PHYSICS: NON-INVASIVE IMAGING

PHYSICS: NON-INVASIVE IMAGING

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Overview

• Medical imaging is the technique and process used to create images of the human body for medical purposes
• Non-invasive imaging is the method of producing images of internal tissues without surgical procedures
• Non invasive imaging techniques can be used to produce anatomical assessment of tissues (such as X-rays) as well as functional assessments (such as MRI)
• As a discipline, it includes radiology, nuclear medicine, endoscopy, thermography etc
• Non-invasive imaging is a vast field with differing technologies such as X-rays, tomography, MRI etc
• Non-invasive imaging provide highly valuable diagnostic tools for diagnosing and treating varied ailments such as cancer, fractures, etc
• Imaging technologies can be broadly classified into two categories
  • Anatomical imaging modalities: these imaging techniques provide information on the anatomy i.e. the physical structure of the organ/tissue under study
  • Functional imaging modalities: these imaging techniques provide information on the physiological functioning of the organ/tissue under study

X-RAYS

• X-rays were discovered by Wilhem Conrad Rontgen (Germany) in 1895. He won the Nobel in Physics 1901
• Radiography is the imaging process that uses X-rays to capture images
• In conventional radiography, X-rays from a X-ray tube pass through the patient and are captured by an X-ray sensitive film screen
• Nowadays, digital radiography (DR) is becoming popular, in which x-rays strike an array of sensors that convert the signal to digital mode and displays the images on a computer screen
• X-rays are the preferred diagnostic tool for studying lungs, heart and skeleton (including fractures) due to their simplicity, available and low cost
• X-rays is an anatomical imaging technology

Fluoroscopy

• Fluoroscopy is used to obtain real time moving images of the internal structures
• Fluoroscope systems consist of an X-ray source and a fluorescent screen connected to a closed circuit TV. The patient is position between the source and the screen
• Fluoroscopes use low x-ray radiation doses
• Fluoroscopy also involves use of radiocontrast agents that increase the contrast of a specific tissue w.r.t. surrounding tissuesby strongly absorbing or scattering the x-rays
• The radiocontrast agents enable visualization of dynamic processes such as peristalsis in the digestive tract of blood flow in arteries and veins
• Commonly used contrast agents include Barium and Iodine. These may be administered orally or rectally or injected into the blood stream
• Used mainly for investigating gastrointestinal functions, orthopaedic surgery and urological surgery
• Fluoroscopy is a functional imaging technology

Computed Tomography (CT)

• Computed Tomography uses X-rays in conjunction with software algorithms to image the body
• CT generates a three-dimensional image of an object using a large series of X-ray images taken around a single axis of rotation
• CT produces a volume data which can be manipulated in order to demonstrate various body functions
• Compared to traditional radiography, CT produces 3d information and has much higher contrast and resolution, but also uses much higher doses of radiation
• CT scanners were first developed by Sir Godfrey Hounsfield (Britain) in 1972. He won Nobel in Medicine in 1979
• CT is used primarily for detecting cerebral haemorrhage, pulmonary embolism, aortic dissection, appendicitis and kidney stones
• CT is an anatomical imaging technology

Ultrasound

• Ultrasound was first developed for medical use by John Wild (Britain) in 1949
• Ultrasonography uses ultrasound (high frequency sound waves) to visualize soft tissues in the body in real time
• Ultrasound does not involve any ionizing radiation, hence it considered safer than X-rays or CT and is used for obstetrical imaging
  
• Ultrasound is limited by its inability to image through air or bone, and by the skill of the examiner
• Ultrasound is used primarily to study the development of foetus
• A variant of ultrasound, the colour flow Doppler ultrasound is used in cardiology for diagnosing peripheral vascular disease
• Ultrasound is a functional imaging technology

Magnetic Resonance Imaging (MRI)

• MRI was invented by Paul Lauterbur (USA) and Sir Peter Mansfield (Britain) in the 1970s. They won Nobel in Medicine in 2003
• MRI uses strong magnetic fields to align atomic nuclei within body tissues, and then uses a radio signal to disturb this alignment and observes the signals generated as the atoms return to their original states
• The working principle of MRI is called Nuclear Magnetic Resonance (NMR)
• MRI scans give the best soft tissue contrast of all imaging modalities
• MRI does not use any ionizing radiation. However, it does use powerful magnetic fields
• A variant of MRI called Functional MRI measures signal changes in the brain due to neural activity
• MRI is used primarily for neurological (brain), musculoskeletal, cardiovascular and oncological (cancer) imaging
• MRI is an anatomical imaging technology

Nuclear medicine

• Nuclear medicine uses radioactive isotopes and the principle of radioactive decay to study body functions
• Nuclear medicine involves the administration into the patients of radio-pharmaceuticals.
  Radio-pharmaceuticals are substances with affinity for certain body tissues that have been labelled with radioactive tracers (called radio-nuclides)
  
• The radio-pharmaceuticals administered into the body emit radiation which is detected and converted into images.
  
• The radio-pharmaceuticals, once administered, localise (i.e. attach) to specific organs or cell receptors, meaning those particular organs or cells can be studied in isolation
  
• Commonly used tracers include Technetium, iodine, gallium and thalium
• Nuclear medicine is used mainly to study the heart, lungs, thyroid, liver and gallbladder
• Nuclear medicine mainly provides information about the physiological function of these tissues
• Since the radio isotopes decay over a period a time, they do not pose a significant threat to normal human functioning
• Nuclear medicine is a functional imaging technology

Positron Emission Tomography (PET)

• PET uses nuclear medicines to produce three dimensional images
• The PET system detects gamma rays emitted by positron emitting radio-nuclides. Images of the nuclide concentration are reconstructed in 3d by computer algorithms
• PET is a functional imaging technology
• PET is often combined with CT and MRI scans, enabling both anatomical and functional imaging simultaneously
• PET was first developed by David Kuhl (USA) and Roy Edwards (USA) in the 1950s
• PET is mainly used in oncology (cancer) and neurology (especially dementias)
• A variant of PET, called Single Positron Emission Computed Tomography (SPECT) detects gamma rays emitted directly by the radio-nuclides

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JOHAR