Digital Technique and It’s Relation to Patient Dose

Digital imaging technique has been a topic that is widely discussed and has much importance in the world of radiography.  Technique plays many roles in radiography such as image quality, diagnostic quality, and the most important; patient dose.  Technique is the number one factor that contributes to patient dose and is therefore of the utmost importance.  Throughout the rest of the paper, a brief history of digital imaging as it relates to technique and patient dose will be given as well as; different exposure systems, variance in recommended techniques from vendors, and collimation creep.

To truly understand how digital imaging technique is related to patient dose, a brief history must be given first.  After the discovery of x-rays, the first system to capture the image was film-screen radiography.  With film-screen radiography, things like techniques, patient positioning, imaging processing, and others needed to be precise in order to receive an optimal image.  If one, or many, of these things were off it could result in a repeat image that would cause more dose to the patient.  In 1980, digital imaging was introduced and quickly changed the world of radiography (Alexander, 2016, p. 55).  While techniques and patient positioning still needed to be precise, there was a little more wiggle room to play with.  Now there was no need for a repeat image if the chemicals to process the image were off, because chemicals were no longer used.  If an image with film-screen radiography was too over or under exposed it would be cause for a repeat.  Now images that are under or over exposed are so subtle that it’s hard to see if it really is, and there are also window and leveling options that give even more room for error.  It has been said that it is usually better to overexpose a patient than to underexpose, because there is more useful information in the image that is overexposed, and because there are window and leveling options to see more of what is on the image.  But with that new philosophy came a common problem of dose creep, where if a little is good a lot is better.  Radiologic technologists seem to get in the habit of erring on the side of overexposed and then continually increase the technique, so the patient receives more dose.  As Low As Reasonably Achievable (ALARA) is easy to ignore when a patient is overexposed and it is hardly noticeable on the image.  With that problem came the solution of an exposure index (EI) number that shows how much dose the imaging receptor (IR) has received.  The dose that the IR receives is comparable to how much a patient receives and is a good indicator for that.

Most systems have an EI range that is used to help determine how over or underexposed the image is and if the image is within acceptable range.  For example, a system using a range of 1300-1700 would have the ideal EI be close to 1500, signaling that it is an optimal image.  If it is above or below that range it could be cause for a repeat or as a signal that the technique needs to be changed for the next images that are taken.  These EI numbers help radiologic technologists know appropriate techniques for different sizes, age ranges, and body types.  There are also new systems being developed such as the DigiBit, that can accurately adjust exposure factors such as milliampere seconds (mAs), kilovoltage peak (kVp), and source-to-image distance (SID) for patients of differing size.  The DigiBit systems is specifically designed for when automatic exposure control (AEC) is not available.

The AEC device controls x-ray exposure during radiography.  It controls the amount of radiation and the time it takes to reach the detector by adjusting the milliampere seconds (mAs) at the x-ray tube and by terminating the exposure once a preset amount of radiation has been detected by the AEC chamber(s) (Ching, Robinson, & McEntee, 2015, p. 614-615).

Other factors such as kVp and SID are controlled by the radiologic technologist and may vary.  AEC exposure will also vary depending on patient size, if the patient is correctly positioned over the correct cells, and if the correct cells are chosen.

According to a study by Gibson and Davidson, the number of optimal exposure factors selected decreased over 2 years in intensive and critical care units and emergency departments where an AEC was not available.  This decrease occurred because radiographers overestimated the exposure factor adjustment required for changes in patient size.  According to a study by Mothiram et al, between 77% and 82% of mobile chest radiographs were overexposed compared to less than 2% of radiographs taken in a department where an AEC was available.  To avoid systematic overestimation of exposure when an AEC is unavailable, radiographers need a system to adjust exposure (Ching, Robinson, & McEntee, 2015, p. 615).

As was shown in the study, the common problem of dose creep is even more present without the help of things such as an AEC. That is why, making new systems to adjust exposure is just one way that can help reduce patient dose and over exposed images.

Other factors to consider with technique and EI are the vendor’s recommended techniques depending on body part and size.  A study was done to determine the amount of mAs acceptable for an anteroposterior (AP) pelvis and lumbar spine using a phantom model of each of them.  The AP pelvis was imaged using a fixed kVp of 81 and variable mAs of 6.3, 8, 12.5, 16, 20, 25, 32, 40, and 50.  The suggested mAs for the AP pelvis according to the manufacturers was 25.  For the AP lumbar spine, it was imaged at a fixed kVp of 81 and variable mAs of 16, 20, 25, 32, 40, 50, 63, 80, and 100.  The suggested mAs according to the manufacturers was 50.  Four images were acquired using the same technique and each image was reviewed by a selected group of radiologists.  After reviewing the images, it was found that the lowest mAs that could be used without compromising image quality was 16 mAs for the AP pelvis and 32 mAs for the AP lumbar spine (Seeram, Davidson, Bushong, & Swan, 2016, p. 380-381).  It is obvious to see that the difference between the recommended amounts and the amounts that could be used have a large difference. This difference in mAs saves patient dose while still receiving an optimal image.  When performing an exam, it is important to keep this in mind so appropriate techniques are chosen for patients and patient dose is kept to a minimum.

Another important thing to keep in mind is proper collimation.  With the use of digital radiography came another use of the cropping feature that is offered.  This cropping feature allows radiologic technologists to cut down on their image after it is has been taken, giving the appearance of a collimated image.  Some radiologic technologists have reported that they use the cropping feature to give their image a “neat” and “tidy” appearance, whereas others use it as a correction to an image that is not collimated enough (Casey, 2019, para. 9).  This creates a problem because radiologic technologists that leave the collimation open and then correct after, are giving their patients more dose and creating what is called ‘collimation creep’.  ‘Collimation creep’ happens when radiologic technologists are trying to create an ideal image by leaving the collimation open when exposing the patient, and then cropping the image after it has been taken to make it look more pleasing to the eye.  While some radiologic technologists report using the cropping feature, others think that it is a bad practice.  One reported saying ‘“If you do your radiography well, you know your positioning, you do your collimation right — why should you do the cropping?”’ (Casey, 2019, para. 11).  With the option of cropping the image after or correctly collimating in the first place, it is always important to remember to protect your patient by using ALARA and keeping dose creep and collimation creep to a minimum.

As reviewed, technique plays a critical role in patient dose as do many other things such as ideal EI’s, new exposure systems, and collimation.  While it is the radiation from the x-rays that gives patients their dose, it is the radiologic technologist that sets the technique and is therefore their responsibility to determine how much is received. Throughout all of these things, it is ultimately up to the radiologic technologist to properly use the systems that are available, set correct techniques, and protect their patients.  Radiologic technologists must always remember the importance of their duties to their patients and do all that they can to keep them safe.

Reference

• Alexander, S. (2016). Image Acquisition and Quality in Digital Radiography. American Society of Radiologic Technologists, 88, 53-65.
• Casey, B. (2019, June 25). Digital radiography may be leading to ‘collimation creep’. Retrieved June 28, 2019, from https://www.auntminnie.com/index.aspx?sec=sup&sub=xra&pag=dis&ItemID=125810
• Ching, W., Robsinson, J., & McEntee, M. (2015). DigiBit: A System for Adjusting Radiographic Exposure Factors in the Digital Era. American Society of Radiologic Technologists, 86, 614-621.
• Seeram, E., Davidson, R., Bushong, S., &Swan, H. (2016). Optimizing the Exposure Indicator as a Dose Management Strategy in Computed Radiography. American Society of Radiologic Technologists, 87, 380-390.