Tomosynthesis

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Tomosynthesis of a lung with chronic fibrosing pulmonary aspergillosis. Tomosynthesis of chronic fibrosing pulmonary aspergillosis.gif
Tomosynthesis of a lung with chronic fibrosing pulmonary aspergillosis.

Tomosynthesis, also digital tomosynthesis (DTS), is a method for performing high-resolution limited-angle tomography at radiation dose levels comparable with projectional radiography. It has been studied for a variety of clinical applications, including vascular imaging, dental imaging, orthopedic imaging, mammographic imaging, musculoskeletal imaging, and chest imaging. [1]

Contents

History

The concept of tomosynthesis was derived from the work of Ziedses des Plantes, who developed methods of reconstructing an arbitrary number of planes from a set of projections. Though this idea was displaced by the advent of computed tomography, tomosynthesis later gained interest as a low-dose tomographic alternative to CT. [2]

Reconstruction

Tomosynthesis reconstruction algorithms are similar to CT reconstructions, in that they are based on performing an inverse Radon transform. Due to partial data sampling with very few projections, approximation algorithms have to be used. Filtered back projection and iterative, expectation-maximization algorithms have both been used to reconstruct the data. [3]

Reconstruction algorithms for tomosynthesis are different from those of conventional CT because the conventional filtered back projection algorithm requires a complete set of data. Iterative algorithms based upon expectation maximization are most commonly used, but are computationally intensive. Some manufacturers have produced practical systems using off-the-shelf GPUs to perform the reconstruction in a few seconds.

Differences from other imaging modalities

Digital tomosynthesis combines digital image capture and processing with simple tube/detector motion as used in conventional computed tomography (CT). However, though there are some similarities to CT, it is a separate technique. In modern (helical) CT, the source/detector makes at least a complete 180-degree rotation about the subject obtaining a complete set of data from which images may be reconstructed. Digital tomosynthesis, on the other hand, only uses a limited rotation angle (e.g., 15-60 degrees) with a lower number of discrete exposures (e.g., 7-51) than CT. This incomplete set of projections is digitally processed to yield images similar to conventional tomography with a limited depth of field. Because the image processing is digital, a series of slices at different depths and with different thicknesses can be reconstructed from the same acquisition. However, since fewer projections are needed than CT to perform the reconstruction, radiation exposure and cost are both reduced. [4]

Applications

Breast

Digital breast tomosynthesis (DBT) [5] is Food and Drug Administration (FDA) approved for use in breast cancer screening. [6] The benefit for screening has been debated, [7] but consensus is being reached that the technology is improving sensitivity compared to two-view digital mammography at the cost of a slightly reduced specificity (increased recall rates). [8] Because the data acquired are 85 - 160 micron typical resolution, much higher than CT, DBT is unable to offer the narrow slice widths that CT offers (typically 1-1.5 mm). However, the higher resolution detectors permit very high in-plane resolution, even if the Z-axis resolution is less. Another interesting property of breast tomosynthesis is that image quality may vary substantially through the imaging volume. [9]

Photon-counting breast tomosynthesis has been investigated, [10] and spectral imaging applications, such as breast density measurement and lesion characterization, [11] [12] have been investigated on that platform.

Chest

Musculoskeletal imaging

Tomosynthesis has a much more limited depth of field than does CT. For this reason, it likely will not be able to replace CT for the evaluation of the deeper organs of the body. However, since bones are often near the skin, multiple musculoskeletal applications of tomosynthesis have been studied, most of which have mostly been used in research with limited use in everyday practice.

Evaluation of fractures

Tomosynthesis has been compared to both radiography and CT for the evaluation of healing fractures, especially in the presence of hardware. In a study of patients with wrist fractures, digital tomosynthesis was shown to enable detection of more fractures than radiography while simultaneously providing lower metal artifact than radiography. [4]

Evaluation of erosions in rheumatoid arthritis

Tomosynthesis has been compared to digital radiography, with CT as the standard, for the detection of erosions associated with rheumatoid arthritis. The radiation dose of digital tomosynthesis was very close to that of digital radiography. However, tomosynthesis showed sensitivity, specificity, accuracy, positive predictive value, and negative predictive value of 80%, 75%, 78%, 76%, and 80%, compared to digital radiography were 66%, 81%, 74%, 77%, and 71%. [13] The slight benefit digital tomosynthesis in this application may or may not justify the slightly increased cost of the modality compared to digital radiography.

Electronics

Tomosynthesis is also used for x-ray inspection of electronics, [14] particularly printed circuit board assemblies and electronic components. Tomosynthesis is usually used where a CT slice is required at high magnification, where conventional CT would not allow the sample to be located close enough to the x-ray source.

Related Research Articles

Positron emission tomography Medical imaging technique

Positron emission tomography (PET) is a functional imaging technique that uses radioactive substances known as radiotracers to visualize and measure changes in metabolic processes, and in other physiological activities including blood flow, regional chemical composition, and absorption. Different tracers are used for various imaging purposes, depending on the target process within the body. For example, 18F-FDG is commonly used to detect cancer, NaF-F18 is widely used for detecting bone formation, and oxygen-15 is sometimes used to measure blood flow.

CT scan Medical imaging procedure using X-rays to produce cross-sectional images

A CT scan or computed tomography scan is a medical imaging technique that uses computer-processed combinations of multiple X-ray measurements taken from different angles to produce tomographic (cross-sectional) images of a body, allowing the user to see inside the body without cutting. The personnel that perform CT scans are called radiographers or radiologic technologists.

Radiography Imaging technique using ionizing and non-ionizing radiation

Radiography is an imaging technique using X-rays, gamma rays, or similar ionizing radiation and non-ionizing radiation to view the internal form of an object. Applications of radiography include medical radiography and industrial radiography. Similar techniques are used in airport security. To create an image in conventional radiography, a beam of X-rays is produced by an X-ray generator and is projected toward the object. A certain amount of the X-rays or other radiation is absorbed by the object, dependent on the object's density and structural composition. The X-rays that pass through the object are captured behind the object by a detector. The generation of flat two dimensional images by this technique is called projectional radiography. In computed tomography an X-ray source and its associated detectors rotate around the subject which itself moves through the conical X-ray beam produced. Any given point within the subject is crossed from many directions by many different beams at different times. Information regarding attenuation of these beams is collated and subjected to computation to generate two dimensional images in three planes which can be further processed to produce a three dimensional image.

Mammography Process of using low-energy X-rays to examine the human breast for diagnosis and screening

Mammography is the process of using low-energy X-rays to examine the human breast for diagnosis and screening. The goal of mammography is the early detection of breast cancer, typically through detection of characteristic masses or microcalcifications.

Single-photon emission computed tomography Nuclear medicine tomographic imaging technique

Single-photon emission computed tomography is a nuclear medicine tomographic imaging technique using gamma rays. It is very similar to conventional nuclear medicine planar imaging using a gamma camera, but is able to provide true 3D information. This information is typically presented as cross-sectional slices through the patient, but can be freely reformatted or manipulated as required.

Tomography Imaging by sections or sectioning using a penetrative wave

Tomography is imaging by sections or sectioning through the use of any kind of penetrating wave. The method is used in radiology, archaeology, biology, atmospheric science, geophysics, oceanography, plasma physics, materials science, astrophysics, quantum information, and other areas of science. The word tomography is derived from Ancient Greek τόμος tomos, "slice, section" and γράφω graphō, "to write" or, in this context as well, " to describe." A device used in tomography is called a tomograph, while the image produced is a tomogram.


Computed tomography laser mammography (CTLM) is the trademark of Imaging Diagnostic Systems, Inc. for its optical tomographic technique for female breast imaging.

Projectional radiography Formation of 2D images using X-rays

Projectional radiography, also known as conventional radiography, is a form of radiography and medical imaging that produces two-dimensional images by x-ray radiation. The image acquisition is generally performed by radiographers, and the images are often examined by radiologists. Both the procedure and any resultant images are often simply called "X-ray". Plain radiography or roentgenography generally refers to projectional radiography. Plain radiography can also refer to radiography without a radiocontrast agent or radiography that generates single static images, as contrasted to fluoroscopy, which are technically also projectional.

Image-guided radiation therapy is the process of frequent two and three-dimensional imaging, during a course of radiation treatment, used to direct radiation therapy utilizing the imaging coordinates of the actual radiation treatment plan. The patient is localized in the treatment room in the same position as planned from the reference imaging dataset. An example of IGRT would include localization of a cone beam computed tomography (CBCT) dataset with the planning computed tomography (CT) dataset from planning. IGRT would also include matching planar kilovoltage (kV) radiographs or megavoltage (MV) images with digital reconstructed radiographs (DRRs) from the planning CT. These two methods comprise the bulk of IGRT strategies currently employed circa 2013.

Automatic exposure control

Automatic Exposure Control (AEC) is an X-ray exposure termination device. A medical radiographic exposure is always initiated by a human operator but an AEC detector system may be used to terminate the exposure when a predetermined amount of radiation has been received. The intention of AEC is to provide consistent x-ray image exposure, whether to film, a digital detector or a CT scanner. AEC systems may also automatically set exposure factors such as the X-ray tube current and voltage in a CT.

Flat-panel detector Class of solid-state x-ray digital radiography devices

Flat-panel detectors are a class of solid-state x-ray digital radiography devices similar in principle to the image sensors used in digital photography and video. They are used in both projectional radiography and as an alternative to x-ray image intensifiers (IIs) in fluoroscopy equipment.

Cone beam computed tomography

Cone beam computed tomography is a medical imaging technique consisting of X-ray computed tomography where the X-rays are divergent, forming a cone.

Positron emission mammography Imaging procedure used to detect breast cancer

Positron emission mammography (PEM) is a nuclear medicine imaging modality used to detect or characterise breast cancer. Mammography typically refers to x-ray imaging of the breast, while PEM uses an injected positron emitting isotope and a dedicated scanner to locate breast tumors. Scintimammography is another nuclear medicine breast imaging technique, however it is performed using a gamma camera. Breasts can be imaged on standard whole-body PET scanners, however dedicated PEM scanners offer advantages including improved resolution.

Photon counting

Photon counting is a technique in which individual photons are counted using a single-photon detector (SPD). In contrast to a normal photodetector, which generates an analog signal proportional to the photon flux, a single-photon detector emits a pulse of signal every time a photon is detected. The total number of pulses is counted, giving an integer number of photons detected per measurement period. The counting efficiency is determined by the quantum efficiency and any electronic losses that are present in the system.

Breast imaging

In medicine, breast imaging is a sub-speciality of diagnostic radiology that involves imaging of the breasts for screening or diagnostic purposes. There are various methods of breast imaging using a variety of technologies as described in detail below. Traditional screening and diagnostic mammography uses x-ray technology. Breast tomosynthesis is a new digital mammography technique that produces 3D images of the breast using x-rays. Xeromammography and Galactography also use x-ray technology and are also used infrequently in the detection of breast cancer. Breast ultrasound is another technology employed in diagnosis & screening and specifically can help differentiate between fluid filled and solid lumps that can help determine if cancerous. Breast MRI is, yet, another technology reserved for high-risk patients and can help determine the extent of cancer if diagnosed. Lastly, scintimammography is used in a subgroup of patients who have abnormal mammograms or whose screening is not reliable on the basis of using traditional mammography or ultrasound.

4DCT

Four-dimensional computed tomography (4DCT) is a type of CT scanning which records multiple images over time. It allows playback of the scan as a video, so that physiological processes can be observed and internal movement can be tracked. The name is derived from the addition of time to traditional 3D computed tomography. Alternatively, the phase of a particular process, such as respiration, may be considered the fourth dimension.

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Photon-counting computed tomography (CT) is a computed tomography technique currently under research and development, both within academia and by major vendors of CT systems. Photon-counting CT has the potential both to offer significant improvements to existing CT imaging techniques and to make possible completely novel applications. A photon-counting CT system employs a photon-counting detector (PCD) which registers the interactions of individual photons. By keeping track of the deposited energy in each interaction, the detector pixels of a PCD each record an approximate energy spectrum, making it a spectral or energy-resolved CT technique. In contrast, typical CT scanners use energy-integrating detectors (EIDs), where the total energy deposited in a pixel during a fixed period of time is registered. Typical CT detectors thus register only photon intensity, comparable to black-and-white photography, whereas photon-counting detectors register also spectral information, similar to colour photography.

Spectral imaging is an umbrella term for energy-resolved X-ray imaging in medicine. The technique makes use of the energy dependence of X-ray attenuation to either increase the contrast-to-noise ratio, or to provide quantitative image data and reduce image artefacts by so-called material decomposition. Dual-energy imaging, i.e. imaging at two energy levels, is a special case of spectral imaging and is still the most widely used terminology, but the terms "spectral imaging" and "spectral CT" have been coined to acknowledge the fact that photon-counting detectors have the potential for measurements at a larger number of energy levels.

Photon-counting mammography was introduced commercially in 2003 and was the first widely available application of photon-counting detector technology in medical x-ray imaging. Photon-counting mammography improves dose efficiency compared to conventional technologies, and enables spectral imaging.

References

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  11. Fredenberg, Erik; Berggren, Karl; Bartels, Matthias; Erhard, Klaus (2016), Tingberg, Anders; Lång, Kristina; Timberg, Pontus (eds.), "Volumetric Breast-Density Measurement Using Spectral Photon-Counting Tomosynthesis: First Clinical Results", Breast Imaging, Cham: Springer International Publishing, 9699, pp. 576–584, doi:10.1007/978-3-319-41546-8_72, ISBN   978-3-319-41545-1 , retrieved 2020-12-30
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