An adaptive technique for digital noise suppression in on-line portal imaging

Improved visualisation of choroidal neovascularisation by scanning laser ophthalmoscope using image averaging

AIMS

To improve visualisation of angiographic features in patients with age related macular degeneration associated with choroidal neovascularisation (CNV) and related complications. To evaluate if image averaging can achieve this goal.

METHODS

27 eyes of 20 sequential patients with age related macular degeneration over a 3 month period were studied. Indocyanine green angiograms (ICGA), fluorescein angiograms (FA), and oral fluorescein angiograms were recorded with a confocal scanning laser ophthalmoscope. Software was used to average multiple images from a 10-20 image series (over 0.5-1.0 seconds). Image quality was assessed by two masked observers and graded on a scale of 0-3. A more quantifiable grading method was devised by adding a variable amount of Gaussian noise to the improved image until the original and image averaged image appeared equal.

RESULTS

Masked review showed mild to strong improvement of visualisation of structures including borders of CNV. Improvement varied depending on the type and phase of the angiogram. Improvement was highest in late phase FA, mid and late phase ICGA, and all phases of oral FA.

CONCLUSION

Image averaging using software based algorithms improves the quality of angiographic images, particularly late ICGA images and oral FAs. This method may assist in the visualisation of choroidal neovascularisation in age related macular degeneration.

Preprocessing of control portal images for patient setup verification during the treatments in external radiotherapy

The main problem in processing the control portal images is their poor quality. We have developed a way of improving the image quality to allow a segmentation stage. Three items were studied for this purpose the background gradient of intensity correction, the noise reduction, and the image restoration. The background was corrected by subtracting a smoothed version of the image from the original. We tested 15 noise reduction methods. The most appropriate for control portal images was found to be the truncated average. Finally, four restoration techniques were compared. The maximum a posteriori (MAP) algorithm was the most efficient. The algorithms were tested over a wide range of conditions (image quality). They produced a great improvement in anatomic detail for all the imaging systems, energies, and anatomical zones tested. For example, the signal-to-noise ratio of a SRI-100 pelvis image, acquired with 4 monitor units (MU) at 10 MV (very low quality image), increased from 0.97 to 42.84 after preprocessing. We found that the improvement in image quality facilitated or even enabled segmentation of the control portal images. The percentage of segments belonging to a structure increased from 30% to 65% in the example cited. The preprocessing of control portal images is the first step in checking the patient setup.

The consequences of fixed-pattern noise and image movement on electronic portal images

Fixed-pattern noise in electronic portal images (EPIs) is normally eliminated by dividing raw image data by an open-field calibration image (OFCI). However, for successful elimination, there must be exact registration between the two image sets. Any movement within the imaging system, such as that which occurs with gantry angle, will result in misregistration and a subsequent increase in noise. This paper describes, both qualitatively, by way of example, and quantitatively, by way of variance analysis, the consequences of misregistration and its effects on image quality. Our results show that image quality is found to degrade significantly with change in gantry angle, when a single OFCI is used, with a loss of low-contrast, fine detail. Variance is observed to increase over 2.5-fold. A simple solution of using multiple OFCIs is described, along with a technique for optimizing the number of images required, and the gantry angles at which they are acquired. When five OFCIs are used, the variance changes with gantry angle are limited to less than 20%. These changes are observed in both long and very short exposures (5 MU or 1 s).

The use of an electronic portal imaging device for exit dosimetry and quality control measurements

PURPOSE

To determine ways in which electronic portal imaging devices (EPIDs) could be used to (a) measure exit doses for external beam radiotherapy and (b) perform quality control checks on linear accelerators.

METHODS AND MATERIALS

When imaging, our fluoroscopic EPID adjusts the gain, offset, and frame acquisition time of the charge coupled device (CCD) camera automatically, to allow for the range of photon transmissions through the patient, and to optimize the signal-to-noise ratio. However, our EPID can be programmed to act as an integrating dosemeter. EPID dosemeter measurements were made for 20 MV photons, for different field sizes and thicknesses of unit density phantom material placed at varying exit surface to detector distances. These were compared with simultaneous Silicon diode exit dose measurements. Our exit dosimetry technique was verified using an anthropomorphic type phantom, and some initial measurements have been made for patients treated with irregularly shaped 20 MV x-ray fields. In this dosimetry mode, our EPID was also used to measure certain quality control parameters, x-ray field flatness, and the verification of segmented intensity modulated field prescriptions.

RESULTS

Configured for dosimetry, our EPID exhibited a highly linear response, capable of resolving individual monitor units. Exit doses could be measured to within about 3% of that measured using Silicon diodes. Field flatness was determined to within 1.5% of Farmer dosemeter measurements. Segmented intensity modulated fields can be easily verified.

CONCLUSIONS

Our EPID has the versatility to assess a range of parameters pertinent to the delivery of high quality, high precision radiotherapy. When configured appropriately, it can measure exit doses in vivo, with reasonable accuracy, perform certain quick quality control checks, and analyze segmented intensity modulated treatment fields.

A comprehensive system for the analysis of portal images

In recent years, several techniques for the processing and analysis of portal images have been developed. It is the aim of this study to integrate some of these techniques into one comprehensive system. An advantage of this approach is that clinical experience can be obtained with more than one technique and a comparison of the techniques becomes possible. The portal image analysis procedure is implemented in the following steps: preparation of the reference image, portal image field edge detection, field edge match, anatomy match and the presentation of the results. For most of these steps, several alternative methods (e.g., interactive and automatic) are implemented. In addition, two new visualisation techniques have been incorporated. The first is a method for combining the results of the analysis of multiple fields in two dimensions, e.g., large and boost fields. The second is a method for three-dimensional reconstruction of beam setup data, as derived from portal image analysis, on arbitrary reconstructed slices of a CT scan. With the latter method, the effect of setup errors on complex treatments (e.g., matching fields) can be studied. The new system has been in clinical use in our institution for two years and has been used to analyse about 5000 clinical portal images. The operators could choose freely from several matching methods. For 83% of the images our automatic matching algorithm was used. When required, the result of this method was corrected using the interactive drawing on image match. Significant corrections (more than 1 mm translation or 1 degree rotation) were applied to 27% of the automatically analysed images.(ABSTRACT TRUNCATED AT 250 WORDS)

Measurement possibilities using an electronic portal imaging device

A vital role in the quality control of radiotherapy is the use of portal imaging for verifying field size, shape, orientation and patient set-up. Coincidence of treated volume and target volume is imperative. Electronic portal imaging devices are effective at providing this verification. However, these devices are versatile enough to be used in other ways pertinent to the delivery of high quality, high precision radiotherapy. This paper examines two such ways: in assessing the reproducibility of a multileaf collimator system, and in determining exit doses in vivo. Configured as a dosimeter, the system shows a linear response with good dynamic range. Its high spatial resolution was used to show that leaf positioning was reproducible to within 0.5 mm for all tested gantry and collimator angles. Our preliminary results from this exit dosimetry technique demonstrate that, under specific conditions, doses can be determined to within 2.5% of that measured using silicon diodes or ion chambers.

Interactive use of on-line portal imaging in pelvic radiation

We have evaluated a fluoroscopic on-line portal imaging system in routine clinical radiotherapy, involving the treatment of 566 pelvic fields on 13 patients. The image was typically generated by delivering a radiation dose of 6-8 cGy. Comparison between portal image and simulator film was done by eye and all visible errors were corrected before continuing irradiation. If possible, these corrections were performed from outside the treatment room by moving the patient couch by remote control or by changing collimator parameters. Adjustments were performed on 289/530 (54.5%) evaluable fields or 229/278 (82.4%) evaluable patient set-ups. The lateral couch position was most frequently adjusted (n = 254). The absolute values of the adjustments were 6.8 mm mean (SD 6.6 mm) with a maximum of 40 mm. All absolute values of adjustments exceeding 25 mm were recorded in one patient and those exceeding 15 mm were observed in two patients. Both patients were obese females. Adjustments exceeding 5 mm were observed in all 13 patients. Related to the use of on-line portal imaging, treatment time was increased by a median of 36.5% (mean 45.8%; SD 42.1%). The range was 7.7 to 442%. The fraction of the total treatment time to perform corrections was 22.7% median (mean: 26.0; SD: 11.8%). Statistically significant systematic in-plane errors were found in 7/13 patients. A systematic error was detected on the lateral position of the field in five patients. In one patient a systematic error of the longitudinal field position and in one patient a rotational error was detected. For adjustments in the lateral direction the present method does not allow to detect lateral shifts of less than 2 mm. For adjustments in the longitudinal direction the sensitivity could not be estimated but the available data suggest that 80% of errors < or = 5 mm were not adjusted. In obese patients, random errors may be surprisingly large.

Routine clinical on-line portal imaging followed by immediate field adjustment using a tele-controlled patient couch

We have evaluated the fluoroscopic on-line portal imaging (OPI) system developed by Siemens (Beamview-1, Concord, CA, U.S.A.) in routine clinical radiotherapy, involving the treatment of 883 fields (559 patient set-ups for treatment) on 21 patients. The image was typically generated by delivering 10 monitor units when used in single exposure or 1-2 monitor units on a large open field followed by 8-10 monitor units on the actual field when double exposure was used. Comparison between the portal image and the simulator film was done by eye. A region of tolerance was drawn on the simulator film and the field edges on the portal image had to project within this region. If this criterion was not met, adjustments followed by verification portal images were done before the remaining field dose was delivered. If possible, these adjustments were performed by moving the patient couch by remote control. The image quality was insufficient for evaluation in 75/883 (8.5%) fields. The abovementioned criterion was not met in 95/808 (11.8%) of the evaluable fields (26/559 patient set-ups were not evaluable). Of the 533 evaluable patient set-ups, 92 had to be adjusted (17.2%) including three (pelvic irradiations) set-ups that were adjusted on both field irradiated during the same radiotherapy session. In one case an incorrect tray (with wrong blocks) was detected and replaced. In one case (a 5.5 x 6.0 cm rectangular larynx field) the x and y axis of the field were interswitched. In one case incorrect focusing of a block was shown by the portal image. To make adjustments, the couch longitudinal position was changed 20 times (range -10 to +15 mm). The lateral position was changed 73 times (range -15 to +16 mm). The height position was changes 6 times (range -7 to +6 mm). Diaphragma rotation changes were performed 5 times (1 degree). The fraction of treatment time that was related to the use of OPI was 30.7% median (mean 32.4%, S.D. 14.1%). The range was 4.1 to 78.6%. On the basis of calculations assuming no OPI would have been used, field treatment time was increased by a median of 44.2% (mean 55.8%; S.D. 41.2%) by using OPI. The fraction of monitor units (fraction of the dose) to generate a satisfactory image was 10% median.(ABSTRACT TRUNCATED AT 400 WORDS)