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This assignment will discuss five of the currently available imaging techniques. The five techniques that will be discussed are MRI, Ultrasound, CT, X-ray, and PET. Magnetic resonance imaging (MRI) or nuclear magnetic resonance (NMR) is an imaging technique which uses a magnetic field to produce a picture of the anatomy of the body. The human body is primarily of water molecules, which consists of two hydrogen nuclei. When a patient undergoes an MRI scan, they are placed in the centre of a homogenous magnetic field magnet. This magnet has a magnetic field which aligns the hydrogen nuclei in the direction of the magnetic field lines. A radio frequency is then used to turn the nuclei 90 degrees away from the magnet field. Coils are then placed beside the patient and a second gradient magnet is turned on. This gradient magnet sends out pulses of very strong to weak and in different directions. The pulses that return are recorded and a Fourier transform is used to determine the strength of the signals and the image is formed.
MRI scans can be used to look at all aspects of the body from organs to blood vessels. It can be used to diagnose and monitor a range of diseases. This type of imaging technique means the patient is not exposed to ionizing radiation and "is more likely in some instances to identify and accurately characterize diseases than other imaging methods."  A MRI is just a giant magnetic all metal objects such as jewellery and watches must be removed before the scan. Due to this reason MRI is not suitable for patients with pacemakers. In order to achieve high-quality images ,patients must remain extremely still for long periods of time ranging "in length from 20 minutes to 90 minutes or more"  , with the main causes for image distortion arising from breathing and bowel motion creating artefacts . Safety issues concerned with MRI involve the magnet never been turned off and that if any metal objects comes with the magnetic field the object may become a missile. Also the noise from the machine is extremely loud and patients must wear earplugs will under the scanner. "MRI enables the discovery of abnormalities that might be obscured by bone with other imaging methods." 
Ultrasound is a non-invasive and inexpensive imaging technique. This imaging system uses the principle of periodic motion whereby it "causes pressure waves in surrounding physical media."  Where the "ultrasonic waves are made of high pressure and low pressure pulses travelling through the medium".  Ultrasound speed is determined by the "mass and spacing of the tissue molecules and the attracting force between the particles of the medium."  In dense materials the waves will travel faster while in compressible materials the waves will travel slower. "Ultrasound is reflected at the boundaries between different materials"  where these materials have different acoustic impedance. Acoustic impedance is the "interference with the passage of sound waves by objects in the path of those waves. It equals the velocity of sound in a medium multiplied by the density of the medium." 
The main part of the ultrasound machine is the transducer. This device is used to make the sound waves and it receives the waves that are reflected. In order to produce these waves it uses a principle know as piezoelectric effect. This principle involves using crystals known as piezoelectric crystals, when an electric current is applied to these crystals they change shape rapidly causing them to vibrate. It is these vibrations which create the ultrasonic wave. These crystals when they receive sound waves generate electrical currents.
The ultrasound is used to look at many of the internal organs of the body for example the heart and gallbladder. It can also be used to help doctors evaluate symptoms like pain and swelling. Ultrasound is also useful for guide procedures such as needle biopsies and also to diagnose a number of heart conditions. Ultrasound is a non-invasive procedure and does not involve the use of ionizing radiation meaning it is safe for pregnant women. It is also provides a clear image of soft tissues which is not picked up by x-rays and also provides real-time imaging. "For standard diagnostic ultrasound there are no known harmful effects on humans."  The main disadvantage of ultrasound is the fact that it is completely operator dependent as the images are completed in real-time and if the "operator is not competent enough, then certain information can be missed." 
Computed Tomography (CT)
Computed tomography (CT) "scanning combines special x-ray equipment with sophisticated computers to produce multiple images or pictures of the inside of the body".  It takes cross-sectional images or slices of the body and looks at the soft tissue which cannot be picked up in regular x-ray imaging. The CT scanner consists of three main sections; table, gantry and the array. The patient is placed on the table and this is then moved in or out of the gantry depending on what part of the body is being imaged. The gantry contains the equipment used to produce and detect the x-rays. The x-ray tubes and detectors are placed directly opposite each other and spin around the aperture of the gantry. As the x-rays pass through the body the detectors measure the x-ray attenuation. Where x-ray attenuation is "the decrease in the number of photons in an X-ray beam due to interactions with the atoms of a material substance."  Fourier transform is then used to construct the image for the information obtained from the detector.
CT scanning is "one of the best and fastest tools for studying the chest, abdomen and pelvis because it provides detailed, cross-sectional views of all types of tissue"  and is "often the preferred method for diagnosing many different cancers"  . The benefits of CT scanning include it being; "painless, non-invasive and accurate, provides very detailed images of many types of tissue as well as the lungs, bones, and blood vessels. CT examinations are fast and simple."  As images are taken in real time, it can be used for guiding procedures and it also less sensitive to patient movement. With the use of radiation from the x-rays there is always the "chance of cancer from excessive exposure to radiation."  CT scanning is not suitable for pregnant women and should not be done on children unless absolutely necessary. Even though CT scanning involves the use of radiation, the benefits of using this type of imaging technique can be clearly seen.
X-ray is an imaging technique which uses x-rays which are electromagnetic waves which are shorted then visible which are able to penetrate the body. In order to create x-rays, an x-ray tube is used. This tube contains a pair of electrodes called the cathode and the anode. The cathode is a filament which releases energy which the introduction of electrical current, the energy that is then released is in the form of electrons. The anode is located on the opposite end of the x-ray tube is a disc of tungsten which is a material which attracts electrons. When the electron released from the cathode comes in contact with the tungsten, they release energy in the form of photons. These highly energised photons are channelled through a lead cylinder and a series of filters creating an x-ray beam. The x-ray beam which can only be absorbed by dense body tissues like bone. Radiographic film or digital recording plate is placed behind the patient. X-ray beam is then focused on a specific part of the patient's body. The area where the x-ray energy passes through the body becomes black while an area where the energy is absorbed becomes white. Soft tissue shows up in shades of grey and air is black on an x-ray image.
The big advantage of x-ray imaging is its ability to penetrate the body in order to give clear images of the bones. "Bone x-rays are the fastest and easiest way for a physician to view and assess broken bone and joint abnormalities, such as arthritis and spine injuries."  X-ray imaging is fast and inexpensive and is widely available. X-ray imaging is also used to look at the chest where it is able to "identify lung diseases"  and look at fluid collection. X-rays can also be used to discover stones in the gallbladder. X-rays like CT scans can be used for real-time imaging and are used for "Surgical Interventions involving the blood vessels, urinary system, and skeletal system"  . Disadvantage of using x-rays is the exposure of the patient to ionizing radiation and also when compared to other imaging techniques the information obtained is less in comparison to CT which can look at detail at the tissue and organs.
Positron Emission Tomography (PET)
Positron Emission Tomography (PET) is a medical imaging technique based on the detection of gamma rays from electron positron annihilation from within the body. In order to produce these gamma rays a short lived radioactive tracer is injected into the body. This tracer is incorporated into a biologically active molecule. As cancerous cells metabolize glucose a lot faster than other cells, for this reason biologically active molecule is usually a sugar like fluorodeoxyglucose (FDG). In order to ensure accurate results from the scan, after the patient has received the tracer, there is a rest period of one hour order where the patient is usually placed in a chair and does not move in order to reduce the amount of glucose being used by muscles in the body. This process allows the biologically active molecule to become concentrated in the tissue. Once the rest period has been reached the patient is now placed in the PET scanner. Here the positron electron annihilation can be detected when the gamma ray creates a burst of light which is picked up by the photomultiplier tubes.
PET scanning differs from other medical imaging techniques as it used to study the physiology of the patient this means it can detect disease before any changes in anatomy occurs. One of the main advantages of the PET scan is its ability to detect cancer and to look at the spread of cancer; it is also used to analyze the blood flow to the heart. Performing a PET scan can sometimes means unnecessary procedures such as surgery can be avoided. The scan is particularly good at identifying early heart disease in patients. In order to increase the accuracy of the PET scan it is usually compared to other imaging techniques such as MRI or CT. This type of scan is safe and although patients are given a radioactive tracer; it will have decayed after several hours and risks no further harm to the patient or others.
Currently there is no imaging technique that is capable of addressing all the diagnostic imaging requirements. From the various technique discussed above it can be clear