What is Segmentation (image processing)?

In computer vision, segmentation refers to the process of partitioning a digital image into multiple regions (sets of pixels). The goal of segmentation is to simplify and/or change the representation of an image into something that is more meaningful and easier to analyze.Image segmentation is typically used to locate objects and boundaries (lines, curves, etc.) in images.
The result of image segmentation is a set of regions that collectively cover the entire image, or a set of contours extracted from the image (see edge detection). Each of the pixels in a region are similar with respect to some characteristic or computed property, such as color, intensity, or texture. Adjacent regions are significantly different with respect to the same characteristic(s).

3D rendering techniques of CT

Surface rendering
A threshold value of radiodensity is chosen by the operator (e.g. a level that corresponds to bone). A threshold level is set, using edge detection image processing algorithms. From this, a 3-dimensional model can be constructed and displayed on screen. Multiple models can be constructed from various different thresholds, allowing different colors to represent each anatomical component such as bone, muscle, and cartilage. However, the interior structure of each element is not visible in this mode of operation.
Volume rendering
Surface rendering is limited in that it will only display surfaces which meet a threshold density, and will only display the surface that is closest to the imaginary viewer. In volume rendering, transparency and colors are used to allow a better representation of the volume to be shown in a single image - e.g. the bones of the pelvis could be displayed as semi-transparent, so that even at an oblique angle, one part of the image does not conceal another.
Image segmentation
Where different structures have similar radiodensity, it can become impossible to separate them simply by adjusting volume rendering parameters. The solution is called segmentation, a manual or automatic procedure that can remove the unwanted structures from the image.

Three dimensional (3D) image reconstruction

The principle
Because contemporary CT scanners offer isotropic, or near isotropic, resolution, display of images does not need to be restricted to the conventional axial images. Instead, it is possible for a software program to build a volume by 'stacking' the individual slices one on top of the other. The program may then display the volume in an alternative manner.

Multiplanar reconstruction

Typical screen layout for diagnostic software, showing one 3D and three MPR views
Multiplanar reconstruction (MPR) is the simplest method of reconstruction. A volume is built by stacking the axial slices. The software then cuts slices through the volume in a different plane (usually orthogonal). Optionally, a special projection method, such as maximum-intensity projection (MIP) or minimum-intensity projection (mIP), can be used to build the reconstructed slices.
MPR is frequently used for examining the spine. Axial images through the spine will only show one vertebral body at a time and cannot reliably show the intervertebral discs. By reformatting the volume, it becomes much easier to visualise the position of one vertebral body in relation to the others.
Modern software allows reconstruction in non-orthogonal (oblique) planes so that the optimal plane can be chosen to display an anatomical structure. This may be particularly useful for visualising the structure of the bronchi as these do not lie orthogonal to the direction of the scan.
For vascular imaging, curved-plane reconstruction can be performed. This allows bends in a vessel to be 'straightened' so that the entire length can be visualised on one image, or a short series of images. Once a vessel has been 'straightened' in this way, quantitative measurements of length and cross sectional area can be made, so that surgery or interventional treatment can be planned.
MIP reconstructions enhance areas of high radiodensity, and so are useful for angiographic studies. mIP reconstructions tend to enhance air spaces so are useful for assessing lung structure.

What are the Artifacts of CT?

Although CT is a relatively accurate test, it is liable to produce artifacts, such as the following.

Example of Beam Hardening
Aliasing Artifact or Streaks
These appear as dark lines which radiate away from sharp corners. It occurs because it is impossible for the scanner to 'sample' or take enough projections of the object, which is usually metallic. It can also occur when an insufficient X-ray tube current is selected, and insufficient penetration of the x-ray occurs. These artifacts are also closely tied to motion during a scan. This type of artifact commonly occurs in head images around the pituitary fossa area.
Partial Volume Effect
This appears as 'blurring' over sharp edges. It is due to the scanner being unable to differentiate between a small amount of high-density material (e.g. bone) and a larger amount of lower density (e.g. cartilage). The processor tries to average out the two densities or structures, and information is lost. This can be partially overcome by scanning using thinner slices.
Ring Artifact
Probably the most common mechanical artifact, the image of one or many 'rings' appears within an image. This is usually due to a detector fault.
Noise Artifact
This appears as graining on the image and is caused by a low signal to noise ratio. This occurs more commonly when a thin slice thickness is used. It can also occur when the kV or mA of the X-ray tube is insufficient to penetrate the anatomy.
Motion Artifact
This is seen as blurring and/or streaking which is caused by movement of the object being imaged.
Windmill
Streaking appearances can occur when the detectors intersect the reconstruction plane. This can be reduced with filters or a reduction in pitch.
Beam Hardening
This can give a 'cupped appearance'. It occurs when there is more attenuation in the center of the object than around the edge. This is easily corrected by filtration and software.

Definition of Low-Dose CT Scan

The main issue within radiology today is how to reduce the radiation dose during CT examinations without compromising the image quality. Generally, a high radiation dose results in high quality images. A lower dose leads to increased image noise and results in unsharp images. Unfortunately, as the radiation dose increases, so does the associated risk of radiation induced cancer. However, there are several methods that can be used in order to lower the exposure to ionizing radiation during a CT scan.
New software technology can significantly reduce the radiation dose. The software works as a filter which reduces random noise and enhances structures. In that way, it is possible to get high quality images and at the same time lower the dose by as much as 30 to 70 percent.
Individualize the examination and adjust the radiation dose to the body type and body organ examined. Different body types and organs require different amounts of radiation.
Prior to every CT examination, evaluate the appropriateness of the exam whether it’s motivated or if another type of examination is more suitable.

Adverse reactions to contrast agents

Because CT scans rely on intravenously administered contrast agents in order to provide superior image quality, there is a low but non-negligible level of risk associated with the contrast agents themselves. Certain patients may experience severe and potentially life-threatening allergic reactions to the contrast dye.
The contrast agent may also induce kidney damage. The risk of this is increased with patients who have preexisting renal insufficiency, preexisting diabetes, or reduced intravascular volume. In general, if a patient has normal kidney function, then the risks of contrast nephropathy are negligible. Patients with mild kidney impairment are usually advised to ensure full hydration for several hours before and after the injection. For moderate kidney failure, the use of iodinated contrast should be avoided; this may mean using an alternative technique instead of CT e.g. MRI. Perhaps paradoxically, patients with severe renal failure requiring dialysis do not require special precautions, as their kidneys have so little function remaining that any further damage would not be noticeable and the dialysis will remove the contrast agent.

Radiation exposure

CT is regarded as a moderate to high radiation diagnostic technique. While technical advances have improved radiation efficiency, there has been simultaneous pressure to obtain higher-resolution imaging and use more complex scan techniques, both of which require higher doses of radiation. The improved resolution of CT has permitted the development of new investigations, which may have advantages; compared to conventional angiography for example, CT angiography avoids the invasive insertion of an arterial catheter and guidewire; CT colonography may be as useful as a barium enema for detection of tumors, but may use a lower radiation dose.
The greatly increased availability of CT, together with its value for an increasing number of conditions, has been responsible for a large rise in popularity. So large has been this rise that, in the most recent comprehensive survey in the United Kingdom, CT scans constituted 7% of all radiologic examinations, but contributed 47% of the total collective dose from medical X-ray examinations in 2000/2001. Increased CT usage has led to an overall rise in the total amount of medical radiation used, despite reductions in other areas. In the United States and Japan for example, there were 26 and 64 CT scanners per 1 million population in 1996. In the U.S., there were about 3 million CT scans performed in 1980, compared to an estimated 62 million scans in 2006.
The radiation dose for a particular study depends on multiple factors: volume scanned, patient build, number and type of scan sequences, and desired resolution and image quality. Additionally, two helical CT scanning parameters that can be adjusted easily and that have a profound effect on radiation dose are tube current and pitch.The radiation from current CT-scan use may cause as many as 1 in 50 future cases of cancer.

What are the Advantages and hazards of CT?

Advantages over projection radiography
First, CT completely eliminates the superimposition of images of structures outside the area of interest. Second, because of the inherent high-contrast resolution of CT, differences between tissues that differ in physical density by less than 1% can be distinguished. Third, data from a single CT imaging procedure consisting of either multiple contiguous or one helical scan can be viewed as images in the axial, coronal, or sagittal planes, depending on the diagnostic task. This is referred to as multiplanar reformatted imaging.

Extremities

CT is often used to image complex fractures, especially ones around joints, because of its ability to reconstruct the area of interest in multiple planes. Fractures, ligamentous injuries and dislocations can easily be recognised with a 0.2 mm resolution.

Abdominal and Pelvic

CT is a sensitive method for diagnosis of abdominal diseases. It is used frequently to determine stage of cancer and to follow progress. It is also a useful test to investigate acute abdominal pain (especially of the lower quadrants, whereas ultrasound is the preferred first line investigation for right upper quadrant pain). Renal stones, appendicitis, pancreatitis, diverticulitis, abdominal aortic aneurysm, and bowel obstruction are conditions that are readily diagnosed and assessed with CT. CT is also the first line for detecting solid organ injury after trauma.
Oral and/or rectal contrast may be used depending on the indications for the scan. A dilute (2% w/v) suspension of barium sulfate is most commonly used. The concentrated barium sulfate preparations used for fluoroscopy e.g. barium enema are too dense and cause severe artifacts on CT. Iodinated contrast agents may be used if barium is contraindicated (for example, suspicion of bowel injury). Other agents may be required to optimize the imaging of specific organs, such as rectally administered gas (air or carbon dioxide) or fluid (water) for a colon study, or oral water for a stomach study.
CT has limited application in the evaluation of the pelvis. For the female pelvis in particular, ultrasound and MRI are the imaging modalities of choice. Nevertheless, it may be part of abdominal scanning (e.g. for tumors), and has uses in assessing fractures.
CT is also used in osteoporosis studies and research alongside dual energy X-ray absorptiometry (DXA). Both CT and DXA can be used to assess bone mineral density (BMD) which is used to indicate bone strength, however CT results do not correlate exactly with DXA (the gold standard of BMD measurement). CT is far more expensive, and subjects patients to much higher levels of ionizing radiation, so it is used infrequently.

Cardiac

With the advent of subsecond rotation combined with multi-slice CT (up to 64-slice), high resolution and high speed can be obtained at the same time, allowing excellent imaging of the coronary arteries (cardiac CT angiography). Images with an even higher temporal resolution can be formed using retrospective ECG gating. In this technique, each portion of the heart is imaged more than once while an ECG trace is recorded. The ECG is then used to correlate the CT data with their corresponding phases of cardiac contraction. Once this correlation is complete, all data that were recorded while the heart was in motion (systole) can be ignored and images can be made from the remaining data that happened to be acquired while the heart was at rest (diastole). In this way, individual frames in a cardiac CT investigation have a better temporal resolution than the shortest tube rotation time.

CT pulmonary angiogram (CTPA)

CT pulmonary angiogram (CTPA) is a medical diagnostic test used to diagnose pulmonary embolism (PE). It employs computed tomography to obtain an image of the pulmonary arteries.
It is a preferred choice of imaging in the diagnosis of PE due to its minimally invasive nature for the patient, whose only requirement for the scan is a cannula (usually a 20G). Before this test is requested, it is usual for the referring clinician to have carried out a D-dimer blood test and requested a chest X-Ray to rule out any other possible differential diagnosis.
MDCT (multi detector CT) scanners give the optimum resolution and image quality for this test. Images are usually taken on a 0.625mm slice thickness, although 2mm is sufficient. 50 - 100 mls of contrast is given to the patient at a rate of 4 ml/s. The tracker/locator is placed at the level of the Pulmonary Arteries, which sit roughly at the level of the carina. Images are acquired with the maximum intensity of radio-opaque contrast in the Pulmonary Arteries. This is done using bolus tracking.
CT machines are now so sophisticated that the test can be done with a patient visit of 5 minutes with an approximate scan time of only 5 seconds or less.

Example of a CTPA, demonstrating a saddle embolus
A normal CTPA scan will show the contrast filling the pulmonary vessels, looking bright white. Ideally the aorta should be empty of contrast, to reduce any partial volume artefact which may result in a false positive. Any mass filling defects, such as an embolus, will appear dark in place of the contrast, filling / blocking the space where blood should be flowing into the lungs.

Chest

CT is excellent for detecting both acute and chronic changes in the lung parenchyma. (parenchyma means internals, in this case, of the lungs. It is relevant here because normal two dimensional x-rays do not show up defects in the internals of the lungs). A variety of different techniques are used depending on the suspected abnormality. For evaluation of chronic interstitial processes (emphysema, fibrosis, and so forth), thin sections with high spatial frequency reconstructions are used - often scans are performed both in inspiration and expiration. This special technique is called High resolution CT (HRCT). HRCT is normally done with thin section with skipped areas between the thin sections. Therefore it produces a sampling of the lung and not continuous images. Continuous images are provided in a standard CT of the chest.
For detection of airspace disease (such as pneumonia) or cancer, relatively thick sections and general purpose image reconstruction techniques may be adequate. IV contrast may also be used as it clarifies the anatomy and boundaries of the great vessels and improves assessment of the mediastinum and hilar regions for lymphadenopathy; this is particularly important for accurate assessment of cancer.
CT angiography of the chest is also becoming the primary method for detecting pulmonary embolism (PE) and aortic dissection, and requires accurately timed rapid injections of contrast (Bolus Tracking) and high-speed helical scanners. CT is the standard method of evaluating abnormalities seen on chest X-ray and of following findings of uncertain acute significance.

What are the Diagnostic use of Computed Tomography?

Since its introduction in the 1970s, CT has become an important tool in medical imaging to supplement X-rays and medical ultrasonography. Although it is still quite expensive, it is the gold standard in the diagnosis of a large number of different disease entities. It has more recently begun to also be used for preventive medicine or screening for disease, for example CT colonography for patients with a high risk of colon cancer. Although a number of institutions offer full-body scans for the general population, this practice remains controversial due to its lack of proven benefit, cost, radiation exposure, and the risk of finding 'incidental' abnormalities that may trigger additional investigations.

What is X-ray Tomography ?

X-ray Tomography is a branch of X-ray microscopy. A series of projection images are used to calculate a three dimensional reconstruction of an object. The technique has found many applications in materials science and later in biology and biomedical research. In terms of the latter, the National Center for X-ray Tomography (NCXT) is one of the principal developers of this technology, in particular for imaging whole, hydrated cells.

Synchrotron X-ray tomographic microscopy

Synchrotron X-ray tomographic microscopy is a 3-D scanning technique that allows non-invasive high definition scans of objects with details as fine as 1,000th of a millimetre, meaning it has two to three thousand times the resolution of a traditional medical CT scan.
Synchrotron X-ray tomographic microscopy has been applied in the field of palaeontology to perform non-destructive internal examination of fossils, including fossil embryos to be made. Scientists feel this technology has the potential to revolutionize the field of paleontology. The first team to use the technique have published their findings in Nature, which they believe "could roll back the evolutionary history of arthropods like insects and spiders."
Archaeologists are increasingly turning to Synchrotron X-ray tomographic microscopy as a non-destructive means to examine ancient specimens.

Peripheral Quantitative Computed Tomography (pQCT)

pQCT or QCT devices are optimized for high precision measurements. of physical properties of bone such as bone density and bone geometry. In comparison to the commonly used DXA system which measures bone mass only (BMD). QCT systems can determine bone strength as a mechanical property and the resulting fracture risk. Hence one outcome parameter is the Stress-Strain Index (SSI) comparing bone strength to results of three point bending tests commonly used for mechanical material tests.

Inverse geometry CT (IGCT)

Inverse geometry CT (IGCT) is a novel concept which is being investigated as refinement of the classic third generation CT design. Although the technique has been demonstrated on a laboratory proof-of-concept device, it remains to be seen whether IGCT is feasible for a practical scanner. IGCT reverses the shapes of the detector and X-ray sources. The conventional third-generation CT geometry uses a point source of X-rays, which diverge in a fan beam to act on a linear array of detectors. In multidetector computed tomography (MDCT), this is extended in 3 dimensions to a conical beam acting on a 2D array of detectors. The IGCT concept, conversely, uses an array of highly collimated X-ray sources which act on a point detector. By using a principle similar to electron beam tomography (EBCT), the individual sources can be activated in turn by steering an electron beam onto each source target.
The rationale behind IGCT is that it avoids the disadvantages of the cone-beam geometry of third generation MDCT. As the z-axis width of the cone beam increases, the quantity of scattered radiation reaching the detector also increases, and the z-axis resolution is thereby degraded - because of the increasing z-axis distance that each ray must traverse. This reversal of roles has extremely high intrinsic resistance to scatter; and, by reducing the number of detectors required per slice, it makes the use of better performing detectors (e.g. ultra-fast photon counting detectors) more practical. Because a separate detector can be used for each 'slice' of sources, the conical geometry can be replaced with an array of fans, permitting z-axis resolution to be preserved.

256+ slice CT

At RSNA 2007, Philips announced a 256 slice scanner, while Toshiba announced a "dynamic volume" scanner based on 320 slices. The majority of published data with regard to both technical and clinical aspects of the systems have been related to the prototype unit made by Toshiba Medical Systems. The recent 3 month Beta installation at Johns Hopkins Press Release using a Toshiba system tested the clinical capabilities of this technology JHU Gazette. The technology currently remains in a development phase but has demonstrated the potential to significantly reduce radiation exposure by eliminating the requirement for a helical examination in both cardiac CT angiography and whole brain perfusion studies for the evaluation of stroke.

Dual-source CT

Siemens introduced a CT model with dual X-ray tube and dual array of 64 slice detectors, at the 2005 Radiological Society of North America (RSNA) medical meeting. Dual sources increase the temporal resolution by reducing the rotation angle required to acquire a complete image, thus permitting cardiac studies without the use of heart rate lowering medication, as well as permitting imaging of the heart in systole. The use of two x-ray units makes possible the use of dual energy imaging, which allows an estimate of the average atomic number in a voxel, as well as the total attenuation. This permits automatic differentiation of calcium (e.g. in bone, or diseased arteries) from iodine (in contrast medium) or titanium (in stents) - which might otherwise be impossible to differentiate. It may also improve the characterization of tissues allowing better tumor differentiation

Multislice CT

Multislice CT scanners are similar in concept to the helical or spiral CT but there are more than one detector ring. It began with two rings in mid nineties, with a 2 solid state ring model designed and built by Elscint (Haifa) called CT TWIN, with one second rotation (1993): It was followed by other manufacturers. Later, it was presented 4, 8, 16, 32, 40 and 64 detector rings, with increasing rotation speeds. Current models (2007) have up to 3 rotations per second, and isotropic resolution of 0.35mm voxels with z-axis scan speed of up to 18 cm/s.. This resolution exceeds that of High Resolution CT techniques with single-slice scanners, yet it is practical to scan adjacent, or overlapping, slices - however, image noise and radiation exposure significantly limit the use of such resolutions.
The major benefit of multi-slice CT is the increased speed of volume coverage. This allows large volumes to be scanned at the optimal time following intravenous contrast administration; this has particularly benefitted CT angiography techniques - which rely heavily on precise timing to ensure good demonstration of arteries.
Computer power permits increasing the postprocessing capabilities on workstations. Bone suppression, volume rendering in real time, with a natural visualization of internal organs and structures, and automated volume reconstruction really change the way diagnostic is performed on CT studies and this models become true volumetric scanners. The ability of multi-slice scanners to achieve isotropic resolution even on routine studies means that maximum image quality is not restricted to images in the axial plane - and studies can be freely viewed in any desired plane.

Helical cone beam computed tomography

Helical (or spiral) cone beam computed tomography is a type of three dimensional computed tomography (CT) in which the source (usually of x-rays) describes a helical trajectory relative to the object while a two dimensional array of detectors measures the transmitted radiation on part of a cone of rays eminating from the source. Willi Kalender, who is credited with the invention prefers the term Spiral scan CT, arguing that spiral is synonymous with helical: for example as used in 'spiral staircase'.
In practical helical cone beam x-ray CT machines, the source and array of detectors are mounted on a rotating gantry while the patient is moved axially at a uniform rate. Earlier x-ray CT scanners imaged one slice at a time by rotating source and one dimensional array of detectors while the patient remained static. The helical scan method reduces the x-ray dose to the patient required for a given resolution while scanning more quickly. This is however at the cost of greater mathematical complexity in the reconstruction of the image from the measurements.

Design and advatage of EBT

The principal application advantage of EBT tomographic CT machines and the reason for the invention, is that the X-Ray source is swept electronically, not mechanically, and can thus be swept with far greater speed than with conventional CT machines based on mechanically spun X-Ray tubes.
The major medical application for which this design technology was invented in the 1980s, namely for imaging the human heart. The heart never stops moving, and some important structures, such as arteries, move several times their diameter during each heartbeat. Rapid imaging is, thus, important to prevent blurring of moving structures during the scan. The most advanced current commercial designs can perform image sweeps in as little as 0.025 seconds. By comparison, the fastest mechanically swept X-Ray tube designs require about 0.33 seconds to perform an image sweep. For reference, current coronary artery angiography imaging is usually performed at 30 frames/second or 0.033 seconds/frame; EBT is far closer to this than mechanically swept CT machines.

Electron beam tomography

Electron beam tomography (EBT) is a specific form of computed axial tomography (CAT or CT) in which the X-Ray tube is not mechanically spun in order to rotate the source of X-Ray photons. This different design was explicitly developed to better image heart structures which never stop moving, performing a complex complete cycle of movement with each heart beat.
As in conventional CT technology, the X-ray source still rotates around the circle in space containing an object to be imaged tomographically, but the X-Ray tube is much larger than the imaging circle and the electron beam current within the vacuum tube is swept electronically, in a circular (partial circle actually) path and focused on a stationary tungsten anode target ring.

What is Radiography?

Radiography is the use of X-rays to view unseen or hard-to-image objects. The use of non-ionizing radiations (visible light and ultraviolet light) to view objects should be considered as a normal “optical” method (e.g., light microscopy). The modification of an object through the use of ionizing radiation is not radiography. Depending on the nature of the object and the intended outcome it can be radiotherapy, food irradiation, or radiation processing

DRR

A Digitally Reconstructed Radiograph is a simulation of a conventional 2D x-ray image, created from computed tomography (CT) data. A radiograph, or conventional x-ray image, is a single 2D view of total x-ray absorption through the body along a given axis. Two objects (say, bones) in front of one another will overlap in the image. By contrast, a 3D CT image gives a volumetric representation. (Earlier CT data sets were better thought of as a set of 2D cross sectional images.) Sometimes one must compare CT data to a classical radiograph, and this can be done by comparing a DRR based on the CT data. An early example of their use is the beam's eye view (BEV) as used in radiotherapy planning. In this application, a BEV is created for a specific patient and is used to help plan the treatment.
DRRs are created by summing CT intensities along a ray from each pixel to the simulated x-ray source.
Since 1993, the Visible Human Project (VHP) has made full body CT data available to researchers. This has allowed several universities and commercial companies to try and create DRR's. These have been suggested as useful for training simulations in Radiology and Diagnostic Radiography. It takes a significant number of calculations to create a summative 3D image from a large amount of 2D data. This is an area of medical science and education that has benefited from the advancing of graphics card technology, driven by the computer games industry.
Another novel use of DRR's is in identification of the dead from old radiographic records, by comparing them to DRR's created from CT data.

Helical cone beam computed tomography

Helical (or spiral) cone beam computed tomography is a type of three dimensional computed tomography (CT) in which the source (usually of x-rays) describes a helical trajectory relative to the object while a two dimensional array of detectors measures the transmitted radiation on part of a cone of rays eminating from the source. Willi Kalender, who is credited with the invention prefers the term Spiral scan CT, arguing that spiral is synonymous with helical: for example as used in 'spiral staircase'.
In practical helical cone beam x-ray CT machines, the source and array of detectors are mounted on a rotating gantry while the patient is moved axially at a uniform rate. Earlier x-ray CT scanners imaged one slice at a time by rotating source and one dimensional array of detectors while the patient remained static. The helical scan method reduces the x-ray dose to the patient required for a given resolution while scanning more quickly. This is however at the cost of greater mathematical complexity in the reconstruction of the image from the measurements.

Cine

A cine acquisition is used when the temporal nature is important. This is used in Perfusion applications to evaluate blood flow, blood volume and mean transit time. Cine is a time sequence of axial images. In a Cine acquisition the cradle is stationary and the gantry rotates continuously. Xray is delivered at a specified interval and duration

What is Tomographic reconstruction?

The mathematical basis for tomographic imaging was laid down by Johann Radon. It is applied in Computed Tomography to obtain cross-sectional images of patients. This article applies in general to tomographic reconstruction for all kinds of tomography, but some of the terms and physical descriptions refer directly to X-ray computed tomography.

Figure 1: Parallel beam geometry. Each projection is made up of the set of line integrals through the object.
The projection of an object at a given angle θ is made up of a set of line integrals. In X-ray CT, the line integral represents the total attenuation of the beam of x-rays as it travels in a straight line through the object. As mentioned above, the resulting image is a 2D (or 3D) model of the attenuation coefficient. That is, we wish to find the image μ(x,y). The simplest and easiest way to visualise method of scanning is the system of parallel projection, as used in the first scanners. For this discussion we consider the data to be collected as a series of parallel rays, at position r, across a projection at angle θ. This is repeated for various angles. Attenuation occurs exponentially in tissue

Axial

In axial "step and shoot" acquisitions each slice/volume is taken and then the table is incremented to the next location. In multislice scanners each location is multiple slices and represents a volume of the patient anatomy. Tomographic reconstruction is used to generate Axial images.

Dynamic volume CT

During the Radiological Society of North America (RSNA) in 2007, Toshiba Medical Systems introduced the world's first dynamic volume CT system, Aquilion ONE. This 320-slice CT scanner, with its 16 cm anatomical coverage, can scan entire organs such as heart and brain, in just one single rotation, thereby also enabling dynamic processes such as blood flow and function to be observed.
Whereas patients exhibiting symptoms of a heart attack or stroke have until now normally had to submit to a variety of examinations preparatory to a precise diagnosis, all of which together took up a considerable amount of time, with dynamic volume CT this can be decreased to a matter of minutes and one single examination. Functional imaging can thus be performed rapidly, with the least possible radiation and contrast dose combined with very high precision

What is Tomosynthesis?

Digital tomosynthesis combines digital image capture and processing with simple tube/detector motion as used in conventional radiographic tomography - although there are some similarities to CT, it is a separate technique. In CT, the source/detector makes a complete 360 degree rotation about the subject obtaining a complete set of data from which images may be reconstructed. In digital tomosynthesis, only a small rotation angle (e.g. 40 degrees) with a small number of discrete exposures (e.g. 10) are used. This incomplete set of data can be digitally processed to yield images similar to conventional tomography with a limited depth of field. However, because the image processing is digital, a series of slices at different depths and with different thicknesses can be reconstructed from the same acquisition, saving both time and radiation exposure.
Because the data acquired is incomplete, tomosynthesis is unable to offer the extremely narrow slice widths that CT offers. However, higher resolution detectors can be used, allowing very-high in-plane resolution, even if the Z-axis resolution is poor. The primary interest in tomosynthesis is in breast imaging, as an extension to mammography, where it may offer better detection rates, with little extra increase in radiation exposure.
Reconstruction algorithms for tomosynthesis are significantly different from conventional CT, as the conventional filtered back projection algorithm requires a complete set of data. Iterative algorithms based upon expectation maximization are most commonly used, but are extremely computationally intensive. Some manufacturers have produced practical systems using commercial GPUs to perform the reconstruction.

What is Tomography?

CT's primary benefit is the ability to view the brain and or head only. A form of tomography can be performed by moving the X-ray source and detector during an exposure. Anatomy at the target level remains sharp, while structures at different levels are blurred. By varying the extent and path of motion, a variety of effects can be obtained, with variable depth of field and different degrees of blurring of 'out of plane' structures.
Although largely obsolete, conventional tomography is still used in specific situations such as dental imaging (orthopantomography) or in intravenous urography.

What is Computed Tomography?

Computed tomography (CT) is a medical imaging method employing tomography. Digital geometry processing is used to generate a three-dimensional image of the inside of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation. The word "tomography" is derived from the Greek tomos (slice) and graphein (to write).
Computed tomography was originally known as the "EMI scan" as it was developed at a research branch of EMI, a company best known today for its music and recording business. It was later known as computed axial tomography (CAT or CT scan) and body section röntgenography.
CT produces a volume of data which can be manipulated, through a process known as windowing, in order to demonstrate various structures based on their ability to block the X-ray/Röntgen beam. Although historically (see below) the images generated were in the axial or transverse plane (orthogonal to the long axis of the body), modern scanners allow this volume of data to be reformatted in various planes or even as volumetric (3D) representations of structures.