Advances in image enhancement for sarcoma surgery.

The recurrence rate of soft tissue and bone sarcomas strongly correlates to the status of the surgical margin after excision, yet excessive removal of tissue may lead to distinct, otherwise avoidable morbidity. Therefore, adequate margination of sarcomas both pre- and intra-operatively is a clinical necessity that has not yet fully been met. Current guidance for soft-tissue sarcomas recommends an ultrasound scan followed by magnetic resonance imaging (MRI). For bone sarcomas, two plane radiographs are required, followed similarly by an MRI scan. The introduction of more precise imaging modalities may reduce the morbidity associated with sarcoma surgery; the PET-CT and PET-MRI approaches in particular demonstrating high clinical efficacy. Despite advancements in the accuracy in pre-operative imaging, translation of an image to surgical margins is difficult, regularly resulting in wider resection margins than required. For soft tissue sarcomas there is currently no standard technique for image guided resections, while for bone sarcomas fluoroscopy may be used, however margins are not easily discernible during the surgical procedure. Near infra-red (NIR) fluorescence guided surgery offers an intra-operative modality through which complete tumour resection with adequate tumour-free margins may be achieved, while simultaneously minimising surgical morbidity. NIR imaging presents a potentially valuable adjunct to sarcoma surgery. Early reports indicate that it may be able to provide the surgeon with helpful information on anatomy, perfusion, lymphatic drainage, tumour margins and metastases. The use of NIR fluorochromes have also been demonstrated to be well tolerated by patients. However, prior to widespread implementation, studies related to cost-effectiveness and the development of protocols are essential. Nevertheless, NIR imaging may become ubiquitous in the future, carrying the potential to transform the surgical management of sarcoma.

Effect of defocus on blur thresholds and on thresholds of perceived change in blur: comparison of source and observer methods.

The defocus levels required for normal observers to notice the first perceptible blur of a clear test target (blur threshold) and the least perceptible change in the degree of blurriness of an already blurry target (threshold of perceived change in blur) were measured using both the source and observer methods. In the source method observers viewed defocused stimuli presented on a projection screen, whereas in the observer method focused stimuli were presented to observers who were defocused using lenses placed in the spectacle plane. Blur thresholds were found to be dependent on target size and when the Landolt ring targets were near threshold acuity size blur thresholds were as small as 0.10 D. For larger target sizes (0.6 log min arc or more above threshold acuity size) the blur thresholds remained relatively unchanged and were about 0.18 D. Thresholds of perceived change in blur were found to be independent of the initial defocus level. Measurements of the threshold of perceived change in blur were found to be 0.05 to 0.07 D, which is much smaller than the blur threshold values. Comparison of results from the two methods of producing defocus indicate that the source and observer methods can be used interchangeably. However, for the same angular blur disc diameter, the blur thresholds found with the source method were significantly lower than those found with the observer method.

Effect of defocus on visual acuity as measured by source and observer methods.

The relation between refractive error and visual acuity has been measured by two very different methods. In one called "source methods," emmetropes or corrected ametropes view defocused stimuli presented on projection screens or photographs. In the type called "observer methods," focused stimuli are presented to the observers who are either uncorrected ametropes or emmetropes defocused by lenses placed (usually), in the spectacle plane. The study reported in this paper demonstrates for the first time that these two methods of defocusing retinal images and their effects on visual acuity can be correlated. Results show that the source method of producing defocus could be used interchangeably with the observer method in investigating the rates of change of visual acuity with defocus for young normal observers. The angular diameter of the defocused image of a point, the blur disc diameter in object space, allows the two methods to be compared. Although the results show that the two methods are highly correlated, they show that the source method gives a statistically but not clinically significant lower acuity. The results of both methods are used to derive an equation linking refractive error, visual acuity, and pupil diameter.