Digital picture (Raster image)

4.1. Introduction

The computer tomography (CT, computerized tomography) was created from the concurrent application and combination of the X-ray analysis technique and the computer technology.

Godfrey Hounsfield and Allan Mc. Cormack received the Nobel Prize for medicine in 1979 for developing the CT. The CT scan is basically a new, spatial approach taken in the

radiology.

4.2. The CT Imaging

X-rays suffer attenuation when passing through the human body, with the help of computer application the mathematical methods can convert these losses into visible images.

The process consists of two parts. The first part is the measurement and data collection phase, and the second is the image reconstruction phase, which ends with image capture.

4.2.1. The basics of CT imaging

In 1917 Radon formulated one of the underlying principles in CT imaging:

"A three-dimensional body composed by an infinite number of points can be mathematically reconstructed, and produced at any time."

A narrow X-ray beam scans across the transverse section of the body part which will be examined.

The difference in the amount of radiation entering and leaving the body is known as absorption profile. The quintessence of tomographic imaging is that we can determine the value of radiation absorption for each of the different space elements in a slice with a sufficiently large number of absorption profiles taken from different directions.

We detect the weakened radiation leaving the body with detector-rows.

The detectors convert the radiation to electronic signals, which is analyzable with digital data processing systems, and can be changed into numerical data.

The CT-image is a sectional image calculated from multi-directionally measured radiation attenuation values.

4.2.2. Digital picture (Raster image)

One voxel is a volume element of the same size in an irradiated slice of the body. It is a prismatic formation, which base is a pixel (the spatial resolution of the CT is 300 micrometer on the average). The height of the prism is basically determined by the chosen slice thickness.

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4.2.3. Basic concepts of CT

Gantry - ring shaped gear, which encompasses the X-ray tube and the detectors Table motion - periodic or continuous

Matrix (raster) - 512x512, 1024x1024 Density - tissue "solidity"

 -1000 HU vacuum

 70-100 HU fresh bleeding in soft tissues

 100-1000 HU contrast agent, calcification

 3000 HU total radiation absorption

A CT scan can theoretically produce 4000 shades. The extent of the attenuation is expressed as so called Hounsfield units (HU), which is characteristic to the through radiated material density. This scale has a negative endpoint (-1000 HU) accordingly to the attenuation of the vacuum, the positive endpoint (3000 HU) fit the total attenuation. The null-point (0 HU) is the density of the water.

4.2.4. Windowing

The human eye can recognize only 40-60 shades of gray. The CT can measure up to 3000 different density. To avoid that it will be all uniformly gray, we will narrow the visible gray scale to the target density, underneath all density will be set to black, and above all to white.

We look at the resulting images with different windows, depending on the tissue structure that is considered a goal.

4.3. CT devices

One-slice (slicing-stepping) CT- the movement of the patient table is periodic, with one measurement there will be mapping of one transverse slice of the body.

Spiral (helical) CT- they appeared from 1990.

The movement of the patient table is continuous, so it is possible to measure whole body volume.

Multislice - multidetector CT- they are since 1992 widespread.

Dual energy, dual-source (with two X-ray tubes) CT- has been used from 2005.

PET-CT is a combined diagnostic method.

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The continuous table movement allows for continuous measurement, so there is no information loss.

A single breath hold is enough for the whole body "scan".

There are less motion artifacts (examination of patient with severe medical conditions).

The thin-slice imaging is more accurate for analysis of density.

Based on the fast, high-volume collection of data is possible to reconstruct in any plane.

The measurement of volume allows spatial visualization.

Favorable radiation

The amount of contrast agent can be reduced.

4.3.2. Dual-Source imaging

Concomitant use of two X-ray sources and two detectors

The two tubes are located perpendicular to each other, and the information is collected by the detectors in sync with each other.

There are two different operating modes.

In dual source mode both X-ray tubes works with the same kV value.

The collection for axial slices requires only 90° rotation.

In dual energy mode the tube voltages are 80 and 140kV, and for one transverse slice the two tubes rotate 180°.

The absorption of X-rays with different energy will be different.

Two sets of data are created, containing different information.

The benefits of dual-source imaging Tissue differentiation in new ways

Blood vessels or bones can be directly subtracted.

Oncological classification of tumors

Vascular plaque characterization, more detailed image quality Body fluid differentiation in the emergency diagnostics Collagen visualization (direct imaging of tendons) Stone analysis

Imaging of heavy elements

Perfusion imaging based on iodine quantification 4.3.3. PET-CT

It is a combined diagnostic method, an alloy of the computer tomography (CT) and the positron emission tomography (PET).

The tracer material is radioactive isotope (18F)-labeled glucose molecule (FDG), which has a very short half-life, and only a small amount is injected.

PET measures the metabolic processes occurring in cells and the CT shows the anatomical structure.

It is used in the first place for early detection of malignant tumors, for determining the stage of them and for following the effectiveness of the treatment.

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