in comparison to computed tomography scans to determine an anatomic correlate. modern scanners may integrate pet, allowing pet-ct, or pet-mri to optimize the image reconstruction involved with positron imaging. this is performed on the same equipment without physically moving the patient off of the gantry. the resultant hybrid of functional and anatomic imaging information is a useful tool in non-invasive diagnosis and patient management. 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 from spect or positron emission tomography can be related to anatomical information provided by magnetic resonance imaging (mri). similarly, fiducial points established during mri can be correlated with brain images generated by magnetoencephalography to localize the source of brain activity. ultrasound main article: medical ultrasound ultrasound image showing the liver, gallbladder and common bile duct. medical ultrasound uses high frequency broadband sound waves in the megahertz range that are reflected by tissue to varying degrees to produce (up to 3d) images. this is commonly associated with imaging 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 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 main article: elastography 3d tactile image (c) is composed from 2d pressure maps (b) recorded in the process of tissue phantom examination (a). 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. 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. 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 main article: photoacoustic imaging in biomedicine 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 basic principle of tomography: superposition free tomographic cross sections s1 and s2 compared with the (not tomographic) projected image p tomography is the imaging by sections or sectioning. the main such methods in medical imaging are: x-ray computed tomography (ct), or computed axial tomography (cat) scan, is a helical tomography technique (latest generation), which traditionally produces a 2d image of the structures in a thin section of the body. in ct, a beam of x-rays spins around an object being examined and is picked up by sensitive radiation detectors after having penetrated the object from multiple angles. a computer then analyses the information received from the scanner's detectors and constructs a detailed image of the object and its contents using the mathematical principles laid out in the radon transform. it has a greater ionizing radiation dose burden than projection radiography; repeated scans must be limited to avoid health effects. ct is based on the same principles as x-ray projections but in this case, the patient is enclosed in a surrounding ring of detectors assigned with 500–1000 scintillation detectors (fourth-generation x-ray ct scanner geometry). previously in older generation scanners, the x-ray beam was paired by a translating source and detector. computed tomography has almost completely replaced focal plane tomography in x-ray tomography imaging. positron emission tomography (pet) also used in conjunction with computed tomography, pet-ct, and magnetic resonance imaging pet-mri. magnetic resonance imaging (mri) commonly produces tomographic images of cross-sections of the body. (see separate mri section in this article.) echocardiography main article: echocardiography 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. functional near-infrared spectroscopy main article: functional near-infrared spectroscopy 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. magnetic particle imaging main article: magnetic particle imagingusing 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, neuroperfusion, and cell tracking. in pregnancy ct scanning (volume rendered in this case) confers a radiation dose to the developing fetus. main article: medical imaging in pregnancy 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 radiography, ct scan and nuclear medicine imaging result some degree of ionizing radiation exposure, but have with a few exceptions much lower absorbed doses than what are associated with fetal harm. at higher dosages, effects can include miscarriage, birth defects and intellectual disability. maximizing imaging procedure use 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. 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. creation of three-dimensional images 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 scans 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 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 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: diffuse optical tomography elastography electrical impedance tomography optoacoustic imaging ophthalmology a-scan b-scan corneal topography optical coherence tomography scanning laser ophthalmoscopy some of these techniques are still at a research stage and not yet used in clinical routines. non-diagnostic imaging 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 not allowed to use in clinical research for patient diagnosis. note that many clinical research studies are not designed for patient diagnosis anyway. archiving and recording further information: picture archiving and communication system 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. dicom 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. compression of medical images 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 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, called jpip, to enable efficient streaming of the jpeg 2000 compressed image data. medical imaging in the cloud there has been 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." use in pharmaceutical clinical trials