medical imaging medical imaging medical imaging is the technique and process of imaging the interior of a body for clinical analysis and medical intervention, as well as visual representation of the function of some organs or tissues (physiology). medical imaging seeks to reveal internal structures hidden by the skin and bones, as well as to diagnose and treat disease. medical imaging also establishes a database of normal anatomy and physiology to make it possible to identify abnormalities. although imaging of removed organs and tissues can be performed for medical reasons, such procedures are usually considered part of pathology instead of medical imaging. this article is about imaging techniques and modalities for the human body. for imaging of animals in research, see preclinical imaging. medical imagingone frame of a ct scan of the chest showing the heart and lungsicd-10-pcsbicd-987-88mesh003952 d 003952ops-301 code3medlineplus007451 measurement and recording techniques that are not primarily designed to produce images, such as electroencephalography (eeg), magnetoencephalography (meg), electrocardiography (ecg), and others, represent other technologies that produce data susceptible to representation as a parameter graph versus time or maps that contain data about the measurement locations. in a limited comparison, these technologies can be considered forms of medical imaging in another discipline of medical instrumentation. as of 2010, 5 billion medical imaging studies had been conducted worldwide. radiation exposure from medical imaging in 2006 made up about 50% of total ionizing radiation exposure in the united states. medical imaging equipment is manufactured using technology from the semiconductor industry, including cmos integrated circuit chips, power semiconductor devices, sensors such as image sensors (particularly cmos sensors) and biosensors, and processors such as microcontrollers, microprocessors, digital signal processors, media processors and system-on-chip devices. as of 2015, annual shipments of medical imaging chips amount to 46 million units and $1.1 billion. medical imaging is often perceived to designate the set of techniques that noninvasively produce images of the internal aspect of the body. in this restricted sense, medical imaging can be seen as the solution of mathematical inverse problems. this means that cause (the properties of living tissue) is inferred from effect (the observed signal). in the case of medical ultrasound, the probe consists of ultrasonic pressure waves and echoes that go inside the tissue to show the internal structure. in the case of projectional radiography, the probe uses x-ray radiation, which is absorbed at different rates by different tissue types such as bone, muscle, and fat. the term "noninvasive" is used to denote a procedure where no instrument is introduced into a patient's body, which is the case for most imaging techniques used. types plain x-ray of the wrist and hand in the clinical context, "invisible light" medical imaging is generally equated to radiology or "clinical imaging". "visible light" medical imaging involves digital video or still pictures that can be seen without special equipment. dermatology and wound care are two modalities that use visible light imagery. interpretation of medical images is generally undertaken by a physician specialising in radiology known as a radiologist; however, this may be undertaken by any healthcare professional who is trained and certified in radiological clinical evaluation. increasingly interpretation is being undertaken by non-physicians, for example radiographers frequently train in interpretation as part of expanded practice. diagnostic radiography designates the technical aspects of medical imaging and in particular the acquisition of medical images. the radiographer (also known as a radiologic technologist) is usually responsible for acquiring medical images of diagnostic quality; although other professionals may train in this area, notably some radiological interventions performed by radiologists are done so without a radiographer. as a field of scientific investigation, medical imaging constitutes a sub-discipline of biomedical engineering, medical physics or medicine depending on the context: research and development in the area of instrumentation, image acquisition (e.g., radiography), modeling and quantification are usually the preserve of biomedical engineering, medical physics, and computer science; research into the application and interpretation of medical images is usually the preserve of radiology and the medical sub-discipline relevant to medical condition or area of medical science (neuroscience, cardiology, psychiatry, psychology, etc.) under investigation. many of the techniques developed for medical imaging also have scientific and industrial applications. radiography main article: radiography two forms of radiographic images are in use in medical imaging. projection radiography and fluoroscopy, with the latter being useful for catheter guidance. these 2d techniques are still in wide use despite the advance of 3d tomography due to the low cost, high resolution, and depending on the application, lower radiation dosages with 2d technique. this imaging modality uses a wide beam of x rays for image acquisition and is the first imaging technique available in modern medicine. fluoroscopy produces real-time images of internal structures of the body in a similar fashion to radiography, but employs a constant input of x-rays, at a lower dose rate. contrast media, such as barium, iodine, and air are used to visualize internal organs as they work. fluoroscopy is also used in image-guided procedures when constant feedback during a procedure is required. an image receptor is required to convert the radiation into an image after it has passed through the area of interest. early on this was a fluorescing screen, which gave way to an image amplifier (ia) which was a large vacuum tube that had the receiving end coated with cesium iodide, and a mirror at the opposite end. eventually the mirror was replaced with a tv camera. projectional radiographs, more commonly known as x-rays, are often used to determine the type and extent of a fracture as well as for detecting pathological changes in the lungs. with the use of radio-opaque contrast media, such as barium, they can also be used to visualize the structure of the stomach and intestines – this can help diagnose ulcers or certain types of colon cancer. magnetic resonance imaging main article: magnetic resonance imaging one frame of an mri scan of the head showing the eyes and brain a magnetic resonance imaging instrument (mri scanner), or "nuclear magnetic resonance (nmr) imaging" scanner as it was originally known, uses powerful magnets to polarize and excite hydrogen nuclei (i.e., single protons) of water molecules in human tissue, producing a detectable signal which is spatially encoded, resulting in images of the body. the mri machine emits a radio frequency (rf) pulse at the resonant frequency of the hydrogen atoms on water molecules. radio frequency antennas ("rf coils") send the pulse to the area of the body to be examined. the rf pulse is absorbed by protons, causing their direction with respect to the primary magnetic field to change. when the rf pulse is turned off, the protons "relax" back to alignment with the primary magnet and emit radio-waves in the process. this radio-frequency emission from the hydrogen-atoms on water is what is detected and reconstructed into an image. the resonant frequency of a spinning magnetic dipole (of which protons are one example) is called the larmor frequency and is determined by the strength of the main magnetic field and the chemical environment of the nuclei of interest. mri uses three electromagnetic fields: a very strong (typically 1.5 to 3 teslas) static magnetic field to polarize the hydrogen nuclei, called the primary field; gradient fields that can be modified to vary in space and time (on the order of 1 khz) for spatial encoding, often simply called gradients; and a spatially homogeneous radio-frequency (rf) field for manipulation of the hydrogen nuclei to produce measurable signals, collected through an rf antenna. like ct, mri traditionally creates a two-dimensional image of a thin "slice" of the body and is therefore considered a tomographic imaging technique. modern mri instruments are capable of producing images in the form of 3d blocks, which may be considered a generalization of the single-slice, tomographic, concept. unlike ct, mri does not involve the use of ionizing radiation and is therefore not associated with the same health hazards. for example, because mri has only been in use since the early 1980s, there are no known long-term effects of exposure to strong static fields (this is the subject of some debate; see 'safety' in mri) and therefore there is no limit to the number of scans to which an individual can be subjected, in contrast with x-ray and ct. however, there are well-identified health risks associated with tissue heating from exposure to the rf field and the presence of implanted devices in the body, such as pacemakers. these risks are strictly controlled as part of the design of the instrument and the scanning protocols used. because ct and mri are sensitive to different tissue properties, the appearances of the images obtained with the two techniques differ markedly. in ct, x-rays must be blocked by some form of dense tissue to create an image, so the image quality when looking at soft tissues will be poor. in mri, while any nucleus with a net nuclear spin can be used, the proton of the hydrogen atom remains the most widely used, especially in the clinical setting, because it is so ubiquitous and returns a large signal. this nucleus, present in water molecules, allows the excellent soft-tissue contrast achievable with mri. a number of different pulse sequences can be used for specific mri diagnostic imaging (multiparametric mri or mpmri). it is possible to differentiate tissue characteristics by combining two or more of the following imaging sequences, depending on the information being sought: t1-weighted (t1-mri), t2-weighted (t2-mri), diffusion weighted imaging (dwi-mri), dynamic contrast enhancement (dce-mri), and spectroscopy (mri-s). for example, imaging of prostate tumors is better accomplished using t2-mri and dwi-mri than t2-weighted imaging alone. the number of applications of mpmri for detecting disease in various organs continues to expand, including liver studies, breast tumors, pancreatic tumors, and assessing the effects of vascular disruption agents on cancer tumors. nuclear medicine main article: nuclear medicine nuclear medicine encompasses both diagnostic imaging and treatment of disease, and may also be referred to as molecular medicine or molecular imaging and therapeutics. nuclear medicine uses certain properties of isotopes and the energetic particles emitted from radioactive material to diagnose or treat various pathology. different from the typical concept of anatomic radiology, nuclear medicine enables assessment of physiology. this function-based approach to medical evaluation has useful applications in most subspecialties, notably oncology, neurology, and cardiology. gamma cameras and pet scanners are used in e.g. scintigraphy, spect and pet to detect regions of biologic activity that may be associated with a disease. relatively short-lived isotope, such as 99mtc is administered to the patient. isotopes are often preferentially absorbed by biologically active tissue in the body, and can be used to identify tumors or fracture points in bone. images are acquired after collimated photons are detected by a crystal that gives off a light signal, which is in turn amplified and converted into count data. scintigraphy ("scint") is a form of diagnostic test wherein radioisotopes are taken internally, for example, intravenously or orally. then, gamma cameras capture and form two-dimensional images from the radiation emitted by the radiopharmaceuticals. spect is a 3d tomographic technique that uses gamma camera data from many projections and can be reconstructed in different planes. a dual detector head gamma camera combined with a ct scanner, which provides localization of functional spect data, is termed a spect-ct camera, and has shown utility in advancing the field of molecular imaging. in most other medical imaging modalities, energy is passed through the body and the reaction or result is read by detectors. in spect imaging, the patient is injected with a radioisotope, most commonly thallium 201ti, technetium 99mtc, iodine 123i, and gallium 67ga. the radioactive gamma rays are emitted through the body as the natural decaying process of these isotopes takes place. the emissions of the gamma rays are captured by detectors that surround the body. this essentially means that the human is now the source of the radioactivity, rather than the medical imaging devices such as x-ray or ct. positron emission tomography (pet) uses coincidence detection to image functional processes. short-lived positron emitting isotope, such as 18f, is incorporated with an organic substance such as glucose, creating f18-fluorodeoxyglucose, which can be used as a marker of metabolic utilization. images of activity distribution throughout the body can show rapidly growing tissue, like tumor, metastasis, or infection. pet images can be viewed