Lecture 1 Introduction to Diagnostic Imaging.doc


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Lecture 1 Introduction to Diagnostic Imaging.doc

  1. 1. INTRODUCTION TO DIAGNOSTIC IMAGING How it all began: On November 8 1895, Wilhelm Conrad Roentgen discovered X-rays. He discovered the X ray while working in his laboratory by accident. A cathode ray tube was in the room and he noticed a glow of light coming from a phosphorescent screen. He began placing objects between the CRT and the screen. When he placed his hand between the 2, he could see the bones of his fingers. He noted at that time that platinum and lead would obstruct the rays. One of the first images he obtained using film was of he wife’s hand in which the exposure was 15 minutes long. He knew he had stumbled onto something fascinating and sequestered himself in his lab for the next couple of months. He later published his findings and shared with the intelligent his new finding. In 1901, the discovery of the X ray received a Nobel Prize for Physics. The Radiological Society of North America (RSNA) today has thousands of members which grew from the early 1900’s American Roentgen Ray Society (ARRS). How did it make its way into medicine? In the early years, there were no “real radiologist” only individuals who would look at the plates of images and compare them to autopsy and surgical findings in light of the patient’s clinical complaint. Over time, they could tell what the disease was from looking at the plates – thus the radiology specialty was born. In veterinary medicine, veterinarians who are specifically trained in diagnostic imaging belong to the American College of Veterinary Radiology (ACVR). Currently, radiologists are one of the most sought after specialist not only in academia but also in large private practices. In all of Canada, there are only approximately 10-15 individuals currently working as radiologist. What is Radiology? In the past, radiology was used to denote a branch of medicine that used xrays for the diagnosis and treatment of disease. More recently, with the advent of MRI, nuclear medicine, ultrasound etc. – which do not use x rays to aid in a diagnosis – Diagnostic Imaging is a more appropriate term. So then why is diagnostic imaging so important? A large number of patients that visit general and referral practices undergo some form of diagnostic imaging in the work up of their clinical signs. It is imperative to realize than imaging is an ancillary diagnostic modality which is performed in light of clinical signs, history, physical exam findings, and numerous other diagnostic tests. However, often times, the diagnosis is on the imaging study and even a normal exam can be an important diagnostic aid. During this course, we are going to learn how images from different modalities can help us, help our patients.
  2. 2. Forms of Diagnostic Imaging: These modalities are often complementary in finalizing a diagnosis for the patient. Diagnostic Radiology / Radiography: - uses x rays to produce the image, transmitted through patient - size, shape, density and location of structures can be evaluated - static images can be made with film/cassettes or digitally - dynamic studies can be performed with fluoroscopy - where things are viewed in real time - black = lots of the x rays get through the part to reach the film - white = most of the x rays are attenuated before reaching the film - (digital) – image is made based on the attenuation of the various structures - Contrast Agents o Allows visualization of structures not routinely seen on routine studies o Barium o Iodine o Can be administered IV, orally, per rectum, intrathecally etc  GI series – Barium is given in syringe by mouth or by tube into stomach – allows visualization of GI tract internally  EU – Iodine is given IV so that the kidneys, ureters, and bladder can be seen.  Myelogram – non iodinated contrast is given intrathecally to visualize the spinal cord. o Can be dynamic and static studies Ultrasonography - uses sound waves to produce an image, transmitted from transducer through patient - sending out and listening for echoes - good for evaluating soft tissue structures in the abdomen and also the heart - size, shape, echogenicity (brighter dots and darker dots) and location of structures (will be displayed further from the top of the screen if a structure is deeper because take longer time for echo to return to transducer - the internal architecture can be evaluated - images seen are dynamic, US can not penetrate air or bone - operator dependent and knowledge of anatomy is important Computed Tomography (CT) - use x rays to produce image, transmitted through patient - black and whites are like radiographs - image the body is cross sections (slices), multiple detectors are used - size, shape, density location and no superimposition of structures - available at referral centers, scan times very quick - great for heads, nasal – thing where a soft tissue mass might be causing change to underlying bone for example - requires computer manipulation of images
  3. 3. Nuclear Scintigraphy (nuc med) - gamma rays used to produce image, emitted from patient - radioactive nuclide is given IV, per os, per rectum ect - radiopharmaceutical will undergo decay and radiation is given off – usu in form of gamma rays - isotopes are bound to a specific agent that located to a specific area such as bone or liver - detects abnormal function, metabolic activity or abnormal amt of uptake - nuc med is poor with regards to anatomical information Magnetic Resonance Imaging (MRI) - uses strong magnetic fields and radiofrequency waves to image structures - there is no ionizing radiation – only radiofrequency waves - hydrogen protons in the body are stimulated to align to the direction of the field then a RF pulse is sent in and then turned off – the image is formed as the protons go back to a relaxing state - cross sectional images are obtained (just like CT) - great for soft tissue – spine, brain ect Radiation Therapy - uses radiation to treat and palliate neoplastic and some benign diseases - Cobalt and linear accelerators are generally used - must have special training – they now have their own specialty All of the modalities listed can be used alone or in combination as needed to aid in the diagnosis of the patient. Each modality can provide different types of information about body structures. What is an X ray? - form of electromagnetic radiation - all forms move at the speed of light - vary in energy and wavelength Production of X rays - they can penetrate matter - they can cause fluorescence of some atoms - they can expose film - they can cause biological damage Production of X rays: X rays are produced by the interaction of a rapidly moving stream of electrons with the atoms of a target material. When the electrons interact with the target, they are suddenly decelerated and 1% of their kinetic energy is converted to X rays (the other 99% is lost as heat)
  4. 4. Three elements are required for the production of x-rays. These are 1) a source of electrons 2) a target for the electrons to hit and 3) a way to accelerate those electrons. THE X-RAY TUBE The Cathode - the cathode is the electron source. - The cathode element (filament) is a fine strand of coiled tungsten wire, which resembles the filament of a light bulb. Tungsten is used because it can be formed into a thin, strong wire, has a high melting point (3370 C), and has little tendency to vaporize. - The cathode assembly includes a negatively charged concave cup (“focusing cup”) surrounding the filament. The focusing cup functions to push the electrons into a relatively narrow stream - remember that the negatively charged electrons will be repelled by each other and will have a tendency to spread out. The focusing cup will narrow the stream so it is directed at a small portion of the anode called the “focal spot”. Greater radiographic detail is achieved with a smaller focal spot. Most X ray units have 2 focal spots sizes – a small one and a larger one. If a high exposure is needed to penetrate a part, the larger focal spot is used so as to not burn up the filament. - When current (amperes) is applied to the filament, the wire heats up and electrons escape from the tungsten atoms = thermonic emission. - The number of electrons is directly proportional to the strength of the current, which is applied to the filament. - The strength of the current is determined by the mA(milliamperes) setting of the x-ray machine. The higher the mA setting, the greater the current supplied, the greater the filament heating, the greater number of electrons produced. The Anode - the anode is the target, which the electrons strike. - The anode is composed of a tungsten target - tungsten is used because it is an efficient producer of x-rays, is able to withstand high temperatures, and dissipates heat reasonably well. The anode may be stationary or rotating. - Stationary anodes have the tungsten target embedded in a large block of copper. Continuous bombardment of electrons in the relatively small target area causes significant heat production that is partially dissipated by the copper. However, because of the problem of overheating these anodes are limited to X-ray machines with low mA potential. They are often present in the portable units used in equine radiography. - Rotating anodes have the tungsten target embedded in a narrow band around the circumference of a disc, which rotates. The electron stream bombards the target path but because of rapid rotation of the disc no area is subjected to continuous bombardment. Because of their ability to "spread out" the heat load these units have a much greater mA potential. o Anode Heel Effect: The anode surface is angled. This allows for better cooling of the anode yet maintains good detail.
  5. 5.  The anode angle will cause the beam intensity to be non uniform with x rays produced on the cathode side being more intense and the ones produced on the anode side less intense.  The heel effect is used in practice by placing the thicker part of the structures being imaged on the cathode side of the tube. - Electrons produced at the cathode are accelerated to the anode by the application of a positive charge at the anode (negative flows toward positive). The positive charge applied at the anode can be varied - it is controlled by the kVp (kilovoltage peak) setting of the machine. The higher the kVp setting, the greater the degree of acceleration, the greater the force with which the electrons strike the anode. Exit window - X-rays go in all directions, so shielding is present around the tube except at the exit window (glass). Housing - Glass tube and vacuum. Filtration – primarily for radiation safety - filtration removes the low energy x rays from the beam which do not contribute anything useful to the image. - These low energy x rays get “stuck” in the patient thus increasing dose - Aluminum Collimation – beam restricting device - control the size and shape of the beam - decreases scatter radiation thus improving radiation safety to personnel and improves image quality (less fogging of film) Options for the X-ray photon once it leaves the tube: 1. Hit the patient and be absorbed. 2. Hit the patient and be scattered in another direction (towards the film, the holders, or within the patient). 3. Go through the patient, hit the cassette and be absorbed causing exposure of the film. 4. Go through the patient, through the cassette and table and hit the floor. What are mAs and kVp and why are they important? mAs - milliampere - second - milliampere is current applied to filament, second is time current was applied - higher currents generate more electrons which create more x-rays at target - longer time generates more electrons which do the same *** mAs Determines Quantity of X-rays!!*** 300mA x 1 sec = 600mA x 1/2 sec = 150mA x 2 sec = mAs
  6. 6. kVp - Kilovoltage peak - the potential differences in volts between the anode and the cathode - determines the speed the electrons have as they hit the target - higher speed will increase the penetrating power of the X-rays - higher speed also increases the number of interactions that a given electron can have with target atoms, so increases the number of X-rays. ***kVp Determines Quality of X-rays!!*** kVp also has some effect on quantity of x-rays kVp is maximum potential of X-ray photon, and in most machines the average energy of the beam is 1/3 the kVp
  7. 7. kVp - Kilovoltage peak - the potential differences in volts between the anode and the cathode - determines the speed the electrons have as they hit the target - higher speed will increase the penetrating power of the X-rays - higher speed also increases the number of interactions that a given electron can have with target atoms, so increases the number of X-rays. ***kVp Determines Quality of X-rays!!*** kVp also has some effect on quantity of x-rays kVp is maximum potential of X-ray photon, and in most machines the average energy of the beam is 1/3 the kVp