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To evaluate the effectiveness of PET, SPECT, MRI and X-rays, we first need some basic knowledge on how each of them work.
PET stands for positron emission tomography and works by an instrument collecting radiation emitted from a radioisotope injected the patient’ body. The strengths of emission are recorded by a gamma camera, which has a series of scintillation crystals, each connected to a photomultiplier tube. The crystals convert the gamma rays, emitted from the patient, to photons of light, and the photomultiplier tubes convert and amplify the photons to electrical signals. These electrical signals are then processed by a computer to generate images. The table is then moved, and the process is repeated, resulting in a series of thin slice images of the body over the region of interest (e.g. brain, breast, liver). These thin slice images can be assembled into a three dimensional representation of the patient's body
Nowadays, PET scanning devices are most often used in conjunction with CT scanners, so that a more accurate image can be observed by the doctor for easier diagnosis of diseases or disorders.
SPECT (Single Photon Emission Computed Tomography) works in a way much the same to PET. But the radioactive substances used in SPECT (Xenon-133, Technetium-99, Iodine-123) have longer decay times than those used in PET, and emit single instead of double gamma rays
MRI has a more complex principle for its function; it works by creating a magnetic field so strong that the hydrogen protons in the body are forced into alignment with the magnetic field. Short bursts of radio waves are sent from the scanner into your body. The radio waves knock the protons from their position. When the burst of radio waves stops, the protons go back into position. They realign back to being in parallel with the magnetic field. As the protons realign, in a process known as relaxation, they emit tiny radio signals. A receiving device in the scanner detects these signals. The type of tissue can be interpreted from the strength of the signal emitted.
Most of the hydrogen atoms in the body are in water molecules.
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CT scans work by having many X-ray shots taken. The X-ray tube and detectors are situated oppositely each other and they can take numerous X-ray images as they rotate at all angles around the patient. The machine records X-ray slices across the body in a spiral motion. The computer varies the intensity of the X-rays in order to scan each type of tissue with the optimum power. After the patient passes through the machine, the computer combines all the information from each scan to form a detailed image of the body. It's not usually necessary to scan the entire body, of course. More often, doctors will scan only a small section.
X-ray images are essentially photos that use X-rays instead of visible light to expose the film. When the X-rays hit the film, they expose it just as light would. Since bone, fat, muscle, tumours and other masses all absorb X-rays at different levels; the image on the film lets you see different (distinct) structures inside the body because of the different levels of exposure on the film. An x-ray tube releases the x-rays at the patient while an x-ray camera on the other side of the patient records the pattern of X-ray light that passes all the way through the patient's body. The X-ray camera uses the same film technology as an ordinary camera, but X-ray light sets off the chemical reaction instead of visible light. Doctors can bring different materials into focus by varying the intensity of the X-ray beam.
The provision of materials used, that is the isotopes needed for the diagnosis and their half lives greatly effects peoples judgement on how effective the mechanism is. MRI requires no radioisotopes for its use, which is a major benefit. Xrays and CT scans also do not require a radioisotope for injection.
PET and SPECT machines, however, do require radioactive isotopes for diagnosis. The isotopes used in PET generally are ones with extremely short half lives whilst those used in SPECT are usually ones that have longer half lives, but are still relatively short.
Isotopes used in PET include:
· Carbon-11 Half Life: 20.4 minutes
· Fluorine-18 Half Life: 109.8 minutes
· Nitrogen-13 Half Life: 9.96 minutes
· Oxygen-15 Half Life: 2.03 minutes
Isotopes used in SPECT include
· Iodine-123 Half Life: 13.2 hours
· Technetium-99m Half Life: 6.02 hours
· Xenon-133 Half Life: 5.245 days
· Indium-111 Half Life: 2.83 days
It is these half live that greatly affect the efficiency of the mechanisms as radioactive decay damages tissues and so obviously the machine that utilizes the radioisotopes with shorter half lives is better for our health as these radioisotopes can make people quite sick.
Other aspects that greatly affect our judgment on the effectiveness of the machine include the time it takes for a scan the accuracy and nature of diagnosis, the skills needed to run the machine, the cost of the machine and materials and therefore the cost passed on to the patient and finally, the facilities needed to be used by the machine.
In the case of MRI, a normal scan usually takes between 30 and 60 minutes, of course this varies depending on the body part being scanned, the immobility of the patient, and many other external factors. In a study undertaken that compared full-body dual-modality positron emission tomography and computed tomography (PET/CT) scan technology with full-body MRI in 98 patients. Eighty-two percent of the patients were referred for primary tumour staging, the rest were referred for suspected tumour recurrence. The researchers found that MRI was found to be accurate in only 52 percent of the patients, and in most cases the scans took longer for MRI than PET.
Although, this is probably not the case in most day-to-day situations. MRI and PET are used for many different problems. After proper consultancy with a doctor the best mechanism for diagnosis should be decided upon, as there is little variation in the cost of a scan. MRI is ideal for:
· Diagnosing multiple sclerosis (MS)
· Diagnosing tumours of the pituitary gland and brain
· Diagnosing infections in the brain, spine or joints
· Visualizing torn ligaments in the wrist, knee and ankle
· Visualizing shoulder injuries
· Diagnosing tendonitis
· Evaluating masses in the soft tissues of the body
· Evaluating bone tumours, cysts and bulging or herniated discs in the spine
· Diagnosing strokes in their earliest stages
The average cost of an MRI scan is currently around $518. Medicare rebates over $350 of this leaving patients paying an average of $168 out of pocket expenses for a scan. The average out of pocket expenses for a scan was $55 cheaper, but medicare decided to lower the amount rebated in June this year in order to fund 23 new machines. Each MRI machine cost about 2 million dollars.