(a) Axial noncontrast CT showing densities of different structures. Note: Gray matter is hyperdense (brighter) relative to white matter. (b) Noncontrast Head CT with hyperdense hematoma in the left putamen with mean density of 64 HU
MRI Axial T2 (a) and FLAIR (b) images- Gray matter is hyperintense (bright) relative to white matter. CSF is hyperintesne (bright) on T2 and hypointense (dark) on FLAIR
Sagittal (a) and axial (b) T1 WI- gray matter is hypointense (dark) relative to white matter, CSF is dark, and fat is very hyperintense (bright)
The axis in which the image is captured is also important when interpreting images. The common planes of image acquisition are:
Axial: transverse imaging plane that is perpendicular to the long axis of the body
Sagittal: longitudinal plane that divides the body into right and left sections.
Coronal: An imaging plane obtained through subsequent sections made from front to back.
In addition to understanding the plane of the image, a basic knowledge of imaging terminology is necessary to understand and be able to articulate what is being viewed.
Non-contrasted images – These are images obtained without the administration of extrinsic contrast medium.
Contrasted images – These are images obtained with the use of intravenous contrast medium. Contrast is used to facilitate visualization of adjacent body tissue as well as to highlight pathology which may have increased density/intensity due to increased vascularity or the breakdown of blood brain barrier. In CT scan imaging, the contrast material contains iodine and is denser (brighter) than the brain . MRI contrast media are Gadolinium-based.
Gray matter – Anatomic areas of the nervous system where the nerve fibers are unmyelinated. Contains the cell body and dendrites and nuclei of the nerve cells. i.e. cerebral cortex and deep gray structures (basal ganglia, thalami)
White matter – Anatomic areas of the nervous system where the myelinated axons and white matter tracts occur, e.g.- corpus callosum. On CT, gray matter is hyperdense relative to white matter.
3.3 Basic Brain Imaging
3.3.1 Computerized Tomography (CT) Scan
Sir Godfrey Hounsfield, working at EMI Laboratories, first conceived of the idea of CT scan imaging in 1967. The first prototype was finished in 1971 and was only designed to scan the head. The prototype took several hours to get the data required to scan a single “slice” and required several days to compile the information into a single image. The images produced were very crude by today’s standards, but allowed physicians to see soft tissue structures by imaging for the first time. This technology has evolved to the point of including vascular and perfusion CT scan technology.
Tomography stems from the Greek tomos, meaning “section”. Like conventional x-rays, CT scans measure the density of studied tissues. The difference from conventional x-ray is that rather than taking one view, the x-ray beam is rotated around the patient to take many different views of a single slice of anatomical structures. As the beam passes through the patient, the x-rays are partially absorbed by the tissues encountered. The amount of energy absorbed is determined by the density of the tissue traversed. Once obtained, the images are reconstructed by the computer to reflect detailed images of all the structures including air, bone, liquid, and soft tissue. With the addition of advanced computational resources, multiple slices can now be acquired simultaneously. Spiral (helical) CT can acquire data continuously without stops. This technology reduces radiation exposure and increases the resolution and speed .
Images obtained are displayed with different densities. Dense structures, like bone or calcifications, appear white on images. Less dense material, such as air, appears black. CT density is often expressed in Hounsfield units (HU) (Table 3.2).
Common CT attenuation values
Attenuation value in HU
From −500 to −1,000 HU
From −10 to −150 HU
From 0 to 20 HU
From 32 to 45 HU
From 25 to 32 HU
From 60 to 90 HU
More than 100 HU
From 200 HU and above
CT is the preferred initial screening technique acute and hyperacute pathologies, including brain trauma, stroke, and altered mental status. It is also the preferred first line study for the identification of space occupying lesions. The speed, accessibility, relative safety, and high sensitivity to cortical bone and acute blood make CT imaging ideally suited for trauma evaluation. CT is not very sensitive for the detection of hyperacute strokes (less that 3 h from symptom onset); however, loss of gray/white differentiation may be seen very early in some cases. Beyond anatomic information of parenchymal structures, useful indirect information about the status of the brain vessels can sometimes be obtained from a plain head CT. One example is the “hyperdense vessel sign,” that reflects occlusion or extremely slow flow of a large vessel, usually the middle cerebral artery. CT is also helpful in assessing for space occupying lesions and evaluation of ventricle size and herniation.
A CT angiogram (see below) expands the role of the plain head CT in the acute setting by offering accurate details of the major brain vessels after a bolus of intravenous contrast. This technology serves to quickly detect proximal large vessel occlusions or injury in stroke and trauma patients . It can also be used to identify underlying vessel abnormalities (i.e. arteriovenous malformations or aneurysms), vascular collateral vessels, vasospasm, or tumor vascularization patterns.
Typical indications for CT scan:
Hyperacute and acute stroke (both ischemic and hemorrhagic)
Head trauma (Fig. 3.4)
Suspected subarachnoid hemorrhage (Fig. 3.5)
Vascular occlusive disease or vasculitis (including use of CT angiography and/or venography)
Detection or evaluation of a calcification
Immediate postoperative evaluation following surgical treatment of tumor, intracranial hemorrhage, or hemorrhagic lesions
Treated or untreated vascular lesions
Suspected shunt malfunctions, or shunt revisions
Mental status change
Hydrocephalus and other causes of increased intracranial pressure
Suspected intracranial infection, especially to rule space occupying lesions before a lumbar tap in suspected meningitis
Congenital lesions (such as, but not limited to, craniosynostosis, macrocephaly, and microcephaly)
Evaluation of psychiatric disorders
Suspected mass or tumor
When magnetic resonance imaging (MRI) is unavailable or contraindicated
Axial CT showing large left convexity biconvex epidural hematoma (a) confined by sutures and non-displaced fracture seen on bone windows (b)
Axial CT with blood in different locations. (a) At the gray-white matter- characteristic of shearing (traumatic axonal) injury. (b). Intraventricular layering in the occipital horn in dependent portion. (c) Subarachnoid blood in quadrigeminal cistern. (d) Subarachnoid blood in prepontine cistern
The potential advantages and disadvantages of CT imaging are summarized below:
Widespread availability, relatively low cost, and rapid acquisition time.
Evaluation in patients with contraindications to MR scanning or when screening for MRI cannot be obtained.
Speed and open set up of equipment creates ease of patient placement while alleviating the potential of claustrophobia
Good for cortical bone pathology- best modality to assess for fractures
High sensitivity to detect acute hemorrhage makes it the corner stone of imaging in stroke treatment and trauma. Blood remains hyperdense on CT (HU 60–100) for 7–10 days.
Uses ionizing radiation and hence risk of radiation exposure. Special considerations for pediatrics and pregnant women. For the latter, the maximum risk occurs during the first 8–11 weeks of pregnancy.
Radiation dose is additive so the more images obtained, the higher the dose of radiation to which the patient is exposed
Posterior fossa structures of the brain are not as well visualized due to beam hardening from the dense petrous bones 
Relatively poor soft tissue visualization due to inferior contrast resolution compared to MRI
Iodinated IV contrast is required for CT with contrast, CTA, CTP and CTV imaging, which can be nephrotoxic and cause contrast-induced nephropathy
Contrast allergies may exist to iodine-based contrast media
3.3.2 Magnetic Resonance Imaging (MRI)
MRI is an imaging modality that uses non-ionizing radiation to create diagnostic images. The concept of MRI was first described by Felix Bloch and Edward Purcell in 1946 and was initially called Nuclear Magnetic Resonance Imaging after its early use for chemical analysis. It was not until 1971 when the potential medical uses of this technology were realized.
A MRI scanner consists of a large and powerful magnet. A radio wave antenna is used to send signals to the body and returning signals are converted into images by a computer.
MRI images are created based on the absorption and emission of radiofrequency energy, without using ionizing radiation. MRI scanning involves the use of primary and secondary magnetic fields. The use of 1.5 or 3 Tesla (T) terminology refers to the strength of the magnetic field. The current FDA approval in terms of upper limit of field strength for adults is 8T and 4T for adults and children, respectively . Most scanners in current clinical use are “electromagnets” or super conducting magnets, which implies that the static magnetic field is always turned “on” even when a patient is not being scanned. Safety precautions are to be strictly followed at all times. “Quenching,” the process of turning the scanner off, is rarely performed. “Open” MRI are useful in claustrophobic patients, although the upper field strength is usually limited to 1T.
For all practical purposes, if a patient develops a medical emergency while being scanned, it is best to get the patient out of the scanner before starting resuscitation. All equipment inside the scanner room must be MR compatible (non- ferromagnetic).
In addition to the main magnetic field, secondary fields are created using radio frequency pulses (RFP) and gradient pulses emitted from the scanner to spatially encode the signal in the x-, y- and z-axis. These secondary gradients cause the loud metallic banging noise inside the scanner. Image density depends on several contrast parameters intrinsic to the tissue being scanned (T1 recovery time, T2 decay time, Proton density, Flow and Apparent diffusion coefficient), as well as extrinsic parameters that are varied by the radiologist to change image quality (TR, TE, Flip angle, TI or inversion time, Turbo factor/Echo train length).
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