Computed tomography and Hounsfield units

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'Computed tomography the name comes from the Greek tomeo, which means to cut(also Computed Tomography, CT, or computed tomography) is one of the most important discoveries in X-ray diagnostics. It is actually a mathematical reconstruction of an image obtained from a series of X-ray projections made at different angles. With its help, we can visualize soft tissues such as the brain, spleen, pancreas, kidneys or muscles in a non-aggressive way. However, we can only detect pathological processes that differ in their density from the surroundings. Often, the patient is also given a contrast agent to better distinguish the pathological tissue.



CT History and Generation
'''The theory of reconstructing a tomographic section from many summation images was developed by the American physicist Allan Cormack already in 1963, but with the use of gamma radiation. However, the first usable tomograph, EMI mark I, was built by the Englishman Godfrey Newbold Hounsfield in 1972. In 1979, both Cormack and Housfield were awarded the Nobel Prize for the discovery of computed tomography.'''

From the point of view of the design of the radiation source/detector system, it is possible to divide the devices into several generations. We know of six CT generation systems, of which third to sixth generation systems are in use today.
 * 1) generation: Houndsfield system that, after being rotated by 10°-15°, moved linearly across the full width of the patient in a given plane. (reconstruction within 5 minutes),
 * 2) generation: It also uses rotational translational movement, a smaller angle between individual frames (3°-15°) and a larger number of detectors (6-60). (reconstruction 10s–20s),
 * 3) generation: This generation is the most used today. It works with 360° x-ray rotation, with a wide beam of radiation and using many detectors (400-600) on an opposite matrix. Scanning is performed in increments of 1° to 0.5°. The recording is taken during the entire time of a smooth rotation, with pulse operation of the X-ray machine. Scan time up to 1 second. Spiral (helical) CT is a continuation of the 3rd generation devices. The first CT system with continuous rotation was developed by Bio-Imaging Research in 1986. Previously, each subsequent rotation had to be performed in the opposite direction to the previous one, meanwhile the table with the patient moved. The introduction of continuous rotation allowed for smooth movement of the table with the patient while continuously recording images. This method made it possible to create better 3D reconstructions and speed up the whole process of acquiring images. Helical CT is mainly used in CT angiology.,
 * 4) generation: Uses a rotating-stationary system: Wide beam, thousands of stationary detectors in a ring. Only the X-ray machine rotates, 360°. The problem arises when exposing edge detectors that are affected by scattered radiation. However, this system is demanding and has not spread much in practice.
 * 5) generation: It is a nutation system, which consists of a matrix of fixed detectors and a rotating X-ray unit. Depending on its position, the detectors are deflected from the perpendicular so that the rays fall on them perpendicularly. Image processing offers many possibilities: 3D reconstruction, reconstruction of sections in planes other than the axial plane. What is new are multi-slice CTs (Toshiba, Aquillion, 1999), which are equipped with several circular detector systems and thus enable the acquisition of multiple slices in several adjacent planes at one time, thus further speeding up the entire process, and less demands on the patient.
 * 6) generation: The source of radiation here is an electron gun. The Imatron-type device differs in that the massive anode is oriented as a slice around part of the patient's circumference and has several annular foci. No folder moves here. The device wakes up simultaneously at several focal points and hits two rings of detectors - obtaining several layer records at the same time, for extremely short exposures of 50ms.

Principle of computed tomography
During the examination itself, the patient is fixed on a sliding bed, which gradually passes through the scanning stand. On one side of the stand is a slit source x-ray (x-ray) and on the opposite side scintillation detectors. In older tomographs, the detectors are placed against the X-ray tube, move with it and are firmly connected to it. In more modern tomographs, the stationary detectors are arranged in a plane. y around the patient's entire body. The most modern technique is spiral CT, where both the examination table and the tunnel of the device rotate. The patient is gradually enlightened point by point and the result is a transverse section of the body. The resulting section is therefore a computer reconstruction of many ``classical'' X-ray images of a certain plane. The radiation is captured using a system of detectors connected to a computer.

The X-ray works in pulses (1 pulse lasts 1-4 ms). The X-ray beam is fan-shaped. X-ray then enters the patient's body and is partially absorbed. Scintillation detectors record the rate (coefficient) of attenuation of radiation μ' and transfer the data to the computer memory. Then the x-ray-detector system is rotated by a certain angle and the process repeats itself. At the end of the examination, the computer processes the data and displays the tomogram, which is given by the values of 'absorption coefficients μ from the individual locations of the given section. We therefore determine the value of X-ray absorption in small volumetric particles, which we call voxels (volume matrix element) - it is actually an analogy of a pixel in a planar image, but each voxel does not represent a two-dimensional unit, but also has its depth given by the thickness of the section. The result is the reconstruction of the transverse layer through the patient's body = AXIAL LAYER, while in classic tomography, FRONTAL or SAGITTAL LAYERS are obtained depending on the position of the patient.



Hounsfield units, image formation
The transverse section of the imaged object by computed tomography is made up of more than 250,000 "voxels", i.e. small units of tissue volume with different average absorption and dispersion of the X-ray radiation used. In order to determine the absorption of a voxel, it is necessary for the radiation to pass through it repeatedly and at different angles. The resulting voxel absorption value (i.e. the rate of absorption and scattering of the passing radiation) is subsequently expressed using the density unit - Hounsfield unit [HU] (CT number)', which expresses the radiation absorption of a given voxel relative to the absorption of water radiation (for water is paid by HU = 0). The detectors then determine the sum of the absorptions of all the voxels through which the beam passed.

Hounsfield units [HU] (CT numbers) are therefore an expression of the density (rate of absorption and scattering of radiation) of specific voxels. For each voxel, the corresponding HU is calculated from the measured absorption value, which is related to the value of the absorption of X-ray radiation by water.

The HU value is defined according to the relationship:

$$ HU = \frac{\mu - \mu_w} {\mu_w} \cdot k$$

where k – contracted constant of size 1000

where μ is the attenuation coefficient of the examined tissue

where μw – water attenuation coefficient (adsorption coefficient μw = 0.22 cm−1)

In practice, Hounsfield units can take on values from −1000 (air) to approximately 1000 (compact bone). Approximately 2000 numbers are therefore available for diagnostic use and for displaying the resulting image in shades of gray on the monitor (see below).

The computer uses the Fourier transform to determine the absorption values, necessary for the calculation of Hounsfield units. A particular section is taken very quickly (5–7 s), which facilitates the visualization of some organs (for example, the intestines, where peristalsis leads to a blurring of the image) and allowed the examination of the heart and large arteries. 'The image is created on the monitor already during the cut, but it is completed only after the entire scanning is finished. The computer must further make a number of corrections, for example to remove artifacts that arise at the interface of bone and soft tissue (the so-called Hounsfield effect), which is caused by the fact that bone absorbs more soft radiation than harder.

about measuring the absorption values of individual voxels and calculating the relevant Hounsfield units, these values are transferred to the monitor, where specific shades of gray correspond to specific values of Hounsfield units.

So the computer converts the analog signal into digital, which it further processes and finally converts it back into analog (resulting image). Like X-ray, CT is an imaging of densities. The measured data (individual images) are subsequently reconstructed into the resulting matrix by complex mathematical procedures.

CT image processing
As mentioned above, approximately 2000 HU values can be used to display the resulting CT image on the monitor, which can theoretically be converted to the corresponding number of shades of gray. However, the human eye can distinguish a maximum of around 25 shades. The full range of Hounsfield units is therefore far from being used when displaying a CT image.

In practice, most soft tissue values lie in the HU range of 0–100. In practical use, shades of gray for display on the monitor are assigned only in the so-called absorption window of values - i.e. in the range in which the HU values are authoritative and representative for the displayed organ (by default, the range about HU 0–100). Thanks to the adsorptive window of values, only the desired image is displayed in detail and at the same time it is safely "understandable to the human eye, because it uses such an amount of shades of gray that the eye can safely distinguish.

Advantages of CT imaging

 * Enables visualization of low-contrast soft tissue, including tumors, thanks to:


 * 1) high sensitivity of scintillation detectors,
 * 2) very fast processing of data from the scintillation detector.


 * The method is also advantageous for planning surgical interventions and radiotherapy of malignant diseases,
 * very good resolution and contrast → the image is sharper,
 * elimination of reflections and their interference,
 * the results can be saved on the computer, part of the image can be enlarged and examined in detail,
 * We can compare the patient's radiation load during CT with the radiation dose during a classic examination.

Disadvantages of CT imaging

 * High purchase price of equipment,
 * their operation requires the presence of highly trained personnel.

CT Components



 * Examination table with storage plate for the patient' - the plate is sliding and its movement can be controlled by the computer,
 * portal (gantry) - an examination tunnel through which the board on which the patient is placed passes; it is a cabinet containing the X-ray tube, detectors and the mechanism along which the X-ray tube and possibly the detectors move, and other equipment, including the cooling system,
 * – in the middle there is a circular opening for the examination table, which is movable vertically, lengthwise and laterally, so it allows the patient to be inserted into different depths of the gantry,
 * – it must have a sufficient diameter (up to 84 cm) so that even obese patients can comfortably lie in it and maintain a constant position, which is an essential requirement for the examination, because any change in the position of the patient will distort the result of both the given layer and the possibility of comparing the layers between themselves,
 * – the gantry can be tilted to a limited extent (up to 30°) and the cutting plane can be selected (e.g. orthograde projection),


 * high performance x-ray machine',
 * detectors - they are calibrated so that the response of all detectors to the impact of an X-ray photon is uniform,
 * – after the impact of the photon, it must return to the zero value as quickly as possible,


 * high power X-ray generator',
 * – x-ray machines must be powerful and heat-resistant in CT because their load is extremely high, pulsed operation and perfect cooling are necessary for good function,


 * operator desk - contains a keyboard and an image and text monitor,
 * computers - control computer (controls and directs the acquisition of images) and evaluation computer (reconstructs images from raw data - so-called raw data - and further processes them),
 * diagnostic table - intended for the study of images by a doctor,
 * documentation - for example on optical or magnetic disks, diskettes or recording on film material.

Detectors
CT equipment varies in size, type, number and placement of detectors. Greater efficiency of the detectors leads to better resolution of contrasts, and a greater number of detectors leads to better spatial resolution.

Types of detectors:
 * scintillation,
 * proportional (gas),
 * ceramic.

 1. Scintillation Crystals  scintillation in the UV spectrum, space-consuming
 * NaI(Tl) – p
 * CsI(Tl) – scintillation in the visible spectrum, space-saving
 * long "dead time", a highly stable high voltage source is required, it is difficult to unify the properties of the individual detectors.

2. Proportional detectors
 * less dependent on stabilized high voltage, simple construction, uniform properties, thermally stable
 * space-consuming, low resolution, low efficiency.

3. Ceramic detectors
 * carbonated rare earth elements – YGdEu2+O3 (yttrium-gadolinium oxide),
 * scintillation in the visible spectrum,
 * high absorption capacity, suitable mechanical properties,
 * economically unavailable for common applications.

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