Introduction Vision is a vital role in medical imaging and the visual ability of the radiologist interpreting the image is largely overlooked (Halpenny et al. 2012). To understand how vision plays a role in radiography, psychophysics is studied to understand the relationship between the physical stimulus and sensation experienced by a person (Lu et al. 2014). The radiologist has a role of interpreting radiographic images hence eyesight is an important factor that needs to be frequently examined.
There have been significant reports from radiologists about eye strain and fatigue due to the continued hours spent examining digital images (Krupinski 2011). Radiology relies on the detailed examination of an image and the precise diagnosis of clinically relevant information. The regular routine of radiology reporting requires an extended focus on objects at close proximity to the eyes hence placing a considerable amount of pressure on the mechanisms of the eye and the ocular anatomy within (Halpenny et al. 2012).
It is important to maintain optimal vision as the radiologist’s visual performance is expected to be accurate throughout the working day and to last for the rest of their career in the profession. Anatomy and Physiology of the Vision System The anatomy of the eye is shown in figure 1. There are three distinct layers in the eye with the outer layer consisting of the cornea and sclera, the middle layer containing the iris, ciliary body and choroid and lastly the inner layer which has the retina (Galloway et al. 2006).
The cornea’s main functions is to protect the eye against infection and to refract and transmit the light to the lens and retina. The iris controls the size of the pupil, thus limiting the amount of light that reaches the retina. The ciliary body controls the shape of the lens and the choroid provides nutrients and oxygen to the eye. The retina contains neurons that capture and processes light. Light enters the eye via the outer components and travels through the neurons of the retina and is accordingly captured by the photoreceptors present at the back of the retina.
The neurons then translate the visual information received from the eye into nerve impulses that travel from the optic nerve to the lateral geniculate nucleus to be interpreted (Willoughby et al. 2010). Each eye sees a marginally different image which is combined in the brain to become one picture. Figure 1: Different layers of the eye (Galloway et al. 2006, pg. 8) Visual Strain and Decline Vision declines with age and every visual function such as sensitivity of visual field, contrast sensitivity and visual acuity would eventually deteriorate (Salvi et al. 006).
With increasing age, the lens selectively absorbs more blue light as there is an accumulation of yellow pigments in the lens. This absorption in blue light causes the observer to have “blue blindness”. The display screens emit a blue light and with blue blindness, it would affect the radiologist’s ability to diagnose images (Salvi et al. 2006). There are diseases that can affect radiologists’ visual functions including age-related macular degeneration, cataracts (clouding of the lens) and glaucoma (Kaido et al. 2011).
Radiologists looking closely at the display screen for extended periods can overstrain their eyes resulting in asthenopia (eyestrain). With conventional screens, four hours is enough to cause eyestrain to the observer. The first symptom of visual fatigue is the inability to focus on the image. Other physical symptoms from eyestrain include headaches, discomfort around the eyes and double or blurred vision (Krupinski et al. 2009). The eyes have a default position at which the eyes rest at when there is nothing to focus on, this is called the resting point of accommodation (Krupinski et al. 009).
The maximum amount of accommodation is dependent on the firmness of the lens and contractility of the ciliary muscle. As the lens become more rigid with increasing age, accommodation decreases (Galloway et al. 2006). In the study by Krupinski and Berbaum (2009), the visual accommodation of radiologists was measured before and after a day of viewing radiographs. Results from the study demonstrated that observing images on digital displays exhausts the accommodation response in radiologists.
A radiologist tends to work close to the display screen hence the degradation of the eye would be more severe (Krupinski et al. 2009). As the working day progresses, the muscles that are required to accommodate at near distances would consistently be stressed, thus the radiologist would have a harder time focusing on the display. With problems in focusing, abnormalities may be missed which would require extra time to read the images and reduces accuracy in diagnosis. The size of an object that can be resolved with the human eye is called visual acuity.
Visual impairment is measured through tests for visual acuity such as Snellen charts as seen in figure 2 and ETDRS (early treatment diabetic retinopathy study) charts seen in figure 3 (Kaiser 2009). The Snellen chart has a large letter on the top followed by rows of various smaller letters, decreasing in size with each row. The Snellen test measures a small area of the retina called the macula. The macula is specialised to detect fine detail whereas the whole peripheral retina detects shapes and movement (Galloway et al. 2006).
For a radiologist, if the macula in the retina is damaged, they would be unable to see any small pathologies that may be present in the radiograph. However with damage to the macula it does not affect a person’s ability to see an overall image. Snellen charts have isadvantages as the distance between letters and rows are not standardized and some letters are easier to read than others. The ETDRS chart was then developed with a more standardized administration, reducing the variability and enhancing the precision of vision testing (Kaiser 2009).
Figure 2: A typical Snellen chart, uneven distances and charts are not standardised (Galloway et al. 2006, pg. 18) Figure 3: An improved eye test, examples of the ETDRS chart (Kaiser 2009, pg. 315) Reporting – Displays and Ambient Lighting The regular use of visual display monitors at work has been reported to cause fatigue in eyes and eyestrain. The development of PACS (picture archiving and communication system) has revolutionised image viewing in westernised healthcare systems. Compared to film, digital imaging offers the flexibility and convenience for medical professionals.
A consequence to PACS usage is that the radiologist would spend a majority of their working day in front of a computer monitor. Even when the radiologist is not reporting on images, they would still be facing a monitor through various meetings. With the use of the tests mentioned, measuring visual acuity should be mandatory for radiologists and be consistently examined in regular periods (Halpenny et al. 2012). There are different digital displays in use at work such as cathode ray tubes (CRTS), plasma display panel (PDP) and liquid crystal displays (LCDs).
With proper calibration, these displays are able to compensate for varying amounts of ambient lighting in the viewing room. LCDs have a significant advantage over CRTs as they have a low diffuse reflection coefficient and also offers a higher maximum luminance at high luminance ratios compared to CRTS (Amarpreet et al. 2007). These qualities seen in the CRT and LCD displays gives flexibility in their adjustments to ambient light levels whilst achieving efficient contrast for imaging observation.
In a study by Lee et al. 2009) the degree of eyestrain was measured by designing a device which was mounted to the head to capture eye movements. This system measured the observer’s changes in pupil size and eye blinking. The degree of eyestrain was then measured separately calculating the rate of blinking and the pupil accommodation speed (Lee et al. 2009). The results demonstrates that the LCD device caused a greater degree of eyestrain compared to the PDP device with the main reason for the increased eyestrain accounting to the glare of the display screen.
The blinking rate was greater in the LCD device and the accommodation speed was slower compared to the PDP display (Pelli et al. 2013). Ambient lighting in a reading room is preferred over a completely dark room but consequently, an increase in ambient lighting would reflect off the digital (Pollard et al. 2008) display thus reducing image contrast which would affect the radiologist’s ability to detect abnormalities in the image. In a dark room, the radiologist’s pupils dilate and contract to focus between the surrounding dark environment and the high luminance display (Pollard et al. 012). This constant pupillary adjustment causes the eye to be strained and degrades diagnostic performance.
When there is a bright light, the iris closes the pupil and in dim environments such as the reading room, the iris opens the pupil to let in more light. However, a controlled increase in ambient lighting may reduce these adjustments in the pupils hence minimising the luminance level (Pollard et al. 2008) in which the eyes adjust to between the image in the display monitor and the surrounding darkness. The results in the study by Pollard, et al. 2008), demonstrate that an increase in ambient lighting, may improve the radiologist’s ability to detect low contrast objects displayed on the screen. The two experiments in the study suggest that ambient lighting at a range of 50-80 lux would maximise radiologist work performance without forfeiting diagnostic performance, given that the monitor is calibrated accordingly (Pollard et al. 2008).
In another study also done by Pollard, et al. (2012), nodule detection was measured in different lightings and the results suggested that with a controlled increase in ambient lighting would not significantly degrade nodule detection (Pollard et al. 012). Proper DICOM (digital imaging and communications in medicine) calibration can compensate for the ambient lighting increases with little loss to the contrast in the image. This calibration in the display makes it possible for an increase in ambient lighting which would improve performance in radiograph interpretation (Pollard et al. 2008). Conclusion Eye care in the setting of regular visual display unit’s use among radiologists is an important quality control and occupational health issue (Halpenny et al. 2012).
Increasing age and being fatigued is likely to impact the clinician’s ability to effectively and efficiently process image information and render correct diagnoses (Krupinski 2011). Ambient lighting is also a relevant factor in radiologist’s detection performance as studies have shown that there is no degradation of performance if the display is calibrated correctly and the lighting is set in a specific range. It is necessary to understand the importance of visual perception and its role in the image interpretation process. Checking eyesight regularly with the use of the charts and having breaks often would maintain optimal vision