MEDICAL PHYSICS IN CONTEXT
One approach to teaching medical physics is to follow a possible patient scenario. Procedures relevant to the medical physics course can be discussed as they would occur in a possible timeline of diagnosis and treatment:
Patient 1: Peter – colonoscopy and radiotherapy
Peter suspected things were not right when he went to see his doctor. He was suffering form abdominal pain and there was blood in his faeces. He had also noticed there was a change in his bowel habit. Peter was concerned as there is a family history of large bowel or colorectal cancer. His doctor ordered an initial faecal occult blood test, which is an inexpensive method for testing for blood in the bowel motions. It came back positive. Peter then has a colonoscopy, when the tumour was visualised and biopsies conducted.
CT scans of the abdomen and pelvis revealed that the disease had not spread to the other areas, in particular the liver. Peter underwent surgery to remove the tumour. This was followed up by chemotherapy.
Later Peter underwent radiotherapy to prevent local recurrence of the cancer. Peter was concerned about the associated risks of radiotherapy. It has been two years since the tumour was first discovered. Peter is regularly screened and he is believed to be cured.
Patient 2: Vanessa – laser eye surgery
Vanessa suffered from nearsightedness or myopia. After wearing contact lenses for 15 years she decided to undergo laser eye surgery. She met with her ophthalmologist to discuss the procedure.
The procedure is known in the medical field as photorefractive keratectomy (PRK). Vanessa would be treated as an outpatient. The procedure involves local anaesthetic eye drops. Using a cool, computer-controlled ultraviolet beam of light (laser) the cornea is gently reshaped by removing microscopic amounts of tissue from its outside surface. The cornea is responsible for most of the focusing power of the eye and Vanessa’s cornea does not properly bend the light entering her eye for focusing. The image always focuses in front of the retina. The procedure would only take a few minutes and she should be back to normal in one to three days. Before the procedure begins, Vanessa’s eye is measured to determine the extent of the visual problem, and the eye’s surface is mapped. The corneal change is then calculated and entered into the laser’s computer.
Possible side effects could include infection and delayed healing, under- or over-correction, corneal haze, regression (effects of surgery gradually lost over time) and an optical effect known as the halo effect. The halo effect involves a second faded image produced when the pupil enlarges.
Vanessa decided to undergo the laser surgery. The surgery was a success. The recovery period was minimal and Vanessa no longer wears contact lenses. Vanessa is very happy with the outcome.
Patient 3: Robert – X-ray, ultrasound and CT
Robert is brought into the hospital trauma centre (or emergency room, some hospitals just call it casualty) via helicopter from the scene of a motor vehicle accident. The car he was travelling in with friends collided head-on with another car. Robert was pulled from the wreckage by a passer-by, but when he went back to the car to try to rescue his friends, the car exploded, throwing him through the air and causing serious burns to his face and hands.
On arrival at hospital, Robert is taken into the trauma room on a stretcher, sedated and intubated (a tube is put down his trachea) to maintain his airway and allow him to breathe. His injuries are examined by the doctors and nurses of the trauma team. One of the doctors performs an ultrasound on Robert's abdomen. Ultrasound is normally used for seeing images of unborn babies, but this new application allows doctors to check for internal bleeding in the abdomen of trauma patients. Robert's abdomen is okay – he has no internal bleeding and his vital signs appear to be stable
Chest and pelvis X-rays are taken in situ via the overhead X-ray machine. They show no fractures to these parts of Robert’s body; however, his right ankle is badly bruised and swollen and an X-ray reveals a fracture. All hospital trauma rooms have overhead X-ray machines so that the patient can stay where they are while the X-rays are performed. Consequently, the doctors and nurses of the trauma team wear lead-lined aprons to protect them from X-ray exposure.
Robert is wheeled into the CT room next to the trauma room to check if he has a head injury. The head CT appears to be clear so it is back to the trauma room for treatment of his burns, broken ankle and other cuts. Robert is finally taken to intensive care for monitoring and further treatment, then to the ward while he recovers. He eventually makes a full recovery.
Patient 4: John – MRI
John had played a lot of football as a teenager and as a young man, and was mainly injury-free, but in his mid-30s he started to have trouble with his left knee. While working in the garden at home one day, he felt something tear inside the knee. This caused a lot of pain and swelling.
John’s GP referred him to an orthopaedic surgeon who suspected John had a torn meniscus which is a cartilage pad between the bones in the knee. An X-ray revealed no bone damage. An arthroscope operation was performed using key-hole surgery to trim away the torn cartilage and smooth over the surfaces of the bones. John had two tiny wounds on his knee to show where the arthroscopic instruments were inserted, but his recovery was slow.
The knee pain persisted and an MRI scan was performed to check that no floating pieces of cartilage were still inside John's knee. About 180 images of his knee were taken with the MRI scanner and the doctor gave John the all clear. With several months of physiotherapy and reduced activity, John’s knee is almost back to normal.
Patient 5: Tony– PET
Tony has a suspected brain tumour. He suffers memory disorders from an undetermined cause. A CT scan has detected the shape and structure of a possible tumour in Tony’s brain. Tony’s doctor has suggested that the next step in the diagnostic stage is a PET scan or Positron Emission Tomography imaging.
Tony’s doctor explains that PET scans detect biochemical changes in the suspected brain tumour. In particular it will provide information about blood flow and the density of glucose receptors within the suspected tumour. Glucose uptake by the tumour will be analysed. Glucose is tagged with a positron-emitting radioactive isotope, Nitrogen-13, that has a half-life of approximately 10 minutes, and is introduced into Tony’s body via intravenous injection. Glucose, tagged with N-13 accumulates in the region of the brain where the suspected brain tumour is. N-13 emits positrons (positively charged -particles) as it flows through the brain. The emitted positron only travels a few millimetres before it collides with a normal electron. In the collision the positron and electron are annihilated, producing two rays that fly off in opposite directions. A ring of detectors around Tony’s head detects the ray pair. The information is fed into a computer and if the area of abnormality’s glucose uptake deviates from normal functioning, this could be further evidence that a brain tumour is present. The distribution of the radiation is represented by enhanced colours on a television monitor. Areas of higher and lower levels of radiation intensity can then be detected. The PET results will be compared with Tony’s initial CT scan.
The side effects of the procedure are minimal, as the levels of N-13 are very low. There should be no change to the normal functioning to the brain and N-13 has a short half-life, both physical and biological. It should leave Tony’s body via normal biological removal processes, that is, perspiration, respiration, urination and defecation. One major disadvantage of PET scans could be that the resolution of the scan could be poor if the suspected tumour is too deep within the brain. The tests might be inconclusive and further tests including possible MRI, might be necessary. Tony’s future is uncertain.