CT scan (dual energy)

Principle

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Fig. 1 CT apparatus

Computed tomography (CT), originally known as computed axial tomography (CAT or CT scan) and body section röntgenography (see X-ray machine), is a medical imaging method (techniques and processes used to create images of parts of the human body for clinical purposes). It employs tomography (imaging by sections) where digital geometry processing (design of algorithms for 3D-modeling) is used to generate a 3D image of the internals of an object from a large series of 2D X-ray (electromagnetic radiation with 10-0.01 nm wavelengths, see also Spectroscopy) scans. The data of CT scans taken around a single axis of rotation can be manipulated by a process known as windowing (see Imaging: windowing). Standard CT generates images in the axial or transverse plane which can be reformatted in various planes or as volumetric (3D) representations of structures.

Fig. 2 Principle of spiral CT

The principle of operation is an X-ray source that rotates around the object with X-ray sensors positioned on the opposite side of the circle from the X-ray source. X-ray slice scans are taken one after the other as the object is gradually passed through the gantry. The data of the slices are combined together by the mathematical procedure known as tomographic reconstruction, for instance to generate 3D volumetric information (3D-CT scan), which are viewable from multiple different perspectives on CT workstation monitors. This can also be done with helical or spiral CT machines, which generate high-resolution 3D images from the data of the moving individual slices.

Application

Although most common in medicine, CT is also used in other fields, for example in experimental earth sciences, material sciences and archeology.

CT has become an important tool in medical imaging to supplement X-rays and ultrasonography or echography. Although it is still expensive, it is the gold standard in diagnostic. It is also used in preventive medicine or screening. Some important fields of application are described below.

Neurology CT is frequently used in neurology and related disciplines: cerebrovascular accidents, intracranial hemorrhage (stroke). increased intracranial pressure, facial and skull fractures, surgical planning for craniofacial and dentofacial deformities, ophthalmology (orbita), trough, nose and ear medicine, brain tumors with IV contrast (but less sensitive than MRI).

Pulmonology Non-contrast CT scans are excellent for acute and chronic changes in the lung parenchyma (pneumonia, cancer). Thin section CT scans are used for emphysema, fibrosis, etc., IV-contrast for the mediastinum and hilar regions. CT angiography (CTPA) is applied for pulmonary embolism and aortic dissection (helical scanners).

Cardiology Dual Source CT scanners and high resolution (multi-slice) and high speed (subsecond rotation) CT scanners are used for imaging of the coronary arteries.

Abdominal and pelvic region Cancer, acute abdominal pain, organ injury, disorders resulting in morphological changes of internal organs, often with contrast (barium sulfate for fluoroscopy, (see also Fluorescence), iodinated contrast for pelvic fractures).

Further CT is useful for (complex) fractures in the extremities.

CT is inadequate for osteoporosis (radiation doses, costs) compared to DXA scanning for assessing bone mineral density (BMD), which is used to indicate bone strength.

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Fig.3 ..CT scanner with cover removed to show the principle of operation

Working principle

In conventional CT machines, an X-ray tube (see X-ray machine)and detector are physically rotated behind a circular shroud (see Fig. 3). In electron beam tomography (EBT) the tube is far larger and higher power to support the high temporal resolution. The electron beam is deflected in a hollow funnel shaped vacuum chamber. X-rays are generated when the beam hits the stationary target. The detector is also stationary. Contrast materials are used to highlight structures such as blood vessels and to obtain functional information.

The data stream, representing the varying radiographic intensity sensed, reaches the detectors on the opposite side of the circle during each sweep. Then, it is computer-processed to calculate cross-sectional estimations of the radiographic density, expressed in Hounsfield units (HU).

Hounsfield unit

This unit is defined as:

HU_material = 1000(μ_material - μ_water)/μ_water,

where μ is the attenuation coefficient, which is analogue to the parameter A (absorbance or extinction) of the Lambert-Beer law. HU is dependent of the beam intensity of the X-ray source (mostly given in kilo-electron volt, keV).

Since μ_water is by definition zero, the radiodensity of distilled water at STPD conditions (see Gas volume units, STPD, BTPS and ATPS) is defined as 0 HU. At 80 keV, that of air at STPD is ca. -1000 HU, cancellous bone amounts to 400 HU and cranial bone to 2000 HU. The attenuation of metallic implants (dental, extremities) depends on the element’s atomic number. Titanium has ca. 9000 HU (at 80 keV) and iron steel ca. 24500, which can completely extinguish the X-ray and is therefore responsible for well-known line-artifacts in computed tomograms.

Pixels in an image obtained by CT scanning are displayed in terms of HUs (from -1024 to +3071). When the CT slice thickness is also factored in, the volumetric unit is known as a voxel, which is a cubical pixel. The phenomenon that one part of the detector cannot differ between different tissues is called the Partial Volume Effect. That means that a big amount of cartilage and a thin layer of compact bone can cause the same attenuation in a voxel as hyperdense cartilage alone.

Dual source (or energy) scanning offers the potential of differentiating materials beyond the visualization of morphology – for example, direct subtraction of either vessels or bone during scanning. Dual Source CT scanners, allow higher temporal resolution so reduce motion blurring at high heart rates, and potentially require a shorter breath-hold time. Manufacturers are now actively developing 256-slice MSCT (multi-slice CT), true 'volumetric' scanners, primarily for their improved cardiac scanning performance.

Advantages and hazards

Advantages over Projection Radiography (see Radiography)

· CT completely eliminates the superposition of images of structures outside the area of interest.

· Because of the inherent high-contrast resolution of CT, differences between tissues that differ in physical density by less than 1% can be distinguished.

· 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. This is referred to as multiplanar reformatted imaging.

Radiation exposure

CT is regarded as a moderate to high radiation diagnostic technique. Unfortunately the newer CT technology requires higher doses for better resolution. For instance, CT angiography avoids the invasive insertion of an arterial catheter and guide wire and CT colonography may be as good as barium enema for detection of tumors in the large intestines, but at the cost of a higher dose.

Cardiac MSCT is equivalent of 500 chest X-rays in terms of radiation. The risk on breast cancer is not well established. The positive predictive value is approximately 82% while the negative prediction is ca. 93%. Sensitivity is ca. 81% and the specificity is about 94%. The real benefit in the test is the high negative predictive value.

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.

Table 1 Typical scan doses

Examination	Typical effective dose (mSv*)
Chest X-ray	0.02
Head CT	1.5
Abdomen	5.3
Chest	5.8
Chest, Abdomen, Pelvis	9.9
Cardiac CT angiogram	6.7-13
CT colongraphy	3.6 - 8.8

*The sievert (Sv) has the same dimensions as the gray (i.e. 1 Sv = 1 J/kg = 1 m²·s^–2), but the former measures the biological effect and the later the radiated dose.

Adverse reactions to contrast agents

Because CT scans often rely on IV-contrast agents, there is a low but non-negligible level of risk associated with the contrast agents themselves, like (life-threatening) allergic reactions to the contrast dye (e.g. causing kidney damage).