MEDICAL IMAGING Created By : Shale Olagbegi DOD TELEHEALTH RESEARCH TOPIC
What Is Medical Imaging? • Medical imaging is the process by which physicians evaluate an area of the subject's body that is not normally visible. This process could be clinical or research motivated and can also have scientific and industrial applications.
Origin of Medical Imaging • In its most primitive form, imaging can refer to the physician simply feeling an area of the body in order to visualize the condition of internal organs. • It remains an important step today in making initial assessments of potential problems, although additional steps are often used to confirm a diagnosis. • The primary drawback of this approach is that findings are subject to interpretation, and while a recorded image can be produced manually, in practice this is often not done.
Modern Imaging Techniques • Radiographs • Computed Tomography • Magnetic Resonance Imaging • Ultrasound • Mammography • Microwave Imaging
This is the creation of radiographs, photographs made by exposing a photographic film or other image receptor to X-rays. Since X-rays penetrate solid objects, but are slightly attenuated by them, the picture resulting from the exposure reveals the internal structure of the object. The most common use of radiography is in the medical field (where it is known as medical imaging). Radiography
Theory of Radiography • The type of electromagnetic radiation of most interest to radiography is x-ray and gamma radiation. This radiation is much more energetic than the more familiar types such as radio waves and visible light. It is this relatively high energy, which makes gamma rays useful in radiography but potentially hazardous to living organisms. • They are produced by X-ray tubes, high energy X-ray equipment or natural radioactive elements, such as Radium and Radon, and artificially produced radioactive isotopes of elements, such as Cobalt 60 and Iridium 192. Electromagnetic radiation consists of oscillating electric and magnetic fields. It is generally pictured as a single sinusoidal wave. • It is characterized by its wavelength (the distance from a point on one cycle to the point on the next cycle) or its frequency (the number of oscillations per second). All electromagnetic waves travel at the same speed, the speed of light (c). The wavelength (W) and the frequency (ν) are all related by the equation: • Wν = c • This is true for all electromagnetic radiation. • Electromagnetic radiation is known by various names, depending on its energy . The energy of these waves is related to the frequency and the wavelength by the relationship: • E = hν = hc / W • Where h is a constant known as Planck's Constant. • Gamma rays are indirectly ionizing radiation. A gamma ray passes through matter until it undergoes an interaction with an atomic particle, usually an electron. During this interaction, energy is transferred from the gamma ray to the electron, which is a directly ionizing particle. As a result of this energy transfer, the electron is liberated from the atom and proceeds to ionize matter by colliding with other electrons along its path. • For the range of energies commonly used in radiography, the interaction between gamma rays and electrons occurs in two ways. One effect takes place where all the gamma ray's energy is transmitted to an entire atom. The gamma ray no longer exists and an electron emerges from the atom with kinetic (motion in relation to force) energy almost equal to the gamma energy. This effect is predominant at low gamma energies and is known as the photoelectric effect. The other major effect occurs when a gamma ray interacts with an atomic electron, freeing it from the atom and imparting to it only a fraction of the gamma ray's kinetic energy. A secondary gamma ray with less energy (hence lower frequency) also emerges from the interaction. This effect predominates at higher gamma energies and is known as the Compton effect. • In both of these effects the emergent electrons lose their kinetic energy by ionizing surrounding atoms. The density of ions so generated is a measure of the energy delivered to the material by the gamma rays. • The most common means of measuring the variations in a beam of radiation is by utilizing its effects onto a photographic film. This effect is the same as that of light, and the more intense the radiation is, it will produce a darker film, or a more exposed film. Other methods are in use, such as the ionizing effect measured electronically, its ability to discharge an electro statically charged plate or to cause certain chemicals to fluoresce as in fluoroscopy.
What's an X-Ray? • X-rays are basically the same thing as visible light rays. Both are wavelike forms of electromagnetic energy carried by particles called photons. • The difference between X-rays and visible light rays is the energy level of the individual photons. This is also expressed as the wavelength of the rays.
Theory of X-ray • X rays were discovered in 1895 by W. C. Roentgen, who called them X rays because their nature was at first unknown; they are sometimes also called Roentgen, or Röntgen, rays. X-ray line spectra were used by H. G. J. Moseley in his important work on atomic numbers (1913) and also provided further confirmation of the quantum theory of atomic structure. • Also important historically is the discovery of X-ray diffraction by Max von Laue (1912) and its subsequent application by W. H. and W. L. Bragg to the study of crystal structure.
Production of X Rays • An important source of X rays is synchrotron radiation. X rays are also produced in a highly evacuated glass bulb, called an X-ray tube, that contains essentially two electrodes—an anode made of platinum, tungsten, or another heavy metal of high melting point, and a cathode. When a high voltage is applied between the electrodes, streams of electrons (cathode rays) are accelerated from the cathode to the anode and produce X rays as they strike the anode. • Two different processes give rise to radiation of X-ray frequency. In one process radiation is emitted by the high-speed electrons themselves as they are slowed or even stopped in passing near the positively charged nuclei of the anode material. This radiation is often called brehmsstrahlung [Ger.,=braking radiation]. In a second process radiation is emitted by the electrons of the anode atoms when incoming electrons from the cathode knock electrons near the nuclei out of orbit and they are replaced by other electrons from outer orbits. The spectrum of frequencies given off with any particular anode material thus consists of a continuous range of frequencies emitted in the first process, and superimposed on it a number of sharp peaks of intensity corresponding to discrete frequencies at which X rays are emitted in the second process. The sharp peaks constitute the X-ray line spectrum for the anode material and will differ for different materials.
Applications of X Rays • Most applications of X rays are based on their ability to pass through matter. This ability varies with different substances; e.g., wood and flesh are easily penetrated, but denser substances such as lead and bone are more opaque. The penetrating power of X rays also depends on their energy. The more penetrating X rays, known as hard X rays, are of higher frequency and are thus more energetic, while the less penetrating X rays, called soft X rays, have lower energies. X rays that have passed through a body provide a visual image of its interior structure when they strike a photographic plate or a fluorescent screen; the darkness of the shadows produced on the plate or screen depends on the relative opacity of different parts of the body. • Photographs made with X rays are known as radiographs or ski graphs. Radiography has applications in both medicine and industry, where it is valuable for diagnosis and nondestructive testing of products for defects. Fluoroscopy is based on the same techniques, with the photographic plate replaced by a fluorescent screen (see fluorescence; fluoroscope ); its advantages over radiography in time and cost are balanced by some loss in sharpness of the image. X rays are also used with computers in CAT (computerized axial tomography) scans to produce cross-sectional images of the inside of the body. • Another use of radiography is in the examination and analysis of paintings, where studies can reveal such details as the age of a painting and underlying brushstroke techniques that help to identify or verify the artist. X rays are used in several techniques that can provide enlarged images of the structure of opaque objects. These techniques, collectively referred to as X-ray microscopy or microradiograph, can also be used in the quantitative analysis of many materials. One of the dangers in the use of X rays is that they can destroy living tissue and can cause severe skin burns on human flesh exposed for too long a time. This destructive power is used in X-ray therapy to destroy diseased cells.
Medical uses • X-rays have been developed for their use in medical imaging. • Radiology is a specialized field of medicine that employs radiography and other techniques for diagnostic imaging. • The use of X-rays are especially useful in the detection of pathology of the skeletal system, but are also useful for detecting some disease processes in soft tissue. • X-ray, which can be used to identify lung diseases such as pneumonia, lung cancer or pulmonary oedema.
Other X-Ray Uses • The most important contributions of X-ray technology have been in the world of medicine, but X-rays have played a crucial role in a number of other areas as well. • X-rays have been pivotal in research involving quantum mechanics theory, crystallography and cosmology. • In the industrial world, X-ray scanners are often used to detect minute flaws in heavy metal equipment. • And X-ray scanners have become standard equipment in airport security, of course.
Cat Scan • (CT), also known as computed axial tomography or computer-assisted tomography (CAT) and body section roentgenography, is medical imaging method employing tomography where digital processing is used to generate a three-dimensional image of the internals of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation. • The word "tomography" is derived from the Greek tomos (slice) and graphia (describing). • Although most common in healthcare, CT is also used in other fields, e.g. nondestructive materials testing
History of Cat Scan • The CT system was invented in 1972 by Godfrey Newbold Hounsfield of EMI Central Research Laboratories; owned by Creative Technology.) using X-rays. • Allan McLeod Cormack of Tufts University independently invented the same process and they shared a Nobel Prize in Medicine in 1979. The first scanner took several hours to acquire the raw data and several days to produce the images. Modern multi-detector CT systems can complete a scan of the chest in less time than it takes for a single breath and display the computed images in a few seconds.
Principles of Cat Scan • X-ray slice data is generated using an X-ray source that rotates around the object; X-ray sensors are positioned on the opposite side of the circle from the X-ray source. Many data scans are progressively taken as the object is gradually passed through the gantry. They are combined together by the mathematical procedure known as tomographic reconstruction. • Newer machines with faster computer systems and newer software strategies can process not only individual cross sections but continuously changing cross sections as the gantry, with the object to be imaged, is slowly and smoothly slid through the X-ray circle. These are called helical or spiral CT machines. Their computer systems integrate the data of the moving individual slices to generate three dimensional volumetric information, in turn viewable from multiple different perspectives on attached CT workstation monitors.
Principles of Cat Scan (cont’d) • EBT Machine • In conventional CT machines, an X-ray tube is physically rotated behind a circular shroud in the less used electron beam tomography (EBT) • The data stream representing the varying radiographic intensity sensed reaching the detectors on the opposite side of the circle during each sweep—360 degree in conventional machines, 220 degree in EBT—is then computer processed to calculate cross-sectional estimations of the radiographic density, expressed in Hounsfield units. • CT is used in medicine as a diagnostic tool and as a guide for interventional procedures. Sometimes contrast materials such as intravenous iodinated contrast is used. This is useful to highlight structures such as blood vessels that otherwise would be difficult to delineate from their surroundings. Using contrast material can also help to obtain functional information about tissues.
Principles of Cat Scan (cont’d) • Pixels in an image obtained by CT scanning are displayed in terms of relative radio-density. The pixel itself is displayed according to the mean attenuation of the tissue that it corresponds to on a scale from −1024 to +3071 on the Hounsfield scale. Water has an attenuation of 0 Hounsfield units (HU) while air is −1000 HU, bone is typically +400 HU or greater and metallic implants are usually +1000 HU. • Improvements in CT technology have meant that the overall radiation dose has decreased, scan times have decreased and the ability to reconstruct images (for example, to look at the same location from a different angle) has increased over time. Still, the radiation dose from CT scans is several times higher than conventional X-ray scans. • Presently, the cost of an average CT scanner is US$1.3 million.
Diagnostic use of Cat Scan • Since its introduction in the 1970’s , CT has become an important tool in medical imaging to supplement X – rays and medical ultrasonography. Although it is still quite expensive, it is the gold standard in the diagnosis of a large number of different disease entities. • Cranial CT • Diagnosis of cerebra vascular accidents and interracial hemorrhage is the most frequent reason for a "head CT" or "CT brain". Scanning is done without intravenous contrast agents (contrast may resemble a bleed). CT generally does not exclude infarct in the acute stage, but is useful to exclude a bleed (so anticoagulant medication can be commenced safely). • For detection of tumors, CT scanning with IV contrast is occasionally used but is less sensitive than (MRI). • CT can also be used to detect increases in intracranial pressure, e.g. before lumbar puncture or to evaluate the functioning of a ventriculoperitoneal shunt. • CT is also useful in the setting of trauma for evaluating facial and skull fractures.
Diagnostic use of Cat Scan (cont’d) • Chest CT • CT is excellent for detecting both acute and chronic changes in the lung parenchyma. For detection of airspace disease or cancer, ordinary non-contrast scans are adequate. • For evaluation of chronic interstitial processes. For evaluation of the mediastinum and hilar regions for lymphadenopathy, IV contrast is administered. • CT angiography of the chest (CTPA) is also becoming the primary method for detecting pulmonary embolism (PE) and aortic dissection, and requires accurately timed rapid injections of contrast and high-speed helical scanners. CT is the standard method of evaluating abnormalities seen on chest X-ray and of following findings of uncertain acute significance.
Diagnostic use of Cat Scan (cont’d) • Cardiac CT • With the advent of sub second rotation combined with multi-slice CT (up to 64 slices), high resolution and high speed can be obtained at the same time, allowing excellent imaging of the coronary arteries. It is uncertain whether this modality will replace the invasive coronary catheterization. • Abdominal and pelvic CT • Many abdominal disease processes require CT for proper diagnosis. CT has limited application in the evaluation of the pelvis. For the female pelvis in particular, ultrasound is the imaging modality of choice. Nevertheless, it may be part of abdominal scanning (e.g. for tumors), and has uses is assessing fractures.
Extremities of Cat Scan • CT is often used to image complex fractures, especially ones around joints, because of the ability to reconstruct the area of interest in multiple planes
Magnetic resonance imaging • (MRI) - also called magnetic resonance tomography (MRT) - is a method of creating images of the inside of opaque organs in living organisms as well as detecting the amount of bound water in geological structures.
MRI • To understand how MRI works, let's start by focusing on the "magnetic" in MRI. The biggest and most important component in an MRI system is the magnet. • The magnet in an MRI system is rated using a unit of measure known as a tesla. Another unit of measure commonly used with magnets is the gauss (1 tesla = 10,000 gauss). • The magnets in use today in MRI are in the 0.5-tesla to 2.0-tesla range, or 5,000 to 20,000 gauss. Magnetic fields greater than 2 tesla have not been approved for use in medical imaging, though much more powerful magnets -- up to 60 tesla -- are used in research. Compared with the Earth's 0.5-gauss magnetic field, you can see how incredibly powerful these magnets are.
MRI (cont’d)_ • Numbers like that help provide an intellectual understanding of the magnetic strength, but everyday examples are also helpful. • The MRI suite can be a very dangerous place if strict precautions are not observed. Metal objects can become dangerous projectiles if they are taken into the scan room. For example, paperclips, pens, keys, scissors, hemostats, stethoscopes and any other small objects can be pulled out of pockets and off the body without warning, at which point they fly toward the opening of the magnet (where the patient is placed) at very high speeds, posing a threat to everyone in the room. Credit cards, bank cards and anything else with magnetic encoding will be erased by most MRI systems.
Purpose Of MRI • To obtain two-dimensional views of an internal organ or structure, especially the brain and spinal cord. • To assess response to treatment, especially cancer chemotherapy or radiation therapy. • To assess sports-related injury to bones and joints.
How it works • MRI uses a powerful magnetic field and radio waves to alter the natural alignment of hydrogen atoms within the body. • Computers record the activity of the hydrogen atoms and translate that into images.
Preparation • All jewelry, hair clips, and other metal objects must be removed. • Some facilities ask patients to disrobe and put on a hospital gown; others allow patients to wear clothing so long as it doesn't have metal parts. • A contrast medium may be injected before some studies (e.g., gadolinium may be injected before an MRI study of the brain); people who are claustrophobic or have difficulty lying still may be given a sedative. Otherwise, no special preparation is required.
Test procedure • You will be instructed to lie as still as possible on a narrow table that slides into a tubelike structure that holds the magnet (see figure). • A loud thumping or hammering noise will be heard during the test; you may request earplugs or listen to music with earphones to reduce the noise level. • At certain points during the test, the noise will stop and you will be able to hear instructions from the doctor or technician administering the test.
FIGURE Magnetic Resonance Imaging • Variations: Echoplanar MRI is a new technique that allows for rapid accumulation of data such as cardiac motion. • After the test: You can resume your pretest activities immediately. • Factors affecting results: Movement, extreme obesity, and the presence of metal objects can all affect results. • Interpretation: A radiologist or other medical specialist interprets the results.
FIGURE Magnetic Resonance Imaging (cont’d) • Advantages • MRIoffers increased-contrast resolution, enabling better visualization of soft tissues. Also, it allows for multiplanar imaging, as opposed to CT, which is usually only axial. • It provides highly detailed information without exposing the body to radiation. In many instances, it provides more useful images than CT scanning and ultrasound. • Disadvantages • It is an expensive procedure and not available in many small hospitals and rural areas. • It also cannot be used for patients with implanted pacemakers and certain other metal objects. • MRI systems are very, very expensive to purchase, and therefore the exams are also very expensive.
Ultrasound • This is a technique that uses sound waves to study and treat hard-to-reach body areas. In scanning with ultrasound, high-frequency sound waves are transmitted to the area of interest and the returning echoes recorded.
What is an Ultrasound Test? • An ultrasound test is a radiology technique, which uses high-frequency sound waves to produce images of the organs and structures of the body. The sound waves are sent through body tissues with a device called a transducer. The transducer is placed directly on top of the skin, which has a gel applied to the surface. The sound waves that are sent by the transducer through the body are then reflected by internal structures as "echoes." These echoes return to the transducer and are transmitted electrically onto a viewing monitor. The echo images are then recorded on a plane film and can also be recorded on videotape. After the ultrasound, the gel is easily wiped off. • The technical term for ultrasound testing and recording is "sonography." Ultrasound testing is painless and harmless. Ultrasound tests involve no radiation and studies have not revealed any adverse effects.
Major Uses of Ultrasound • Ultrasound has been used in a variety of clinical settings, including obstetrics and gynecology, cardiology and cancer detection. • The main advantage of ultrasound is that certain structures can be observed without using radiation. • Ultrasound can also be done much faster than X-rays or other radiographic techniques.
Here is a short list of some uses for ultrasound: • Obstetrics and Gynecology • measuring the size of the fetus to determine the due date • determining the position of the fetus to see if it is in the normal head down position or breech • checking the position of the placenta to see if it is improperly developing over the opening to the uterus (cervix) • seeing the number of fetuses in the uterus • checking the sex of the baby (if the genital area can be clearly seen) • checking the fetus's growth rate by making many measurements over time • detecting ectopic pregnancy, the life-threatening situation in which the baby is implanted in the mother's Fallopian tubes instead of in the uterus • determining whether there is an appropriate amount of amniotic fluid cushioning the baby • monitoring the baby during specialized procedures - ultrasound has been helpful in seeing and avoiding the baby during amniocentesis (sampling of the amniotic fluid with a needle for genetic testing). Years ago, doctors use to perform this procedure blindly; however, with accompanying use of ultrasound, the risks of this procedure have dropped dramatically. • seeing tumors of the ovary and breast • Cardiology • seeing the inside of the heart to identify abnormal structures or functions • measuring blood flow through the heart and major blood vessels • Urology • measuring blood flow through the kidney • seeing kidney stones • detecting prostate cancer early
For what purposes are ultrasounds performed? • Ultrasound examinations can be used in various areas of the body for a variety of purposes. These purposes include examination of the chest, abdomen, blood vessels (such as to detect blood clots in leg veins) and the evaluation of pregnancy. • In the chest, ultrasound can be used to obtain detailed images of the size and function of the heart. Ultrasound can detect abnormalities of the heart valves, such as mistral valve prolapse, aortic stenosis, and infection. • Ultrasound is commonly used to guide fluid withdrawal aspiration) from the chest, lungs, or around the heart.
For what purposes are ultrasounds performed? contd For what purposes are ultrasounds performed? • Ultrasounds also commonly used to examine internal structures of the abdomen. Ultrasound can detect fluid, cysts, tumors or abscess in the abdomen or liver. Impaired blood flow from clots or arteriosclerosis in the legs can be detected by ultrasound. Aneurysms of the aorta can also be seen. Ultrasound is also commonly used to evaluate the structure of the thyroid gland in the neck. • During pregnancy, an ultrasound can be used to evaluate the size, gender, movement, and position of the growing baby. The baby's heart is usually visible early, and as the baby ages, body motion becomes more apparent. The baby can often be visualized by the mother during the ultrasound, and the gender of the baby is sometimes detectable.
How do patients prepare for an ultrasound? • Preparation for ultrasound is minimal. Generally, if internal organs such as the gallbladder are to be examined, patients are requested to avoid eating and drinking with the exception of water for six to eight hours prior to the examination. This is because food causes gallbladder contraction, minimizing the size, which would be visible during the ultrasound. • In preparation for examination of the baby and womb during pregnancy, it is recommended that mothers drink at least four to six glasses of water approximately one to two hours prior to the examination for the purpose of filling the bladder. The extra fluid in the bladder moves air-filled bowel loops away from the womb so that the baby and womb are more visible during the ultrasound test.
What is Mammography • This is a specific type of imaging that uses a low-dose x-ray system for examining the breasts. • The images of the breasts can be viewed on film at a view box or as soft copy on a digital mammography work station. • Most medical experts agree that successful treatment of breast cancer often is linked to early diagnosis. • Mammography plays a central part in early detection of breast cancers because it can show changes in the breast up to two years before a patient or physician can feel them.
Procedures involved • A mammography unit is a rectangular box that houses the tube in which x-rays are produced. The unit is a dedicated equipment because it is used exclusively for x-ray exam of the breast, with special accessories that allow only the breast to be exposed to the x-rays. Attached to the unit is a device that holds and compresses the breast and positions it so images can be obtained at different angles. • The breast is exposed to a small dose of radiation to produce an image of internal breast tissue. The image of the breast is produced as a result of some of the x-rays being absorbed (attenuation) while others pass through the breast to expose either a film (conventional mammography) or digital image receptor (digital mammography). The exposed film is either placed in a developing machine—producing images much like the negatives from a 35mm camera—or images are digitally stored on computer
Uses of Mammography • The detection of breast cancer is X-ray imaging of the breasts. • When mammography screening is combined with a follow-up ultrasonic examination of those women whose mammographies show signs of possible cancer
Other common uses of the procedure • Mammography is used to aid in the diagnosis of breast diseases in women. Screening mammography can assist your physician in the detection of disease even if you have no complaints or symptoms. • Initial mammographic images themselves are not always enough to determine the existence of a benign or malignant disease with certainty. If a finding or spot seems suspicious, your radiologist may recommend further diagnostic studies. • Diagnostic mammography is used to evaluate a patient with abnormal clinical findings, such as a breast lump or lumps, that have been found by the woman or her doctor. Diagnostic mammography may also be done after an abnormal screening mammography in order to determine the cause of the area of concern on the screening exam
Screening mammography • Imaging examination of the breast by means of x-rays, of individuals usually without symptoms to detect those with a high probability of having breast disease.
What is Microwave Imaging • The term microwave imaging covers all processes in which measurements of electromagnetic fields in the microwave region from 300 MHz to 30 GHz are used for creating images.
Processes involved in microwave imaging • To create images from microwave measurements, it is necessary to construct a microwave camera, which is able to transmit microwaves and measure the scattered waves at one or more antennas. Different types of microwave cameras are currently being used for imaging in such areas as ground penetrating radar and remote sensing. Depending on the items to be imaged, different types of microwave cameras are needed. These range from small antennas used for near field measurements in ground penetrating radar to the large airborne systems used in remote sensing. There are two key issues to address when designing a microwave cameras. One is the increase of the signal to noise ratio in the system and the other is to assure that the system has a large dynamic range. The importance of both of these is closely related to the fact that the scattered signal is often very weak in comparison to the transmitted signal. This implies that any noise in the system will have a large impact on the image quality and that the system must be able to distinguish even small differences in the received signals. To obtain the maximum amount of information from the microwave measurements, inverse scattering techniques must be applied.
Techniques involved in MI • Inverse scattering is the technique in which the images are created by inverting a model of the scattering mechanisms derived from Maxwell's equations. • The quality of the images when using inverse scattering for microwave imaging are determined by: • The accuracy of the forward model • The accuracy of the inversion algorithm. • By using Maxwell equations, an exact solution to the forward scattering problem can be determined.
References • www.answers.com • www.colorado.edu/physics • www.reference.dictionary.com • www.medicalimaging.org