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Wednesday, March 31, 2010

general science#3

PHYSICS: NON-INVASIVE IMAGING

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 tissues by 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

CHEMISTRY: MEDICINAL CHEMISTRY

  • Medicinal chemistry involves the design, synthesis and development of pharmaceutical drugs
  • Compounds used as medicines are overwhelmingly organic compounds including small molecules and biopolymers. However, some inorganic compounds and metals have been found to have medicinal properties as well
Classes of drugs

Class of drug Application Example Notes
Antipyretics Reduce body temperature Aspirin, paracetamol (acetaminophen) Antipyretics cause the hypothalamus to override an increase in temperature
Taking antipyretics in empty stomach can cause ulcer
Analgesics Pain relief Paracetamol
Non steroidal anti inflammatory drugs (NSAIDS)

Morphine
Some antipyretics act as analgesics as well
Some narcotics (heroin, morphine, marijuana) can also act as analgesics
Tranquilizers Induce sedation Barbiturates, antihistamines Sedatives cause sleep, poor judgement, slow reflexes
Excessive use can cause unconsciousness and even death
Antiseptics Reduce possibility of infection Boric acid, hydrogen peroxide, iodine Antiseptics are applied externally to living tissues
Antiseptics also reduce body odour caused due to bacterial decomposition

They are used in breath freshners and deodorants
Antibiotics Kill bacteria Penicillin, gramicidin, amoxicillin, streptomycin An antibiotic is defined as a substance produced by a microorganism that kills other microorganisms
Antibiotics are considered life-saving drugs
Diuretics Increases rate of urination Amiloride, triamterene
Vasodilators Widen blood vessels Histamine, nitric oxide Decrease blood pressure
Increase blood flow
Vasoconstrictors Narrow blood vessels
Staunch blood loss due to haemorrhage
Antihistamines, cocaine, LSD, caffeine Increase blood pressure
Decrease blood flow

Make skin look paler because less blood reaches the skin
Anaesthetics Cause loss of sensation Cocaine, nitrous oxide, halothane General anaesthetics cause a loss of consciousness
Local anaesthetics cause loss of sensation in a specific part of the body
Antifungals Fungal diseases like ringworm, athlete’s foot, meningitis Ketoconazole, benzoic acid, neem seed oil, tea tree oil Since both fungi and human cells are eukaryotes, the possibility of side effects is higher than in anti-bacterial drugs (like antibiotics)
Antivirals
(Antiretrovirals)
Inhibit growth of virus Zedovudine, lamivudine Unlike antibiotics, antiviral drugs do not destroy target microbes but only inhibit their growth
Designing antiviral drugs is difficult because virus use host’s cells to replicate

Some virus, like influenza and HIV, mutate rapidly which means they can be treated with antivirals only and not be prevented by vaccines

Antiretrovirals are a subclass of antivirals that treat retroviruses such as HIV
Some important common drugs

Drug Classification Application Notes
Penicillin Antibiotic Syphilis, staphylococcal infections (food poisoning) Narrow spectrum antibiotic
(treats only a narrow range of diseases)
Zedovudine Antiviral HIV
Lamivudine Antiviral Hepatitis B
Streptomycin Antibiotic Tuberculosis
Erythromycin Antibiotic Respiratory tract infections
Ciprofloxacin Antibiotic Urinary tract infections, common pneumonia, myoplasmal infections Broad spectrum antibiotic
Amoxicillin Antibiotic Wide range of infections Broad spectrum
Tetracycline Antibiotic Cholera
Chloroquine Antibiotic Malaria
Aspirin Analgesic, Antipyretic Fever, pain One of the most widely used medications in the world
Paracetamol
(Acetaminophen)
Analgesic, antipyretic Fever, pain

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