The technique and process of imaging the interior of a body for clinical examination and medical intervention, as well as visual representation of the function of particular organs or tissues, is referred to as medical imaging (physiology). Medical imaging aims to expose hidden interior structures beneath the skin and bones, as well as diagnose and cure disease. Medical imaging also creates a database of normal anatomy and physiology, allowing abnormalities to be detected. Although medical imaging of excised organs and tissues is possible, such operations are normally classified as pathology rather than medical imaging.
It includes radiology, which uses imaging technologies such as X-ray radiography, magnetic resonance imaging, ultrasound, endoscopy, elastography, tactile imaging, thermography, and medical photography, as well as nuclear medicine functional imaging techniques such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT).
Other technologies that produce data susceptible to representation as a parameter graph versus time or maps that contain data about the measurement locations include electroencephalography (EEG), magnetoencephalography (MEG), electrocardiography (ECG), and others that are not primarily designed to produce images, such as electroencephalography (EEG), magnetoencephalography (MEG), electrocardiography (ECG), and others. These technologies can be considered forms of medical imaging in another discipline in a limited comparison.
Worldwide, 5 billion medical imaging investigations had been completed as of 2010. In 2006, medical imaging exposure accounted for almost half of all ionizing radiation exposure in the United States. 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 are all used in the manufacture of medical imaging equipment. Annual shipments of medical imaging chips totaled 46 million units and $1.1 billion as of 2015[update].
Medical imaging is commonly thought to refer to a collection of noninvasive procedures for creating images of the body's internal structures. Medical imaging can be thought of as the solution of mathematical inverse problems in this limited sense. This suggests that effect infers cause (the qualities of living tissue) (the observed signal). The probe in medical ultrasound is made up of ultrasonic pressure waves and echoes that go into the tissue and reveal the internal structure. The probe used in projectional radiography emits X-ray radiation, which is absorbed at varying rates by different tissue types like bone, muscle, and fat.
The phrase "noninvasive" refers to a method in which no instrument is inserted into a patient's body, as is the case with the majority of imaging techniques.
"Invisible light" medical imaging is commonly associated to radiography or "clinical imaging" in the clinical setting. Medical imaging using "visible light" refers to digital video or still images that may be viewed without the use of special equipment. Visible light imaging is used in dermatology and wound treatment, for example. The interpretation of medical pictures is usually done by a radiologist, but it can also be done by any healthcare worker who has been trained and qualified in radiological clinical evaluation. Non-physicians are increasingly performing interpretation; for example, radiographers frequently receive training in interpretation as part of their extended practice. The technological aspects of medical imaging, particularly the acquisition of medical pictures, are referred to as diagnostic radiography. The radiographer (also known as a radiologic technician) is usually in charge of obtaining diagnostic-quality medical images; while other professions may be trained in this area, some radiological procedures performed by radiologists are done without a radiographer.
Medical imaging, as a subject of study, is classified as a sub-discipline of biomedical engineering, medical physics, or medicine, depending on the context: Instrumentation, image acquisition (e.g., radiography), modeling, and quantification are typically the domains of biomedical engineering, medical physics, and computer science; research into the application and interpretation of medical images is typically the domain of radiology and the medical sub-discipline relevant to the medical condition or area of medical science (neuroscience, cardiology, psychiatry, psychology, and so on) under investigation. Many of the medical imaging techniques developed have scientific and industrial applications as well.
In medical imaging, two types of radiographic pictures are used. Projection radiography and fluoroscopy are both beneficial for catheter guidance, with the latter being particularly useful. Despite the advancement of 3D tomography, these 2D techniques are still widely used due to their low cost, good resolution, and, depending on the application, reduced radiation doses. The first imaging technology available in modern medicine, this imaging modality uses a wide beam of x-rays to acquire images.
A magnetic resonance imaging (MRI) scanner, also known as a "nuclear magnetic resonance imaging (NMR) imaging" scanner, uses powerful magnets to polarize and excite hydrogen nuclei (i.e., single protons) of water molecules in human tissue, resulting in a detectable signal that is spatially encoded and images of the body. A radio frequency (RF) pulse is emitted by the MRI equipment at the resonant frequency of hydrogen atoms on water molecules. The pulse is sent to the area of the body being studied through radio frequency antennas (also known as "RF coils"). Protons absorb the RF pulse, changing their orientation with respect to the fundamental magnetic field. The protons "relax" back into alignment with the primary magnet when the RF pulse is switched off, emitting radio waves in the process. The hydrogen atoms on water emit radio-frequency emission, which is recognized and reconstructed into an image. The Larmor frequency, which is determined by the strength of the main magnetic field and the chemical environment of the nuclei of interest, is the resonant frequency of a spinning magnetic dipole (of which protons are one example). 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, 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. [requires citation]
MRI, like CT, produces a two-dimensional image of a thin "slice" of the body and is thus classified as tomographic imaging technology. Modern MRI machines may provide images in the form of 3D blocks, which can be thought of as a broadening of the single-slice tomographic idea. MRI, unlike CT, does not employ ionizing radiation and, as a result, is not associated with the same health risks. Because MRI has only been in use since the early 1980s, there are no known long-term effects of exposure to powerful static fields (this is a point of contention; see 'Safety' in MRI), and hence, unlike X-ray and CT, there is no limit to the number of scans that an individual can have. However, tissue heating from exposure to the RF field and the existence of implanted devices in the body, such as pacemakers, are well-known health hazards. As part of the instrument's design and scanning methods, these dangers are rigorously controlled. [requires citation]
Because CT and MRI are sensitive to distinct tissue qualities, the images produced by the two modalities look very different. Because X-rays must be stopped by dense tissue in order to obtain a picture in CT, the image quality will be low when looking at soft tissues. While any nucleus with a net nuclear spin can be employed in MRI, the proton of the hydrogen atom is the most commonly used, especially in the clinical context, due to its widespread availability and high signal. The presence of this nucleus in water molecules enables MRI to achieve superior soft-tissue contrast. [requires citation]
For specific MRI diagnostic imaging, a variety of pulse sequences can be used (multiparametric MRI or mpMRI). Depending on the information required, two or more of the following imaging sequences can be used to identify tissue characteristics: T1-weighted (T1-MRI), T2-weighted (T2-MRI), diffusion-weighted imaging (DWI-MRI), dynamic contrast enhancement (DCE-MRI), and spectroscopy are all types of magnetic resonance imaging (MRI) (MRI-S). T2-MRI and DWI-MRI, for example, are more effective than T2-weighted imaging alone in imaging prostate cancers. The use of mpMRI to detect disease in multiple organs is growing, including liver studies, breast tumors, pancreatic tumors, and evaluating the impact of vascular disruption agents on cancer tumors.
Nuclear medicine, often known as molecular medicine or molecular imaging and therapeutics, covers both diagnostic imaging and illness therapy. Nuclear medicine makes use of isotope characteristics and energetic particles released by radioactive material to diagnose and treat a variety of diseases. Nuclear medicine, unlike traditional anatomic radiology, allows for the examination of physiology. Most subspecialties, including oncology, neurology, and cardiology, can benefit from this function-based approach to a medical examination. Scintigraphy, SPECT, and PET all use gamma cameras and PET scanners to detect regions of biologic activity that could be linked to an illness. The patient is given a relatively short-lived isotope, such as 99mTc. Isotopes are frequently absorbed preferentially by biologically active tissue in the body and can be utilized to detect malignancies or fracture spots in bone. After collimated photons are detected by a crystal, which emits a light signal, which is amplified and transformed into count data, images are recorded.
In a variety of medical imaging applications, fiduciary markers are used. By inserting a fiduciary marker in the area imaged by both systems, images of the same subject taken with two distinct imaging systems can be linked (called image registration). In this instance, a marker that can be seen in both imaging modalities' images must be utilized. Functional information from SPECT or positron emission tomography can be linked to anatomical information from magnetic resonance imaging using this method (MRI). Similarly, fiducial points established during MRI can be compared to magnetoencephalography brain images to pinpoint the source of brain activity.
Medical ultrasonography produces (up to 3D) images by using high-frequency broadband sound waves in the megahertz range that are reflected in varying degrees by tissue. This is frequently connected with fetal imaging in pregnant women. Uses of ultrasound are much broader, however. Other important uses include imaging the abdominal organs, heart, breast, muscles, tendons, arteries, and veins. While it may provide less anatomical detail than techniques such as CT or MRI, it has several advantages which make it ideal in numerous situations, in particular, that it studies the function of moving structures in real-time, emits no ionizing radiation, and contains speckle that can be used in elastography. Ultrasound is also used as a popular research tool for capturing raw data, that can be made available through an ultrasound research interface, for the purpose of tissue characterization and implementation of new image processing techniques. The concepts of ultrasound differ from other medical imaging modalities in the fact that it is operated by the transmission and receipt of sound waves. The high-frequency sound waves are sent into the tissue and depending on the composition of the different tissues; the signal will be attenuated and returned at separate intervals. A path of reflected sound waves in a multilayered structure can be defined by an input acoustic impedance (ultrasound sound wave) and the Reflection and transmission coefficients of the relative structures. It is very safe to use and does not appear to cause any adverse effects. It is also relatively inexpensive and quick to perform. Ultrasound scanners can be taken to critically ill patients in intensive care units, avoiding the danger caused while moving the patient to the radiology department. The real-time moving image obtained can be used to guide drainage and biopsy procedures. Doppler capabilities on modern scanners allow the blood flow in arteries and veins to be assessed.
Elastography is a relatively new imaging modality that maps the elastic properties of soft tissue. This modality emerged in the last two decades. Elastography is useful in medical diagnoses, as elasticity can discern healthy from unhealthy tissue for specific organs/growths. For example, cancerous tumors will often be harder than the surrounding tissue, and diseased livers are stiffer than healthy ones. There are several elastographic techniques based on the use of ultrasound, magnetic resonance imaging, and tactile imaging. The wide clinical use of ultrasound elastography is a result of the implementation of technology in clinical ultrasound machines. The main branches of ultrasound elastography include Quasistatic Elastography/Strain Imaging, Shear Wave Elasticity Imaging (SWEI), and Acoustic Radiation Force Impulse imaging (ARFI), Supersonic Shear Imaging (SSI), and Transient Elastography. In the last decade, a steady increase in activities in the field of elastography is observed demonstrating the successful application of the technology in various areas of medical diagnostics and treatment monitoring.
Photoacoustic imaging is a recently developed hybrid biomedical imaging modality based on the photoacoustic effect. It combines the advantages of optical absorption contrast with an ultrasonic spatial resolution for deep imaging in (optical) diffusive or quasi-diffusive regimes. Recent studies have shown that photoacoustic imaging can be used in vivo for tumor angiogenesis monitoring, blood oxygenation mapping, functional brain imaging, and skin melanoma detection, etc.
When ultrasound is used to image the heart it is referred to as an echocardiogram. Echocardiography allows detailed structures of the heart, including chamber size, heart function, the valves of the heart, as well as the pericardium (the sac around the heart) to be seen. Echocardiography uses 2D, 3D, and Doppler imaging to create pictures of the heart and visualize the blood flowing through each of the four heart valves. Echocardiography is widely used in an array of patients ranging from those experiencing symptoms, such as shortness of breath or chest pain, to those undergoing cancer treatments. Transthoracic ultrasound has been proven to be safe for patients of all ages, from infants to the elderly, without risk of harmful side effects or radiation, differentiating it from other imaging modalities. Echocardiography is one of the most commonly used imaging modalities in the world due to its portability and use in a variety of applications. In emergency situations, echocardiography is quick, easily accessible, and able to be performed at the bedside, making it the modality of choice for many physicians.
FNIR Is a relatively new non-invasive imaging technique. NIRS (near-infrared spectroscopy) is used for the purpose of functional neuroimaging and has been widely accepted as a brain imaging technique.
Using superparamagnetic iron oxide nanoparticles, magnetic particle imaging (MPI) is a developing diagnostic imaging technique used for tracking superparamagnetic iron oxide nanoparticles. The primary advantage is the high sensitivity and specificity, along with the lack of signal decrease with tissue depth. MPI has been used in medical research to image cardiovascular performance, neuro perfusion, and cell tracking.
Medical imaging may be indicated in pregnancy because of pregnancy complications, a pre-existing disease or an acquired disease in pregnancy, or routine prenatal care. Magnetic resonance imaging (MRI) without MRI contrast agents as well as obstetric ultrasonography are not associated with any risk for the mother or the fetus and are the imaging techniques of choice for pregnant women. Projectional radiographs, CT scans,s, and nuclear medicine imaging result in some degree of ionizing radiation exposure but have with a few exceptions much lower absorbed doses than what is associated with fetal harm. At higher dosages, effects can include miscarriage, birth defects, and intellectual disability.
The amount of data obtained in a single MR or CT scan is very extensive. Some of the data that radiologists discard could save patients time and money while reducing their exposure to radiation and the risk of complications from invasive procedures. Another approach for making the procedures more efficient is based on utilizing additional constraints, e.g., in some medical imaging modalities, one can improve the efficiency of the data acquisition by taking into account the fact the reconstructed density is positive.
Volume rendering techniques have been developed to enable CT, MRI, and ultrasound scanning software to produce 3D images for the physician. Traditionally CT and MRI scan produced 2D static output on film. To produce 3D images, many scans are made and then combined by computers to produce a 3D model, which can then be manipulated by the physician. 3D ultrasounds are produced using a somewhat similar technique. In diagnosing disease of the viscera of the abdomen, ultrasound is particularly sensitive to imaging of biliary tract, urinary tract, and female reproductive organs (ovary, fallopian tubes). For example, diagnosis of gallstone by dilatation of common bile duct and stone in the common bile duct. With the ability to visualize important structures in great detail, 3D visualization methods are a valuable resource for the diagnosis and surgical treatment of many pathologies. It was a key resource for the famous, but ultimately unsuccessful attempt by Singaporean surgeons to separate Iranian twins Ladan and Laleh Bijani in 2003. The 3D equipment was used previously for similar operations with great success.
Other proposed or developed techniques include:
Some of these techniques[example needed] are still at a research stage and not yet used in clinical routines.
Neuroimaging has also been used in experimental circumstances to allow people (especially disabled persons) to control outside devices, acting as a brain-computer interface.
Many medical imaging software applications are used for non-diagnostic imaging, specifically because they don't have an FDA approval and are not allowed to use in clinical research for patient diagnosis. Note that many clinical research studies are not designed for patient diagnosis anyway.
Used primarily in ultrasound imaging, capturing the image produced by a medical imaging device is required for archiving and telemedicine applications. In most scenarios, a frame grabber is used in order to capture the video signal from the medical device and relay it to a computer for further processing and operations.
The Digital Imaging and Communication in Medicine (DICOM) Standard is used globally to store, exchange, and transmit medical images. The DICOM Standard incorporates protocols for imaging techniques such as radiography, computed tomography (CT), magnetic resonance imaging (MRI), ultrasound, and radiation therapy.
Medical imaging techniques produce very large amounts of data, especially from CT, MRI, and PET modalities. As a result, storage and communications of electronic image data are prohibitive without the use of compression. JPEG 2000 image compression is used by the DICOM standard for the storage and transmission of medical images. The cost and feasibility of accessing large image data sets over low or various bandwidths are further addressed by the use of another DICOM standard, called JPIP, to enable efficient streaming of the JPEG 2000 compressed image data.
There has been a growing trend to migrate from on-premise PACS to a cloud-based PACS. A recent article by Applied Radiology said, "As the digital imaging realm is embraced across the healthcare enterprise, the swift transition from terabytes to petabytes of data has put radiology on the brink of information overload. Cloud computing offers the imaging department of the future the tools to manage data much more intelligently."
Medical imaging has become a major tool in clinical trials since it enables rapid diagnosis with visualization and quantitative assessment.
A typical clinical trial goes through multiple phases and can take up to eight years. Clinical endpoints or outcomes are used to determine whether the therapy is safe and effective. Once a patient reaches the endpoint, he or she is generally excluded from further experimental interaction. Trials that rely solely on clinical endpoints are very costly as they have longer durations and tend to need large numbers of patients.
In contrast to clinical endpoints, surrogate endpoints have been shown to cut down the time required to confirm whether a drug has clinical benefits. Imaging biomarkers (a characteristic that is objectively measured by an imaging technique, which is used as an indicator of pharmacological response to therapy) and surrogate endpoints have been shown to facilitate the use of small group sizes, obtaining quick results with good statistical power.
Imaging is able to reveal subtle change that is indicative of the progression of therapy that may be missed out by more subjective, traditional approaches. Statistical bias is reduced as the findings are evaluated without any direct patient contact.
Imaging techniques such as positron emission tomography (PET) and magnetic resonance imaging (MRI) are routinely used in oncology and neuroscience areas,. For example, the measurement of tumor shrinkage is a commonly used surrogate endpoint in solid tumor response evaluation. This allows for a faster and more objective assessment of the effects of anticancer drugs. In Alzheimer's disease, MRI scans of the entire brain can accurately assess the rate of hippocampal atrophy, while PET scans can measure the brain's metabolic activity by measuring regional glucose metabolism, and beta-amyloid plaques using tracers such as Pittsburgh compound B (PiB). Historically less use has been made of quantitative medical imaging in other areas of drug development although interest is growing.
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