Basic principles of CT brain

a) Principles of CT scan
Computed Tomography (CT) is an imaging modality that uses computer-generated analysis ( i.e. computed ) of the x-ray attenuation through a section of the body tissue to produce cross-sectional images ( i.e. tomography ) with great anatomical details.

Unlike plain X-ray images which are projected images of one plane, CT X-ray tubes produced a number of projections by rotation of the CT X-ray tube around the patient. Images are then produced by computer program which analyzes the sum of projections.

The basic principles of CT are identical to conventional x-ray, with greater x-ray attenuation results in regions of high density; whereas soft tissue structures that have weak attenuation of x-rays, such as organs and air-filled cavities, results in lower density.

b) Modern CT units

The basic components of a CT unit include :
1. Gantry
2. Patient couch
3. X-ray tube
4. High-powered generator
5. Detector and detector electronic
6. Data transmission system
7. Computer system for image reconstruction and manipulation
The overall performance depends on each one of these components.

Multi-detector CT (MDCT) is now readily available in most radiology departments. Single or multiple (ranged from 4 to 320) solid-state detectors, positioned opposite to the x-ray tube, result in multiple slices per beam rotation around the patient. The couch moves continuously through gantry that the rotating x-ray beam stored, generating a continuous helical data that can be reconstructed into various orthogonal planes and slice thicknesses.

Advantages of MDCT include shorter scan times, reduced patient and organ motion, and the ability to acquire images dynamically during the infusion of intravenous iodinated contrast, which can be used to perform CT angiography (CTA) for vascular structures and perfusion images.

The resolution of an image depends on the radiation dose, the detector size, collimation, the field of view, and the matrix size of the display. A modern CT unit is capable of obtaining sections as thin as 0.5-1 mm with 0.4 mm inplane resolution at a speed of 0.3 s per rotation; complete studies of the brain can be completed in 1-10 s.

c) Intravenous Contrast in Brain CT
Intravenous iodinated contrast can reveal the vascular structures clearly.
IV contrast is normally blocked by the blood-brain barrier (BBB) and cannot reach the brain tissue, with an exception for structures with no BBB naturally : 
1. Pituitary gland
2. Choroid plexus
3. Dura

Therefore, any leakage of contrast to tissue with BBB may signify pathologies like :
1. Tumors ( due to angiogenesis )
2. Infarcts and infections ( damage and open of BBB )

 

Noted that the use of iodinated contrast agents may pose a small risk of allergic reaction, contrast nephrotoxicity in renal insufficiency and add additional expenses. Therefore, informed decision must be made in using contrast agent. Therefore, contrast is only administered when benefits in conditions like characterizing mass lesions, acquisition of CT perfusion (CTP) and CTA studies may outweigh the risks.

d) Clinical Applications of Brain CT
– CT offers multiple applications to aid in the detection and characterization of various neurologic disorders, such as :
1. Acute change in mental status
2. Focal neurologic findings
3. Acute stroke
4. Acute trauma to the brain and spine
5. Suspected subarachnoid hemorrhage
6. Conductive hearing loss

– A multimodal approach for CT brain evaluation combines protocols like non-enhanced CT, CTP, and CTA

e) Non-enhanced CT
Non-enhanced CT, the traditional initial screening technique of the brain, can reveal the presence of an infarct or intracranial hemorrhage. Focal edematous hypodensity on CT, in the setting of abrupt onset of stroke symptoms, is specific for tissue with a high probability of irreversible infarction.


f) CT perfusion (CTP)
CTP extends structural imaging technique to evaluation of brain physiology. It measures cerebral perfusion by determining different blood hemodynamic properties :

1. Cerebral blood flow (CBF)
2. Volume and other time-based parameters

In real clinical practice, CTP is mostly used to improve the accuracy of differential diagnosis by excluding stroke mimics, and aids in outcome prediction in acute stroke patients.

g) CT angiogram (CTA)
CTA displays in a three dimensional manner to yield angiogram-like images. CTA enhances the assessment of the cervical and intracranial arterial and venous anatomy for the detection vessel occlusion.

– It becomes an increasingly used technique for acute stroke and aneurysm diagnosis, characterization and treatment planning.

h) Disadvantages of Brain CT
One of the technical disadvantages of CT is the use of ionizing radiation. Although the normal radiation dosage for a routine brain CT study is between 2 and 5 mSv, it is important to note that radiation dose is additive, additional acquisition series during a single exam sitting will increase dose. Care must be taken to reduce exposure when imaging pediatric patients, and ALARA (As low as reasonably achievable) principle should be implemented in all situations to patients and personnel. With the advent of MDCT, CTP and CTA, the benefit must be weighed against the increased radiation doses associated with these techniques.

Advanced noise reduction and image reconstruction technologies result improvement in qualitative and quantitative aspects of CT image quality, with significant reductions in radiation exposure (30–40% lower radiation doses).

Additional complications from CT scan are those associated with the use of intravenous contrast agents mentioned before, which are essential for CTP, CTA, and computed tomography venography. While two broad categories of contrast media, ionic and nonionic, are in use, ionic agents have been largely replaced by safer nonionic compounds.

Clinically, it is important to consider the risks of contrast-induced nephropathy or anaphylactic reaction, particularly in geriatric patients of whom glomerular filtration rate can be elevated despite normal creatinine levels. Optimized CTA protocols can be implemented for the contrast load reduction, while improving arterial opacification, including saline bolus following contrast administration.

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