Principles of Modern Neuroimaging

Radiography

Discovered by Wilhelm Roentgen in 1895, x-rays are photons carrying electromagnetic energy that are created by an anode-cathode system within a vacuum. X-ray photons are of higher energy and shorter wavelength than visible light. When x-ray photons collide with atoms of varying sizes, they either pass through or are absorbed. Larger (heavier, radiopaque) atoms, such as calcium or metals, are more likely to absorb the energy of the photons than smaller (lighter, radiolucent) atoms and small molecules, such as water or air. When a patient is positioned in a beam of x-rays, the x-rays will be differentially absorbed based on the tissue components (bone, soft tissue, aerated sinuses). Photons that pass through the patient strike a detector and create an x-ray image, producing a two-dimensional projected image of the different attenuation properties of body tissue.

Fluoroscopy is a variation of radiography in which images are obtained in rapid succession and displayed in real time on a screen. In angiography, intravenous contrast material is injected into vessels during continuous fluoroscopy. Digital subtraction angiography (DSA) is a technique in which a baseline or mask image is initially obtained of the area of interest. This baseline image is subtracted from subsequent images obtained during intravascular contrast injection, optimizing visualization of the contrast agent itself within opacified vessels. With modern angiography systems, images can be acquired from multiple different projections during a single contrast injection by rapidly rotating the fluoroscopy unit around the patient, allowing reconstruction of a three-dimensional image of the vascular structures.

Computed Tomography

Sir Godfrey Hounsfield and Dr. Allan Cormack invented the first computed axial tomographic scanner in 1972, which earned them the Nobel Prize for medicine in 1979. CT scanners have advanced significantly since that time, rapidly increasing in speed and resolution. Modern scanners use a rotating x-ray tube and detector array that revolve around the body, obtaining tissue attenuation information from beams or rays of tissues within a slab. Standard axial images are obtained by applying a reconstruction algorithm, typically filtered back projection, to reconstruct the two-dimensional image. Sagittal, coronal, or oblique imaging planes can be reconstructed from the axial sequences by computer reformatting. Radiodense contrast material administered intravenously or parenterally can outline hollow structures such as blood vessels or the digestive system.

CT density is quantitatively measured in Hounsfield units (HU). Hounsfield units describe a linear scale of attenuation that is constant across scanner platforms, with water and air given arbitrary values of 0 and −1000, respectively. Materials with increased x-ray attenuation with respect to water have a positive HU value, and those with less x-ray attenuation than water have a negative HU value ( Table 4.1 ).

CT images can be viewed in different ways to accentuate different tissues. Window level describes the center point of the gray scale, and window width describes the range of CT values displayed. For example, gray matter has an attenuation of approximately 35 HU, and white matter attenuation is approximately 45 HU. To differentiate gray matter from white matter, a narrow window is needed to highlight small changes in HU values. On the other hand, if detailed evaluation of dense material such as bone is desired, a wide window better delineates the radiodense edges. Window level and width are easily manipulated using most imaging viewing software.

In addition to window width and level, CT scans are generally processed using different reconstruction filters, frequently referred to as bone and standard algorithms . Both filters can be applied to a single acquisition of data, allowing accentuation of different structures. Standard algorithm is a method of averaging adjacent pixels to accentuate soft tissue detail. Standard algorithm images are useful for evaluating gray-white matter differentiation and for detecting acute blood. Bone algorithm images are processed to maximize edges, thus accentuating high-density materials such as calcium and metal. Bone algorithm images are also useful for evaluating lung parenchyma due to the differences in attenuation between aerated lung and small soft tissue attenuation structures such as blood vessels and pulmonary nodules.

Magnetic Resonance Imaging

Magnetic resonance imaging was developed by a host of innovative scientists over multiple decades of development from a scientific tool to a medical imaging necessity. MRI is based on the principles of nuclear magnetic resonance (NMR), first discovered by Felix Bloch and Edward Purcell, for which they were awarded the Nobel Prize in physics in 1952. NMR can be utilized to characterize and differentiate tissues based on their intrinsic NMR signal. Using NMR techniques, chemist Paul C. Lauterbur and physicist Sir Peter Mansfield developed the gradients and mathematical formulations required for rapid two-dimensional MR images, publishing the first images in 1973 and 1974. Drs. Raymond Damadian, Larry Minkoff, and Michael Goldsmith were also instrumental in the development and refining of this technology for use in humans. For their pioneering work in MRI development, Drs. Lauterbur and Mansfield shared the Nobel Prize in physiology or medicine in 2003.

The detailed physics principles underlying MRI are highly complex and beyond the scope of this chapter. In brief, a powerful electromagnetic field is created within the bore of an MRI machine, typically along the cranial-caudal (z) axis. Protons that are present in the human body predominantly as hydrogen atoms in water molecules reach equilibrium aligned along the direction of this magnetic field (longitudinal magnetization). A radiofrequency (RF) pulse is applied at a resonance frequency specific to the protons within the main magnetic field (B-zero), causing them to absorb energy and change their alignment toward the horizontal/vertical plane (x-y axes), called transverse magnetization. When the RF pulse ends, the protons first dephase in the x-y direction (free-induction decay, the basis of T2 signal) at a rate dependent on the molecular structure of the sample. The protons then realign along the z-axis (spin-lattice relaxation, the basis of T1 signal) at a slower rate, which is dependent on the molecular structure surrounding the proton. As the protons realign toward equilibrium, they emit RF energy that is detected by antennas (receiver coil) surrounding the patient in the scanner. By inducing small changes in frequency and phase of the proton resonance frequency that vary as a function of proton position, the MRI system reconstructs the precise location of each signal within the patient. The MRI system thus produces cross-sectional images through the patient where each pixel (corresponding to a defined volume of tissue, or voxel) depends on the magnetic microenvironment of the corresponding tissue.

From this general principle, a variety of pulse sequences have been developed to emphasize different tissue characteristics. A pulse sequence refers to a specific pattern of RF pulses that may vary in timing, order, repetition, and direction. Basic pulse sequences include spin echo, gradient echo including T2* gradient recalled echo (GRE) and susceptibility-weighted imaging (SWI), inversion recovery including short tau inversion recovery (STIR) and fluid attenuated inversion recovery (FLAIR), and echo planar imaging used in diffusion, perfusion, and functional MR techniques. The clinical applications of those sequences most pertinent to neuroradiology are described in this chapter.

MR angiography (MRA) can be performed by several techni­ques, including time-of-flight, phase-contrast, and gadolinium-enhanced MRA. In time-of-flight angiography, protons in moving blood are tagged in one tissue slab by applying an RF pulse to change their longitudinal magnetization. Tagged protons are subsequently detected in a different tissue slab that has not experienced the RF pulse. The direction of blood flow can be selected by applying a saturation pulse to null the longitudinal magnetization from protons traveling in the opposite direction. For example, to selectively visualize tagged blood protons moving superiorly within the cervical arteries, a saturation pulse is applied superior to the scan volume (eg, within the head) to neutralize the longitudinal magnetization of the tagged protons within the intracranial compartment before they travel inferiorly in the cervical veins.

Phase contrast angiography is another method to detect moving protons such as those in blood or CSF. Phase contrast imaging depends on applying bipolar gradients to protons, so that stationary protons experience both positive and negative gradients, with no net phase change. Moving protons experience only one gradient before moving out of the field, resulting in positive or negative excitation.

Contrast-enhanced angiography uses gadolinium-based contrast agents to highlight blood vessels. Gadolinium is strongly paramagnetic, resulting in significant T1 shortening (high signal on T1-weighted sequences). After intravenous administration of gadolinium, T1-weighted sequences can be obtained during the arterial or venous phase to highlight vascular anatomy. Three-dimensional reformatted sequences can be constructed after any of the angiographic techniques.

Radiography

Once the mainstay of neuroradiology, skull radiography and its permutations have been largely replaced by cross-sectional imaging modalities such as CT and MRI. Radiography is still used in the evaluation of shunts and other neurosurgical implants such as intrathecal pumps and deep brain stimulators.

Shunt series are the most commonly encountered of these studies and include radiographs of the shunt components in two planes. Shunt series are used to evaluate the nature of the shunt, including the location of the ventricular and distal catheters, drainage location (eg, atrial, peritoneal, pleural), and the type and setting of the shunt valve, as well as to identify causes of shunt dysfunction. Shunt catheters, valves, and tubing can be quickly examined for kinks or disruption, without the cost or radiation exposure of CT or the time and cost of MRI. The type and setting of most implanted shunt valves can also be determined based on radiographic appearance.

Radiography is also sometimes used in the evaluation of the bony calvarium. Although CT has largely replaced radiography for evaluation of facial fractures or sinusitis, skull films are occasionally obtained for these purposes. Linear nondisplaced skull fractures that may be missed on axial CT can at times be detected by radiography. Such occult fractures can sometimes be visualized by either examining the scout tomographic view obtained as part of a routine CT scan or by constructing surface-rendered three-dimensional (3D) reformats of CT images. In addition, skull films are sometimes obtained to evaluate for radiolucent bone lesions prior to the placement of stereotactic frames for Gamma Knife treatment or stereotactic biopsy.