profiles from a Profiler™. Finally, to verify our method, the films were exposed to 60° physical wedge fields and the compositive fields, and their relative dose profiles were compared with those from the water phantom measurement. The fitting uncertainty was less than 0.5% due to the many calibration points, and the overall calibration uncertainty was within 3% for doses above 50 cGy, when the average of four films were used for the calibration. According to our study, the non-uniformity calibration factor was found to be independent of the given dose for the EBT2 film and the relative dose differences between the profiles measured by the film and the Profiler were within 1.5% after applying the non-uniformity correction. For the verification tests, the relative dose differences between the measurements by films and in the water phantom, when the average of three films were used, were generally within 3% for the 60° wedge fields and compositive fields, respectively. In conclusion, our method is convenient, time-saving and cost-effective, since no film cutting is needed and only two films with two exposures are needed.

Quality Degradation in Lossy Wavelet Image Compression

The objective of this study was to develop a method for measuring quality degradation in lossy wavelet image compression. Quality degradation is due to denoising and edge blurring effects that cause smoothness in the compressed image. The peak Moran z histogram ratio between the reconstructed and original images is used as an index for degradation after image compression. The Moran test is applied to images randomly selected from each medical modality, computerized tomography, magnetic resonance imaging, and computed radiography and compressed using the wavelet compression at various levels. The relationship between the quality degradation and compression ratio for each image modality agrees with previous reports that showed a preference for mildly compressed images. Preliminary results show that the peak Moran z histogram ratio can be used to quantify the quality degradation in lossy image compression. The potential for this method is applications for determining the optimal compression ratio (the maximized compression without seriously degrading image quality) of an image for teleradiology.

Thresholding Histogram Equalization

The drawbacks of adaptive histogram equalization techniques are the loss of definition on the edges of the object and overenhancement of noise in the images. These drawbacks can be avoided if the noise is excluded in the equalization transformation function computation. A method has been developed to separate the histogram into zones, each with its own equalization transformation. This method can be used to suppress the nonanatomic noise and enhance only certain parts of the object. This method can be combined with other adaptive histogram equalization techniques. Preliminary results indicate that this method can produce images with superior contrast.

Scatter correction for 3D PET using beam stoppers combined with dual-energy window acquisition: a feasibility study.

Fully three-dimensional (3D) positron emission tomography (PET) can achieve high sensitivity of coincidence events, but the absence of inter-slice septa inevitably leads to increased scattered events. The scattered events can represent as much as 50% of the total detected events. In this research, we proposed a scatter correction method for 3D PET based on beam stoppers and dual-energy window acquisition. The beam stoppers were placed surrounding the object to attenuate primary beams. The scatter fractions were directly estimated at those blocked lines of response and then the entire scatter fraction distribution was recovered using the dual-energy window ratio as reference. The performance was evaluated by using Monte Carlo simulations of various digital phantoms. For the Utah phantom study, the proposed method accurately estimated the scatter fraction distribution, and improved image contrast and quantification based on four different quality indices as performance measures. For the non-homogeneous Zubal phantom, the simulated results also demonstrated that the proposed method achieved a better restoration of image contrast than the dual-energy window method. We conclude that the proposed scatter correction method could effectively suppress various kinds of scattered events, including multiple scatter and scatter from outside the field of view.

Determination of beam intensity in a single step for IMRT inverse planning.

In intensity modulated radiotherapy (IMRT), targets are treated by multiple beams at different orientations each with spatially-modulated beam intensities. This approach spreads the normal tissue dose to a greater volume and produces a higher dose conformation to the target. In general, inverse planning is used for IMRT treatment planning. The inverse planning requires iterative calculation of dose distribution in order to optimize the intensity profile for each beam and is very computation intensive. In this paper, we propose a single-step method utilizing a figure of merit (FoM) to estimate the beam intensities for IMRT treatment planning. The FoM of a ray is defined as the ratio between the delivered tumour dose and normal tissue dose and is a good index for the dose efficacy of the ray. To maximize the beam utility, it is natural to irradiate the tumour with intensity of each ray proportional to the value of the FoM. The nonuniform beam intensity profiles are then fixed and the weights of the beam are determined iteratively in order to yield a uniform tumour dose. In this study, beams are employed at equispaced angles around the patient. Each beam with its field size that just covers the tumour is divided into a fixed number of beamlets. The FoM is calculated for each beamlet and this value is assigned to be the beam intensity. Various weighting factors are incorporated in the FoM computation to accommodate different clinical considerations. Two clinical datasets are used to test the feasibility of the algorithm. The resultant dose-volume histograms of this method are presented and compared to that of conformal therapy. Preliminary results indicate that this method reduces the critical organ doses at a small expense of uniformity in tumour dose distribution. This method estimates the beam intensity in one single step and the computation time is extremely fast and can be finished in less than one minute using a regular PC.

An effective method for smoothing the staggered dose distribution of multi-leaf collimator field edge

Purpose: To smooth the staggered dose distribution that occurs in stepped leaves defined by a multi-leaf collimator (MLC). Materials and methods: The MLC Shaper program controlled the stepped leaves, which were shifted in a traveling range, the pattern of shift was from the position of out-bound to in-bound with a one-segment (cross-bound), threesegment, and five-segment shifts. Film was placed at a depth of 1.5 cm and irradiated with the same irradiation dose used for the cerrobend block experiment. Four field edges with the MLC defining at 151 ,3 01 ,4 51 ,6 01 angels relative to the jaw edge were performed, respectively, in this study. For the field edge defined by the multi-segment technique, the amplitude of the isodose lines for 50% isodose line and both the 80% and 20% isodose lines were measured. The effective penumbra widths with 90–10% and 80–20% distances for different irradiations were determined at four field edges with the MLC defining at 151 ,3 01 ,4 51 ,6 01 angels relative to the jaw edge. Results: Use of the five-segment technique for multi-leaf collimation at the 601 angle field edge smoothes each isodose line into an effectively straight line, similar to the pattern achieved using a cerrobend block. The separation of these lines is also important. The 80–20% effective penumbra width with five-segment techniques (8.23 mm) at 601 angle relative to the jaw edge is little wider (1.9 times) than the penumbra of cerrobend block field edge (4.23 mm). We also found that the 90–10% effective penumbra width with five-segment techniques (12.68mm) at 601 angle relative to the jaw edge is little wider (1.28times) than the penumbra of cerrobend block field edge (9.8 9 mm). Conclusion: The multi-segment technique is effective in smoothing the MLC staggered field edge. The effective penumbra width with more segment techniques at larger degree angles relative to the field edge is little wider than the penumbra for a cerrobend block. r 2002 Elsevier Science B.V. All rights reserved.

A Study on the Front-end Region Leakage of Opposed Abutted Multi-leaf Collimator Leaves

The front-end region dose leakage of opposed abutted multi-leaf collimator (MLC) leaf pairs from field center to off-axis distance (OAD) = 12 cm were measured and evaluated to offer the decision of using opposed abutted MLC leaves as a replacement for central block and notation for dynamic conformal radiation therapy (DCRT) and intensity-modulated radiation therapy (IMRT) performance. We found that when the abutted region (OAD) increased, the dose leakage decrease exponentially from +24% to less than +5% when OAD＞12 cm for Varian CL 600C Linac. We recommended that when the MLC leaves are used to replace the cerrobend block as conventional central block, the abutted region of opposed leaves should be: (1) moved out of the field and using backup field jaw to block the leakage, (2) the abutted region should be at OAD＞12 cm, (3) abutted at different OAD for each treatment day-by-day at least 1 cm apart, or (4) using a small customized cerrobend block to block the leakage. The peak leakage at OAD≦12 cm for DCRT and IMRT performance should be take more attention.

Comprehensive measurement for clinical dosimetry of tertiary multileaf collimator.

To initiate the use of a tertiary multileaf collimator (MLC) in the clinic, a set of dosimetry data for clinical use of the MLC, the secondary field size jaw and the MLC tracked by the jaw were measured. The dose calculation technique from the commissioned jaw field data was established. The dosimetry characteristics included absolute output (Dw), collimator scatter factor (Sc), total scatter factor (Scp), phantom scatter factor (Sp), percentage depth dose (PDD), tissue-maximum ratio (TMR), and peak scatter factor (PSF). The absolute output of the MLC field was +5% to +2% greater than that of the same jaw size field from 4 x 4 to 24 x 24 cm2 fields. The variation of Sc and Scp ranged from 4 x 4 to 24 x 24 cm2 fields and were less than that of the jaw fields, while the Sp, PDD and TMR values remained the same. Importantly, when the MLC-only field was performed without the collimator jaws tracking close to the field segments, greater output was delivered, and PSFs should be used to calculate the MLC field output.

Technique of Total Body Irradiation Used in NCKUH

Purpose: We developed a system for total body irradiation (TBI) in an attempt to shorten patient setup and increase the reproducibility. Materials and Methods: A special homemade stand with a quasi-saddle shaped chair and an additional immobilization system for the head and body help to position patient in a reproducible manner. Customized lung shields are used to protect the lung from excess tolerance dose. The blocks are screwed in a customized Aquaplast(superscript TM) Thermoplasties cast which the patient wears like a vest. The reproducibility of the patients positioning and lung shielding was evaluated by comparing the verification films with the simulation films. In vivo dosimetry was performed by placing diodes in the central axis and in several off-axis sites. Full-course verification of the lung shields was also performed. Results: The mean horizontal and vertical deviations were 4.8±1.1mm and 4.3±1.3mm for the anterior fields, and 3.1±1.1mm and 3.4±0.9mm for the posterior fields. The average lung dose was 936±21 cGy. The mean duration per fraction was about 30±5min for linear accelerator and 55±5min for (superscript 60)Co. Conclusions: Our technique can reach a satisfactory level of reproducibility. It can also increase patient comfort during the treatment.