What do ct detectors measure

Richard Ketcham, University of Texas at Austin. X-ray Computed Tomography CT is a nondestructive technique for visualizing interior features within solid objects, and for obtaining digital information on their 3-D geometries and properties.

Sample is 32 cm long. Details A CT image is typically called a slice , as it corresponds to what the object being scanned would look like if it were sliced open along a plane. An even better analogy is a slice from a loaf of bread, because just as a slice of bread has a thickness, a CT slice corresponds to a certain thickness of the object being scanned. So, while a typical digital image is composed of pixels picture elements , a CT slice image is composed of voxels volume elements.

Taking the analogy one step further, just as a loaf of bread can be reconstituted by stacking all of its slices, a complete volumetric representation of an object is obtained by acquiring a contiguous set of CT slices. The gray levels in a CT slice image correspond to X-ray attenuation, which reflects the proportion of X-rays scattered or absorbed as they pass through each voxel.

X-ray attenuation is primarily a function of X-ray energy and the density and composition of the material being imaged. Tomographic imaging consists of directing X-rays at an object from multiple orientations and measuring the decrease in intensity along a series of linear paths. This decrease is characterized by Beer's Law, which describes intensity reduction as a function of X-ray energy, path length, and material linear attenuation coefficient.

A specialized algorithm is then used to reconstruct the distribution of X-ray attenuation in the volume being imaged. In a well-calibrated system using a monochromatic X-ray source i.

This complicates absolute calibration, as effective attenuation is a function of both the X-ray spectrum and the properties of the scan object.

It also leads to beam-hardening artifacts: changes in image gray levels caused by preferential attenuation of low-energy X-rays. Show more about linear attenuation coefficients Hide The dominant physical processes responsible for X-ray attenuation for most laboratory X-ray sources are photoelectric absorption and Compton scattering. Photoelectric absorption occurs when the total energy of an incoming X-ray photon is transferred to an inner electron, causing the electron to be ejected.

In Compton scattering, the incoming photon interacts with an outer electron, ejecting the electron and losing only a part of its own energy, after which it is deflected in a different direction. In general for geological materials, the photoelectric effect is the dominant attenuation mechanism at low X-ray energies, up to approximately keV, after which Compton scatter predominates.

The practical importance of this transition is that the photoelectric effect is proportional to atomic number Z , whereas Compton scattering is proportional only to Z , or, to first order, mass density. As a result, low-energy X-rays are more sensitive to differences in composition than high-energy ones, but are also attenuated much more quickly, limiting the thickness of high-density material that can be penetrated and imaged with them.

The figure on the right shows linear attenuation coefficients as a function of energy for four minerals: quartz, orthoclase, calcite, and almandine garnet.

Quartz and orthoclase are very similar in mass density 2. Thus, these two minerals can be differentiated in CT imagery if the mean X-ray energy used is low enough, but at higher energies they are nearly indistinguishable. Calcite, though only slightly more dense 2. Here the divergence with quartz persists to slightly higher energies, indicating that it should be possible to distinguish the two even in higher-energy scans. High-density, high-Z phases such as almandine are distinguishable at all energies from the other rock-forming minerals examined here.

Show more about CT reconstruction Hide There are a number of methods by which the X-ray attenuation data can be converted into an image, some proprietary. The most frequent approach is called "filtered backprojection," in which the linear data acquired at each angular orientation are convolved with a specially designed filter and then backprojected across a pixel field at the same angle.

This principle is illustrated in the image at right and an animation that can be viewed by clicking on the link below. A hand sample of garnet-biotite-kyanite schist top left is rotated, and its midsection is imaged with a planar fan beam blue. The attenuation of X-rays by the sample as it rotates is shown in the upper right; the more attenuation there is along a beam path leading from the point source bottom to the linear detector top , the fewer X-rays reach the detector.

The data collected at each angle are compiled in the bottom right. In this image the horizontal axis corresponds to detector channel, and the vertical axis corresponds to rotation angle or time , and brightness corresponds to the extent of X-ray attenuation. The resulting image is called a sinogram , as any point in the original object corresponds to a sine curve. First- through fourth-generation computed tomography systems utilize only rays in a single plane: the scan plane.

In first-generation CT Fig. Second-generation CT Fig. In typical third-generation CT Fig. One variation of third-generation scanning offsets the sample from the center of the fan beam so that a part of it is outside of the beam, but the center of rotation is within it Fig.

As the object rotates, all of it passes through the fan beam, which permits reconstruction of a complete image. This technique allows larger objects to be scanned and permits smaller objects to be moved closer to the source into a narrower section of the fan beam, leading to increased resolution through enhanced utilization of detectors to image smaller subsections of the object in any one view.

Third-generation scanning tends to be much faster than second-generation, as X-rays are utilized more efficiently. Most modern medical scanners are fourth-generation devices, consisting of a fixed complete ring of detectors and a single X-ray source that rotates around the object being scanned.

In first- through third-generation scanners the motion between the object being scanned and the source-detector pair is relative, and can be accomplished either by keeping the object stationary and moving the source-detector pair, as is done in medical CT systems, or vice versa as is more common in industrial systems. In volume CT, a cone beam or highly-collimated, thick, parallel beam is used rather than a fan beam, and a planar grid replaces the linear series of detectors. This allows for much faster data acquisition, as the data required for multiple slices can be acquired in one rotation.

However, it is also computationally more intensive, prone to distortion, and in many cases provides lower-resolution images. Whereas volume CT has been largely perfected for some of the most advanced medical systems, and is ideally suited for tomography using parallel-beam synchrotron radiation, for most industrial scanners it does not yet provide the same quality of imagery as single-slice arrangements.

Figure 2: Theoretical energy spectra for a kV X-ray source with a tungsten target, calculated combining 5-keV intervals. The spectra consist of continuous Bremsstrahlung and characteristic K-series peaks at keV and keV.

The upper spectrum is modified only by inherent beam filtration by 3 mm of aluminum at the tube exit port. The mean X-ray energy is keV. The lower curve represents a spectrum that has also passed through 5 cm of quartz. The preferential attenuation of low-energy X-rays causes the average energy to rise to keV. The important variables that determine how effective an X-ray source will be for a particular task are the size of the focal spot, the spectrum of X-ray energies generated, and the X-ray intensity.

The focal-spot size partially defines the potential spatial resolution of a CT system by determining the number of possible source-detector paths that can intersect a given point in the object being scanned. The more such source-detector paths there are, the more blurring of features there will be. The energy spectrum defines the penetrative ability of the X-rays, as well as their expected relative attenuation as they pass through materials of different density.

Higher-energy X-rays penetrate more effectively than lower-energy ones, but are less sensitive to changes in material density and composition. The X-ray intensity directly affects the signal-to-noise ratio and thus image clarity. Higher intensities improve the underlying counting statistics, but often require a larger focal spot.

Many conventional X-ray tubes have a dual filament that provides two focal-spot sizes, with the smaller spot size allowing more detailed imagery at a cost in intensity.

Medical CT systems tend to have X-ray spot sizes that range from 0. The major factors hampering detector efficiency are the loss of x-ray photons in the casing window and the space taken up by the plates. Solid State Crystal Detectors are also called scintillation detectors because they use a crystal that fluoresces when struck by an x-ray photon.

The photodiode is attached to the crystal and transforms the light energy into electrical analog energy. Individual detector elements are affixed to a circuit board. Solid state crystal detectors are made from a variety of materials, like cadmium tungstate, cesium iodide, bismuth germinate and ceramic rare earth compounds like gadolinium of yttrius. Solid state detectors have higher absorption coefficients because these solids have high atomic numbers and high density in comparison to gases.

In case you missed part one-this is what it contained: The Gantry, Slip Rings, Cooling System and the Generator- click here to read it! If you have questions about CT Scanner systems or their components, talk to an expert at Atlantis Worldwide. Meet the author: Vikki Harmonay. Download Your Free eBook Today!