| Medical imaging | |
|---|---|
 One frame of aCT scan of the chest showing the heart and lungs | |
| ICD-10-PCS | B |
| ICD-9 | 87-88 |
| MeSH | 003952 D 003952 |
| OPS-301 code | 3 |
| MedlinePlus | 007451 |
Medical imaging is the technique and process ofimaging 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 treatdisease. Medical imaging also establishes a database of normalanatomy andphysiology to make it possible to identify abnormalities. Although imaging of removedorgans andtissues can be performed for medical reasons, such procedures are usually considered part ofpathology instead of medical imaging.[citation needed]
Measurement and recording techniques that are not primarily designed to produceimages, such aselectroencephalography (EEG),magnetoencephalography (MEG),electrocardiography (ECG), and others, represent other technologies that produce data susceptible to representation as a parameter graph versus time ormaps that contain data about the measurement locations. In a limited comparison, these technologies can be considered forms of medical imaging in another discipline ofmedical instrumentation.
As of 2010, 5 billion medical imaging studies had been conducted worldwide.[1] Radiation exposure from medical imaging in 2006 made up about 50% of total ionizing radiation exposure in the United States.[2] Medical imaging equipment is manufactured using technology from thesemiconductor industry, includingCMOSintegrated circuit chips,power semiconductor devices,sensors such asimage sensors (particularlyCMOS sensors) andbiosensors, and processors such asmicrocontrollers,microprocessors,digital signal processors,media processors andsystem-on-chip devices. As of 2015[update], annual shipments of medical imaging chips amount to 46 million units and$1.1 billion.[3]
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.
In 1972, engineer Godfrey Hounsfield from the British company EMI invented the X-ray computed tomography (CT) device for head diagnosis, which is commonly referred to as CT (computed tomography). The CT nucleus method is based on the projecting X-rays through a section of the human head, which are then processed by computer to reconstruct the cross-sectional image, known as image reconstruction. In 1975, EMI successfully developed a CT device for the entire body, enabling the clear acquisition of tomographic images of various parts of the human body. This revolutionary diagnostic technique earned Hounsfield and physicist Allan Cormack the Nobel Prize in Physiology or Medicine in 1979.[4] Digital image processing technology for medical applications was inducted into theSpace Foundation's Space Technology Hall of Fame in 1994.[5]
By 2010, over 5 billion medical imaging studies had been conducted worldwide.[6][7] Radiation exposure from medical imaging in 2006 accounted for about 50% of total ionizing radiation exposure in the United States.[8] Medical imaging equipment is manufactured using technology from thesemiconductor industry, includingCMOSintegrated circuit chips,power semiconductor devices,sensors such asimage sensors (particularlyCMOS sensors) andbiosensors, as well as processors likemicrocontrollers,microprocessors,digital signal processors,media processors andsystem-on-chip devices. As of 2015[update], annual shipments of medical imaging chips reached 46 million units, generating a market value of$1.1 billion.[9][10]
In the clinical context, "invisible light" medical imaging is generally equated toradiology 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 aradiologist; 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 exampleradiographers frequently train in interpretation as part of expanded practice. Diagnosticradiography 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.[citation needed]
As a field of scientific investigation, medical imaging constitutes a sub-discipline ofbiomedical engineering,medical physics ormedicine 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, andcomputer 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 havescientific andindustrial applications.[11]
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 ofX-rays for image acquisition and is the first imaging technique available in modern medicine.
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 excitehydrogen nuclei (i.e., singleprotons) of water molecules in human tissue, producing a detectable signal which is spatially encoded, resulting in images of the body.[12] 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 theLarmor frequency and is determined by the strength of the main magnetic field and the chemical environment of the nuclei of interest. MRI uses threeelectromagnetic fields: a very strong (typically 1.5 to 3teslas) 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 homogeneousradio-frequency (RF) field for manipulation of the hydrogen nuclei to produce measurable signals, collected through anRF antenna.[citation needed]
LikeCT, MRI traditionally creates a two-dimensional image of a thin "slice" of the body and is therefore considered atomographic 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 ofionizing 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' inMRI) and therefore there is no limit to the number of scans to which an individual can be subjected, in contrast withX-ray andCT. 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.[citation needed]
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.[13][citation needed]
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.[14] The number of applications of mpMRI for detecting disease in various organs continues to expand, includingliver studies,breast tumors,pancreatic tumors, and assessing the effects ofvascular disruption agents on cancer tumors.[15][16][17]
Nuclear medicine encompasses both diagnostic imaging and treatment of disease, and may also be referred to as molecular medicine or molecular imaging and therapeutics.[18] 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 andPET 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-livedisotope, such as99mTc is administered to the patient. Isotopes are often preferentially absorbed by biologically active tissue in the body, and can be used to identify tumors orfracture 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.
Fiduciary markers are used in a wide range of medical imaging applications. Images of the same subject produced with two different imaging systems may be correlated (called image registration) by placing a fiduciary marker in the area imaged by both systems. In this case, a marker which is visible in the images produced by both imaging modalities must be used. By this method, functional information fromSPECT orpositron emission tomography can be related to anatomical information provided bymagnetic resonance imaging (MRI).[21] Similarly, fiducial points established during MRI can be correlated with brain images generated bymagnetoencephalography to localize the source of brain activity.
Medical ultrasound uses high frequencybroadband sound waves in themegahertz range that are reflected by tissue to varying degrees to produce (up to 3D) images. This is commonly associated withimaging the fetus 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 noionizing radiation, and containsspeckle that can be used inelastography. Ultrasound is also used as a popular research tool for capturing raw data, that can be made available through anultrasound 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.[20] 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 tumours will often be harder than the surrounding tissue, and diseased livers are stiffer than healthy ones.[22][23][24][25] 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. Main branches of ultrasound elastography include Quasistatic Elastography/Strain Imaging, Shear Wave Elasticity Imaging (SWEI), Acoustic Radiation Force Impulse imaging (ARFI), Supersonic Shear Imaging (SSI), and Transient Elastography.[23] In the last decade, a steady increase of activities in the field of elastography is observed demonstrating 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 regime. 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.
Tomography is the imaging by sections or sectioning. The main such methods in medical imaging are:
When ultrasound is used to image the heart it is referred to as anechocardiogram. 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, andDoppler 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 offunctional neuroimaging and has been widely accepted as abrain imaging technique.[26]
Usingsuperparamagnetic iron oxide nanoparticles, magnetic particle imaging (MPI) is a developing diagnostic imaging technique used for trackingsuperparamagneticiron oxidenanoparticles. The primary advantage is the highsensitivity and specificity, along with the lack of signal decrease with tissue depth. MPI has been used in medical research to imagecardiovascular performance,neuroperfusion, and cell tracking.
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Medical imaging may beindicated inpregnancy because ofpregnancy complications, apre-existing disease or an acquired disease in pregnancy, or routineprenatal care.Magnetic resonance imaging (MRI) withoutMRI contrast agents as well asobstetric ultrasonography are not associated with any risk for the mother or the fetus, and are the imaging techniques of choice for pregnant women.[27]Projectional radiography,CT scan andnuclear medicine imaging result some degree ofionizing radiation exposure, but have with a few exceptions much lowerabsorbed doses than what are associated with fetal harm.[27] At higher dosages, effects can includemiscarriage,birth defects andintellectual disability.[27]
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 risk of complications from invasive procedures.[28] 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.[29][30]
Volume rendering techniques have been developed to enable CT, MRI and ultrasound scanning software to produce 3D images for the physician.[31] Traditionally CT and MRI scans produced 2D static output on film. To produce 3D images, many scans are made and thencombined 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 on imaging of biliary tract, urinary tract and female reproductive organs (ovary, fallopian tubes). As 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 twinsLadan 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 abrain computer interface.
Many medical imaging software applications are used for non-diagnostic imaging, specifically because they do not have an FDA approval[32] and not allowed to use inclinical research for patient diagnosis.[33] Note that manyclinical research studies are not designed for patient diagnosis anyway.[34]
Used primarily inultrasound imaging, capturing the image produced by a medical imaging device is required for archiving andtelemedicine applications. In most scenarios, aframe 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.[35]
TheDigital 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.[36]
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.[37][38]JPEG 2000 image compression is used by theDICOM standard for 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 use of another DICOM standard, calledJPIP, to enable efficient streaming of theJPEG 2000 compressed image data.
There has been growing trend to migrate from on-premisePACS to acloud-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 ofinformation overload. Cloud computing offers the imaging department of the future the tools to manage data much more intelligently."[39]
Medical imaging has become a major tool in clinical trials since it enables rapid diagnosis with visualization and quantitative assessment.
A typicalclinical 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 onclinical endpoints are very costly as they have long 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. Imagingbiomarkers (a characteristic that is objectively measured by an imaging technique, which is used as an indicator of pharmacological response to a therapy) and surrogate endpoints have shown to facilitate the use of small group sizes, obtaining quick results with good statistical power.[40]
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 aspositron emission tomography (PET) andmagnetic resonance imaging (MRI) are routinely used in oncology and neuroscience areas.[41][42][43][44] For example, measurement oftumour shrinkage is a commonly used surrogate endpoint in solid tumour response evaluation. This allows for faster and more objective assessment of the effects of anticancer drugs. InAlzheimer's disease,MRI scans of the entire brain can accurately assess the rate of hippocampal atrophy,[45][46] while PET scans can measure the brain's metabolic activity by measuring regional glucose metabolism,[40] and beta-amyloid plaques using tracers such asPittsburgh compound B (PiB). Historically less use has been made of quantitative medical imaging in other areas ofdrug development although interest is growing.[47]
An imaging-based trial will usually be made up of three components:
Medical imaging can lead to patient and healthcare provider harm through exposure toionizing radiation,iodinated contrast,magnetic fields, and other hazards.[48]
Lead is the main material used forradiographic shielding against scattered X-rays.
Inmagnetic resonance imaging, there isMRI RF shielding as well asmagnetic shielding to prevent external disturbance of image quality.[49]
Medical imaging are generally covered by laws ofmedical privacy. For example, in the United States theHealth Insurance Portability and Accountability Act (HIPAA) sets restrictions for health care providers on utilizingprotected health information, which is any individually identifiable information relating to the past, present, or future physical or mental health of any individual.[50] While there has not been any definitive legal decision in the matter, at least one study has indicated that medical imaging may contain biometric information that can uniquely identify a person, and so may qualify as PHI.[51]
The UK General Medical Council's ethical guidelines indicate that the Council does not require consent prior to making recordings of X-ray images.[52] However, the same guidance indicates that the images and recordings need to be anonimized, and acknowledges that in deciding whether a recording is anonymised, one should bear in mind that apparently insignificant details may still be capable of identifying a patient. As such, one should be particularly careful about the anonymity of a recordings of an X-ray image before using or publishing them without consent in journals and other learning materials, whether they are printed or in an electronic format.[53]
Organizations in the medical imaging industry include manufacturers of imaging equipment, freestanding radiology facilities, and hospitals.
The global market for manufactured devices was estimated at $5 billion in 2018.[54][55] Notable manufacturers as of 2012 includedFujifilm,GE HealthCare,Siemens Healthineers,Philips,Shimadzu,Toshiba,Carestream Health,Hitachi,Hologic, andEsaote.[56] In 2016, the manufacturing industry was characterized as oligopolistic and mature; new entrants included inSamsung andNeusoft Medical.[57]
In the United States, as estimate as of 2015 places the US market for imaging scans at about $100b, with 60% occurring in hospitals and 40% occurring in freestanding clinics, such as theRadNet chain.[58]
As per chapter 300 of theCompendium of U.S. Copyright Office Practices, "the Office will not register works produced by a machine or mere mechanical process that operates randomly or automatically without any creative input or intervention from a human author" including "Medical imaging produced by X-rays, ultrasounds, magnetic resonance imaging, or other diagnostic equipment."[59] This position differs from the broad copyright protections afforded to photographs. While the Copyright Compendium is an agency statutory interpretation and not legally binding, courts are likely to give deference to it if they find it reasonable.[60] Yet, there is no U.S. federal case law directly addressing the issue of the copyrightability of X-ray images.
An extensive definition of the termderivative work is given by the United States Copyright Act in17 U.S.C. § 101:
A "derivative work" is a work based upon one or more preexisting works, such as a translation...[note 1] art reproduction, abridgment, condensation, or any other form in which a work may be recast, transformed, or adapted. A work consisting of editorial revisions, annotations, elaborations, or other modifications which, as a whole, represent an original work of authorship, is a "derivative work".
The copyright in a compilation or derivative work extends only to the material contributed by the author of such work, as distinguished from the preexisting material employed in the work, and does not imply any exclusive right in the preexisting material. The copyright in such work is independent of, and does not affect or enlarge the scope, duration, ownership, or subsistence of, any copyright protection in the preexisting material.
In Germany,X-ray images as well asMRI,medical ultrasound,PET andscintigraphy images are protected by (copyright-like)related rights or neighbouring rights.[61] This protection does not require creativity (as would be necessary forregular copyright protection) and lasts only for 50 years after image creation, if not published within 50 years, or for 50 years after the first legitimate publication.[62] The letter of the law grants this right to the "Lichtbildner",[63] i.e. the person who created the image. The literature seems to uniformly consider the medical doctor, dentist or veterinary physician as the rights holder, which may result from the circumstance that in Germany many X-rays are performed in ambulatory settings.
Medical images created in the United Kingdom will normally be protected by copyright due to "the high level of skill, labour and judgement required to produce a good quality X-ray, particularly to show contrast between bones and various soft tissues".[64] The Society of Radiographers believe this copyright is owned by employer (unless the radiographer is self-employed—though even then their contract might require them to transfer ownership to the hospital). This copyright owner can grant certain permissions to whoever they wish, without giving up their ownership of the copyright. So the hospital and its employees will be given permission to use such radiographic images for the various purposes that they require for medical care. Physicians employed at the hospital will, in their contracts, be given the right to publish patient information in journal papers or books they write (providing they are made anonymous). Patients may also be granted permission to "do what they like with" their own images.
TheCyber Law in Sweden states: "Pictures can be protected as photographic works or as photographic pictures. The former requires a higher level of originality; the latterprotects all types of photographs, also the ones taken by amateurs, orwithin medicine or science. The protection requires some sort of photographic technique being used, which includes digital cameras as well as holograms created by laser technique. The difference between the two types of work is the term of protection, which amounts to seventy years after the death of the author of a photographic work as opposed to fifty years, from the year in which the photographic picture was taken."[65]
Medical imaging may possibly be included in the scope of "photography", similarly to a U.S. statement that "MRI images, CT scans, and the like are analogous to photography."[66]
:1 was invoked but never defined (see thehelp page).Medical imaging systems produce increasingly accurate images with improved quality using higher spatial resolutions and color bit-depth. Such improvements increase the amount of information that needs to be stored, processed, and transmitted.
Because of the large amount of medical imaging data, the transmission process becomes complicated in telemedicine applications. Thus, in order to adapt the data bit streams to the constraints related to the limitation of the bandwidths a reduction of the size of the data by compression of the images is essential.
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