Unit 1 Physics Detailed Study 3.6 Chapter 15: Medical Physics

Section 15.4 Diagnostic X-rays The discovery of X-rays ∗ The discovery of X-rays was a pure accident. While investigating the newly discovered cathode rays (which as it turns out are a stream of electrons), Wilhelm Konrad Roentgen noticed that a screen with a special fluorescent coating nearby would glow. He assumed that they must be rays, but was not sure what they were called so called the x-rays. ∗ He discovered that these rays would penetrate some materials more easily then others. He used x-rays to produce an image of a human body part within a few weeks of their first discovery. ∗ In the early days of x-rays, not knowing the danger they can present, scientists working with them suffered from hair loss to skin cancers and even death as a result of overexposure. ∗ Today we know that x-rays are part of the electromagnetic spectrum. They wavelength is of order m and are not affected by electric of magnetic fields.

Section 15.4 Diagnostic X-rays X-rays as electromagnetic waves ∗ The production of x-rays requires a very specific apparatus. In the apparatus there is an evacuated tube with a filament from which electrons are ejected as it is heated. The electrons are attracted and accelerated towards the positively charged metal plate. The electrons are then decelerated and emit an x-ray in the process. ∗ In modern x-ray machines, the apparatus is enclosed in a lead casing, except of one particular area where the x-rays are sent out. ∗ The whole process produces a lot of heat, so tungsten which has a very high melting point is commonly used as a target, it is placed at an angle of 45° to direct the x-rays through the given gap. These targets also rotate so heat is not produced in one particular area.

Section 15.4 Diagnostic X-rays The energy of X-rays ∗ The quality of an X-ray refers to its penetrating ability. High energy beams of x-rays have a higher penetrating ability while low energy beams have a lower penetrating ability. ∗ Hard X-rays are the most penetrating type of X-ray, they have wavelengths less then m. ∗ Soft X-rays are less penetrating and have a longer wavelength. ∗ The penetrating ability of an x-ray is related to the energy carried by the x-rays. ∗ The energy of a particular x-ray photon is directly proportional to its frequency (and therefore wavelength). The energy of an X-ray is not measured in Joules as you may expect, it is measured in electronvolts (eV). Electronvolts is the amount of energy given to one electron when moving through a 1 V potential

Section 15.4 Diagnostic X-rays The energy of X-rays ∗ The output of an X-ray machine includes a full range of X-ray frequencies. This is known as a heterogeneous X-ray beam. A heterogeneous beam is not particularly useful to a radiographer, they require specific energy values or a small set of energy values produced by homogeneous beams. ∗ To filter out these lower energy beams, there is a layer of oil and some shielding in place. This removes the lowest energy x-rays, while some extra metal shielding is put in place to remove further low energies. ∗ This process is known as ‘Hardening’ the x-ray beam, as it removes the soft, low energy beams and increases the overall average energy.

Section 15.4 Diagnostic X-rays Attenuation of X-rays and creating contrast. ∗ The gradual reduction in the intensity of an X-ray beam as it travels away from the source is known as the attenuation of an X-ray. ∗ X-rays attenuate and interact differently through and with different materials, this is what makes images using X-rays possible. ∗ The term ‘Half-value thickness’ or HVT refers to the thickness of a material required to reduce the intensity of an X-ray beam by half of its original value. ∗ Dense materials which have high attenuation values (such as bone), have low HVT values. These values are small and often quoted in millimetres.

Section 15.4 Diagnostic X-rays Making an image. ∗ The process used to take an x-ray of, say a broken arm, is known as a beam cross- sectional area. ∗ Little sliding doors act as an aperture similar to a camera. ∗ A normal light is shone on the area to be X-rayed, this light represents where the X-ray beam will go. ∗ The doors are moved so that the beam is as narrow as possible, so as to minimise the patients exposure to x-rays. ∗ The image is captured using a film cassette. The patient is positioned between the cassette and X-ray beam. ∗ The more X-rays incident on the cassette in a particular area, the darker that area. ∗ The regions of low attenuation will result in darker regions, while areas of high attenuation result in brighter, whiter regions.

Section 15.4 Diagnostic X-rays Making an image. ∗ Within the cassette, there is a substance on the screen that fluoresces (shines) when struck by X-rays. It is this that causes the photographic film to go dark in the regions of high X-ray exposure. ∗ Contrast enhancement is required when the attenuation of the part of the body being photographed does not differ much from the surrounding tissue. To investigate a particular region a substance needs to be introduced to increase the contrast. ∗ These substances are relatively dense and therefore are able to block X-rays helping produce a high contrast.

Section 15.4 Diagnostic X-rays CT Scans ∗ Computer Tomography scanning, or CT scans as they are commonly known, take an image of a slice of the body. ∗ CT scans use a whole lot of thin X-ray beams and detectors to take an image of a slice of a patient. The thickness of a slice is determined by the width of the X-ray beam. ∗ The X-ray source and Detectors are in a circle around the patient. Each detector is responsible for around a 1mm x 1mm square piece of tissue. The detector records the intensity od the X-ray, and sends it to the computer, which interprets this intensity as a corresponding shade on the Black-Grey-White scale. ∗ While this technique produces much clearer images, it exposes patients to a higher dosed of radiation than a typical X-ray.

Section 15.4 Diagnostic X-rays Magnetic Resonance Imaging ∗ Magnetic Resonance Imaging or MRI, can be used on elements that have an odd number of nucleons, such as hydrogen, flourine-19, sodium-23 and carbon-13 ∗ There is lots of hydrogen in the body, so MRI’s are used as a way of taking internal images. MRI’s are consider safer then X-ray imaging as they do not expose the patient to any ionising radiation, however, they are quite expensive. ∗ If placed in a strong magnetic field, the protons within the body will align themselves with the magnetic field. ∗ The MRI machine then sends out a radio frequency pulse, and the protons shift their alignment. Once the pulse passes the protons realign with the magnetic field and emit a low frequency which is detected by a detector to produce an image. ∗ Different tissues have different concentration of hydrogen, and therefore produce different contrasting colours. ∗ Like a CT scan and MRI takes a cross section image of the patient.