Hello! I’m a second year STP trainee on the Medical Physics (undefined) stream – undefined just means that I’ll choose what I specialise in after having completed my rotations in Radiotherapy, Radiation Safety, Imaging with Non Ionising Radiation and Imaging with Ionising Radiation.
I’m currently on my Imaging with Ionising Radiation* rotation and in this blog post I’ll be talking you through a day where I visited a hospital imaging department to perform some routine quality assurance testing on an X-ray unit used for general diagnostic radiology. As medical physicists dealing with ionising radiation, it is our job to ensure the safe operation and use of x-ray equipment. Regular tests are required in order to assure us that things are working as they should be and unintended radiation exposures are minimised as much as possible!
Here’s what the X-ray examination room we were performing tests on looked like:
After unpacking the equipment and setting up, we performed the following tests:
1. Alignment of X-rays to light field
A beam of light is used to indicate the area that the X-ray beam is exposed to – this is to ensure that the intended anatomy is selected before the patient is imaged. The alignment of this light beam with the X-ray beam is important – if the light field is larger than the X-ray field, the area of the body that is being imaged is overestimated and so the area that is actually being imaged using the X-rays may not actually include the intended anatomy e.g. the chest. On the other hand, if it is smaller than the radiation field, patients may receive a larger dose than they need for imaging, exposing them to an unnecessary amount of ionising radiation – this is something we try to minimise as much as possible to reduce the long-term effects of exposure to ionising radiation.
The test is carried out using a collimator test tool. The light field is set to fit the rectangle on this tool and, once imaged, the X-ray field is observed to see how well it matches up to it. This unit was aligned and so passed the test!
2. Tube output and Dose Area Produce (DAP) Measurements
X-rays are generated in an X-ray tube where electrons are accelerated towards a target material with a high atomic number and high melting temperature (usually tungsten). The electrons slow down as they encounter the atoms in the target material and X-rays are produced as ‘bremsstrahlung’ radiation (a German word which translates to ‘braking radiation’). The intensity, energy and dose delivered by the X-rays can be changed by adjusting the voltage across the X-ray tube, the number of electrons accelerated across the tube and the amount of time the X-rays are emitted for. Diagnostic X-ray units also have a built-in DAP (Dose Area Product) chamber. This measures the dose that the beam is delivering as it leaves the X-ray tube.
Using a solid-state radiation detector, dose measurements were taken to observe the variation of the dose with varying tube voltage, tube currents and exposure time. This gives information on whether the dose is what is expected for the changing tube parameters. The measured dose is also compared to the DAP chamber reading to ensure it is within tolerance. This was also a pass this time!
3. Image quality testing
This is one of my favourite tests! Image quality is important in diagnostic imaging – poor image quality could be the reason a radiologist would miss a diagnosis or even misdiagnose a patient! Poor image quality can also result in a repeat X-ray – this would increase the patient’s radiation dose. The tests that were performed include a test of the spatial resolution in the image (the ability to distinguish two points on the image) and the threshold contrast (i.e. the minimum difference in brightness that can be seen on the image).
Huttner and TO-12 test objects were placed on top of the digital detector plate and images were acquired. This X-ray unit had sufficient resolution and threshold contrast to pass the test!
4. AEC testing
Automatic Exposure Controls (AECs) are devices that are used to terminate the beam once a pre-set dose has been detected. The dose is set to ensure that the image that is obtained is of a sufficient quality for diagnostic purposes. This is useful as patients come in all shapes and sizes and the densities of the anatomy within the area being imaged vary a lot too. By setting AECs, there is consistency in image quality across the diagnostic images.
Sheets of copper and aluminium were attached to the X-ray window to attenuate the X-rays as a patient would – attenuation describes the changes in intensity and energy of the X-rays as they pass through a material e.g. a patient’s body.
A highly sensitive radiation detector was used to measure the dose to the detector plate to test that the AECs were terminating the beam at the preset intended dose. The AECs were working as expected, so nobody will be receiving inappropriate doses from this X-ray set!
5. Radiation protection and environmental monitoring
Now, this is more the ‘radiation safety’ side of things but since our Trust provides a medical physics service to hospitals in the region, we are responsible for their radiation safety monitoring too. This means checking that all signage outside ‘controlled’ areas is clear, concise and ensures that unauthorised persons do not enter. We also put up some TLDs (themoluminescent dosimeters) around the room to monitor the radiation in the local environment. TLDs measure the total X-ray radiation they’re exposed to over a period of time. Once they’re sent back to us, we can keep a record of the dose rates in the room and confirm that the shielding in the room is sufficient to stop radiation outside the area exceeding dose constraints.
I hope this gives some insight into the sort of work medical physicists are involved in – I know I would have definitely benefitted from a post like this before applying for the STP!
* The Imaging with Ionising Radiation specialism will soon be changing – it currently includes diagnostic radiology which involves any imaging using X-rays, and nuclear medicine. In the 2022 curriculum, the testing discussed above will come under the diagnostic radiology and radiation safety pathway.
STP Perspectives 