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The clinical importance of diagnostic modalities: The Computer Tomography

4.The clinical importance of diagnostic modalities: The Computer Tomography

Written by: Ildikó Kalina

Semmelweis University Department of Radiology, Budapest


4.1 Aim of the chapter

To review the theoretical basics of CT imaging.
To become familiar with the structure and types of CT machines, CT terminology, windowing, postprocessing techniques, process of CT examination, clinical use of CT and its place in the diagnostic algorithm.

4.2. The CT Imaging

X-rays attenuate while passing through the human body, with the help of mathematical methods computer application 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.

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.

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
  • -100 HU fat
  • 0 HU water
  • 0-15 HU clear watery fluid
  • 15-20 HU denser fluid
  • 20-70 HU soft tissue
  • 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 in Hounsfield units (HU), which is characteristic to the material radiodensity. The scale’s negative endpoint is -1000 HU, the attenuation of the vacuum, the positive endpoint is 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 all will be uniformly gray, the gray scale needs to be constrained to the target density, underneath all density will be set to black, and above all to white.
The images are viewed with different windows, depending on the tissue structure.

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.

4.3.1. The benefits of multislice CT

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.

4.4. The CT examination

4.4.1. Patient preparation for CT examination

Patient preparation for CT examination is the referring physician's responsibility.
Before and after the examination, the patient must be hydrated by drinking plenty of fluids or with infusion to avoid the nephrotoxicity of injected intravenous contrast agent.
Intravenous contrast material will be administered only with the knowledge of renal function (serum creatinine, GFR), which will be verified in all cases by the radiologist also. Particular attention should be paid to high-risk patients or patients with chronic kidney disease.
Patients with impaired renal function get special (Iomeron, Visipaque) contrast material, for patients with very low GFR intravenous contrast material is given only in case of vital indication.
A contrast-enhanced CT examination takes place after four hours of fasting to avoid the contrast agent side effects (nausea, vomiting and aspiration).
If the patient’s antidiabetic medication contains metformin, it should be skipped before and after the test for 48 hours to avoid the lactate acidosis, which may occur mainly in patients with impaired renal function.
The intravenous iodinated contrast material changes the serum levels of iodine in patient treated with iodine-containing drug, it can falsify the control examination results.
Before the examination, the patient will be notified about the risks of the examination, she/he must sign the form of consent.

4.4.2. Examination technique

Each test includes two basic types of recording: the topogram (scout view) and the tomogram (slice, layer).
All CT studies begins with an overview picture, this is the topogram. It is a digital X-ray image, which can be taken from postero-anterior or from lateral side, depending on the part of the body which will be examined. The beginning and the end of the examination area is selected. After that the tomograms - the transverse slices are prepared, of the selected part of the body.
The measurement parameters can be chosen from the opportunities offered by the computer, according to the clinical question and the examined region, but it is possible to prepare own protocols.
The tomography usually carried out in two series, first unenhanced and after administration of contrast material contrast enhanced series.

4.4.3. By CT examinations applied contrast materials

A CT scan can be done unenhanced, without injection of contrast material.
However, in most cases different contrast agents are used. For better visualization of the alimentary tract oral contrast material and for the better general tissue resolution intravenous iodinated contrast media can be used, most commonly injected into the cubital vein. In most cases injector is used for the intravenous application of contrast agent. The injector delivers the contrast material into the vascular system smooth, with constant, administered flow. In special cases the contrast media can be administered in other ways, such as through the rectum, or a tube into special anatomical region, or through an opening of a fistula.

4.5. The clinical use of CT

Most common, routine CT examinations:

  • Head CT (and / or CTA) scan (brain, bones, blood vessels)
  • Neck CT (and / or CTA) scan (cervical soft tissue, lymph nodes, blood vessels)
  • Facial CT scan (sinuses)
  • Chest-CT (and / or CTA) scan (lung, mediastinum, great vessels, heart)
  • Abdominal-pelvic CT (and / or CTA) scan (abdominal parenchymal organs, digestive tract, urography, abdominal blood vessels)
  • Lower limb CT angiography
  • CT-scan of bone (e.g. spine)

CT scan of the patient takes place commonly in the examination algorithms after some other imaging methods (X-rays, ultrasound) but in some cases it can be the first modality of choice (stroke, polytrauma, pulmonary embolism, suspected aneurysm rupture).
In all cases both written and pictorial documentation must be prepared. It is the radiologist competence to determine which protocol would be used or to plan out the study based on the clinical question.
The fellow clinical doctors must completely inform the radiologist about all relevant clinical data.
For the radiologist examination-request form and the review of the available clinical data are always essential. This information is necessary for proper planning of the examination with minimum radiation burden for the patient.
For routine scans pre-written protocols are used, which will be tailored and optimized by the patient's individual necessities.
In some cases, it may be enough to prepare only unenhanced series (e.g. consideration of fresh bleeding or urinary tract stones) or only venous phase, but sometimes more than one phase maybe required.
Arterial, parenchymal, venous and late phase images can be produced after administration of intravenous contrast material.
The high-volume, quick data collection allows mapping with various technical parameters and even the whole body can be scanned.

Dynamic CT: after the intravenous injection of the contrast material the same body region will be several times scanned, that means we detect the time course of the contrast enhancement (e.g. by the focal liver lesions).
HRCT (high resolution CT): it visualizes thin layers with high resolution. The measurement time is longer and the radiation exposure is higher (e.g. examination of the lung parenchyma or focused investigation of the inner ear).

Three-dimensional (3D) images can be produced from the large amount of data.
Arterial CT angiography (CTA): the measurement is done by bolus detection at injection of the intravenous contrast material. The flow is high 4-5 ml/sec. In the selected altitude (location) the bolus detection may be done under the control of eye or it is possible to set up automatic bolus detection. With software 3D reconstruction is possible from the axial slice.
Venous CT angiography: with comparison of the arterial angiography more contrast material should be injected with slow flow (1.8-2 ml/sec). Due to the prolonged contrast enhancement automatic 3D representation is not possible; the different planar reconstructions are preferred.
Virtual colonoscopy: instead of colonoscopy it is a CT-scan (with screening purposes, or if the endoscopic examination can not be performed); with this method we can diagnose cancers and pre-cancer conditions (polyps).
Virtual bronchoscopy: a non-invasive assessment of bronchial structures (foreign body, tumor).
CT perfusion: in case of acute ischemic stroke with the help of time-density curves (perfusion maps) it is possible to determine the size of penumbra, and the extent of brain tissue that can be saved.
With the newest machines it is possible to do perfusion examinations in liver, kidneys and lung.
These studies have advanced technical and personnel conditions, the assessment of them is more time-consuming than the average examinations.

The CT can guide invasive diagnostic procedures (FNAB, biopsy) and therapeutic interventions (drainage, RFA) also when it is not possible with ultrasound. In many cases the CT-guided biopsy can be carried out more accurately and safely, than the US-guided procedure.

4.6. Advantages and disadvantages of CT examination

The benefits of CT examination
The CT-scan - against the summation effect of X-ray procedures - has an excellent spatial resolution (better than MRI) and gives a good anatomical orientation.
Contrast to the ultrasound the set of the sections is standard, it has a good reproducibility and it has the ability for cross-sectional imaging of the total body.
The contrast resolution, especially with proper contrast-enhancement techniques - except the magnetic resonance imaging - is over other imaging procedures.
Based on the density measurements (Hounsfield-value) the individual lesions can also be ordered into different tissues.
Short examination time
The CTA takes out the diagnostic angiography.
Based on 3D images it is possible to design recunstructive surgery.

The disadvantage of CT examination
The greatest and almost only disadvantage of the CT is, that it uses ionizing radiation that is even multiple fold bigger than by conventional X-Ray examination (in some cases it can be 5-20 fold bigger). Compounded by direct and scattered (1-2 scale smaller) radiation exposure.
In many cases the radiation doses suffered by the patient aren't in the centre of attention in clinical doctors.

4.7. Summary

The CT is one of the most effective methods in the diagnostic algorithms based on the great diagnostic accuracy, despite the considerable radiation exposure. The resolution can be further enhanced by application of contrast material. The examination time is short; it is possible to scan even the whole body in seconds that is a unique diagnostic ability.

Translated by Csaba Korom

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