Principles+of+Neurological+Imaging

=**Principles of Neurological Imaging**=


 * 1. Define the physical parameters imaged with the various diagnostic imaging techniques.**

__Ultrasound__ Ultrasound uses high frequency sound to image boundaries and echogenic tissue, but can also be used to measure blood flow by exploiting the Doppler effect. It can be used to monitor carotid blood flow, fetal development, and neonate geminal migration.

__X-ray Radiography and Fluoroscopy__ X-rays penetrate and record shadow variations on film (radiography) or in real time (fluoroscopy). Differences in ability to attenuate x-rays are measured; attenuation depends on thickness, density, and atomic number.

__Computed Tomography__ CT are x-rays at many different angles that are reconstructed into cross-sectional slices using computers and brute force number crunching.

__Nuclear Medicine and PET Scanning__ Nuclear medicine and PET scans are emission studies where radioactive substance, usually a gamma radiator, is placed into a patient and images are made of the distribution of the material within the patient. These modalities include planar nuclear imaging, SPECT, coincidence detection imaging, and PET. While most ultrasonic, radiographic, CT, and MRI studies provide visualization of anatomy, most nuclear medicine studies provide information on function or physiology. For example, nuclear medicine can be used to evaluate tumor metabolic activity, blood-brain barrier integrity, or brain death.

__Magnetic Resonance Imaging (MRI)__ MRI images hydrogen nuclei using magnetic fields and radio waves.

__Special Imaging Techniques__ Special MR imaging techniques use MRI but with a variety of additional techniques for particular situations.

Echo Planar Imaging (EPI) is very fast but generally low in quality; it is useful for rapid sequential imaging for flow studies or for blood oxygen level dependent (BOLD) imaging.

MR Angiography (MRA) is similar to images of blood vessels generated by x-ray angiography but may be particularly benefit to patients sensitive to iodinated x-ray contrast media.

Perfusion imaging and diffusion imaging are useful for evaluation of acute stroke and other ischemic events.

Functional MRI (fMRI) measures deoxygenated


 * 2. Identify general limitations to diagnostic images with respect to image contrast, low contrast, detectability, noise, spatial resolution, and image artifacts.**

__Ultrasound__ Ultrasound is relatively inexpensive and widely available. It is non-invasive and is not ionizing radiation. However, it is difficult to use to image past air or bone.

__X-ray Radiography and Fluoroscopy__ X-rays are good for skull radiographs, vertebrae imaging, angiography with iodine contrast media, but it cannot visualize soft tissue differences well. Additionally, x-rays are an ionizing radiation risk.

__Computed Tomography__ CT scans have good sensitivity to small x-ray density differences and can visualize calcified or ossified abnormalities. CT is capable of multiplanar imaging and angiography with iodine contrast media. However, CT is expensive had has lower resolution than radiographs, though usually better than MRI. Because multiple images are taken, radiation dose is relatively high. Additionally, the large number of images generated creates a logistical problem in terms of archiving images.

__Nuclear Medicine and PET Scanning__ Nuclear medicine is generally relatively low resolution and has high image noise. Additionally, it is a modality which uses ionizing radiation. It is not a good modality to evaluate anatomy but is useful in evaluating physiology.

__Magnetic Resonance Imaging (MRI)__ MRI is good for imaging soft tissue with high contrast and detail. It has multiplanar imaging capability, and is non-ionizing radiation. Angiography with paramagnetic contrast media such as gadolinium may be used. There are a variety of MRI techniques for various situations. However, MRI cannot be used with ferromagnetic prosthetic or support hardware because of the use of powerful magnetic fields. Additionally, MRI may be a problem with claustrophobic patients.

__Special MR Imaging Techniques__ Echo Planar Imaging (EPI) is very fast but generally low in quality; it is useful for rapid sequential imaging for flow studies or for blood oxygen level dependent (BOLD) imaging.

MR Angiography (MRA) can produce artifacts as blood flows through vessels, but may be distinguishable from surrounding tissue depending on the pulse sequence.

Perfusion imaging and diffusion imaging are useful for evaluation of acute stroke and other ischemic events.

Functional MRI (fMRI) uses deoxygenated hemoglobin as an endogenous contrast agent to measure blood flow as a surrogate marker for brain activity. This is accomplished using rapid scan techniques such as Echo Planar Imaging, and, therefore, subject to the same limitations of such MRI techniques.

MR Spectroscopy does not produce images but analyzes the molecular concentrations in areas of interest.


 * 3. Understand the basic principles underlying the various diagnostic imaging modalities and how they may provide information on anatomy and pathology.**

__Ultrasound__ Basically, high frequency pressure waves reflect off structures and in homogeneities, permitting the localization and measurement of anatomy, determination of cysts, and guidance for percutaneous needle biopsies. Measurement of frequency shifts or correlational analysis can yield image motion of blood in vessels.

__X-ray Radiography and Fluoroscopy__ X-rays are sent through the patient and recorded on a digital sensor or directly on film. Alternatively, the x-rays can excite electrons which are trapped in fluorescent traps. A laser reads the fluorescent traps and knocks the electrons out to produce an image using less electromagnetic radiation exposure. Contrast media can be injected to visualize blood vessels and images without contrast can be subtracted out to visualize vessels.

__Computed Tomography__ The computer calculates what each cross-sectional slices (0.5-10mm) of the patient must look like in order to produce the set of measured shadows from x-ray transmission. The calculated CT number is a scaled value of “linear attenuation coefficient” or radiodensity which is a function of physical thickness, density, and atomic number. CT numbers are based around water (which is set at 0) with air and fat looking relatively darker and CSF, white matter, gray matter, blood, muscle, and dense bone looking lighter. The CT image range can be adjusted to see a wide CT number range but low contrast, or a narrow CT number range but high contrast.

Scout scans are digital radiographic images taken with a CT system to assist in setup and localization of cross-sectional slices. Spiral or helical scans revolve the tube and detector continuously around the patient as the patient is moved slowly through the x-ray beam; this data acquisition scan allows for rapid acquisition of a series of slices covering a volume of the patient. Multi-slice detector CT has more rows of detectors to allow for faster scanning.


 * **Substance** || **CT Number** ||
 * Air || -1000 ||
 * Fat || -100 ||
 * Water || 0 ||
 * CSF || 10 ||
 * White Matter || 25 ||
 * Gray Matter || 35 ||
 * Blood || 45 ||
 * Muscle || 50 ||
 * Dense Bone || 800-2000 ||

__Nuclear Medicine and PET Scanning__ In nuclear medicine planar imaging, a gamma camera is used to obtain planar or projection views of radioactive material distribution within the body. In single photon emission computed tomography (SPECT), a gamma camera acquires planar gamma images from many different angels around the patient and records the data in a series of cross-sectional slices to depict the radioactive material concentration and distribution in the slice; it is essentially the nuclear medicine equivalent of CT. Positron emission tomography (PET) uses positron emitting radioactive materials and a specialized PET scanner with one or more rings of radiation detectors around the patient; positrons annihilate with electrons to release 2 photons in opposite directions that are detected using coincidence detection and localized to the line the source of the photons.

__Magnetic Resonance Imaging (MRI)__ MRI uses high energy magnetic fields to align hydrogen nuclei (protons) at two possible tip angle orientations. The spinning proton undergoes precession. If a radiofrequency (RF) wave at the precession frequency hits the proton, resonance will occur and the proton absorbs the RF energy, flipping it to the opposite direction of the magnetic field. Over a period of time T1 (spin-lattice relaxation time), the proton returns to its ground state and producing an echo. When the proton flips, it produces some transverse plane dipoles which will go out of alignment or randomize depending on the variations of the local magnetic fields around individual protons; the time for this transverse plane dipole to go out of alignment is T2 (spin-spin relaxation time).

MRI produces images that are generally some combination of T1, T2, and (hydrogen) proton density.

``*`` Very short T2 such as large molecules or solids cannot be imaged.
 * **Molecule** || **T1** || **T2** ||
 * Large, slow, bound molecules || Long T1 || Short T2 ||
 * Medium sized molecules and molecular motion || Short T1 || Short T2 ||
 * Small, rapidly moving, free molecules || Long T1 || Long T2 ||

MRI uses a sequence of RF pulses and short applications of magnetic field gradients to localize tissue that is stimulated and acquire information on the location of signals being emitted in order to form an image of the proton density, T1, and T2 characteristics of the tissues being imaged. Data is generally taken over a series of many pulses. 90 degree RF pulses flip half the available protons while 180 degree RF pulses flip all available protons. TE is the echo time from the 90 degree excitation pulse to the middle of the echo signal when the protons realign at ground state. TR is the repetition time or the time from the start of one 90 degree pulse sequence to the tstart of the next 90 degree pulse.


 * **CSF Color** || **TE** || **TR** || **Weighted Image** || **Terminator** ||
 * Black || Short TE || Short TR || T1 Weighted || Terminator 1 ||
 * -- || Short TE || Long TR || Proton Density Weighted || -- ||
 * White || Long TE || Long TR || T2 Weighted || Terminator 2 ||

__Special Imaging Techniques__ Echo Planar Imaging (EPI) uses very rapid, low quality imaging to capture an entire image data set very quickly.

MR Angiography (MRA) is similar to images of blood vessels generated by x-ray angiography but may be particularly benefit to patients sensitive to iodinated x-ray contrast media.

Perfusion imaging takes many images of the same location over time of injected MR contrast media (iron or gadolinium) to characterize blood flow characteristics and perfusion of tissue. Diffusion imaging uses a special pulse sequence with magnetic field gradients that cancel out for stationary sources but cause dephasing of hydrogen nuclei that are freely diffusible in the direction of the gradient.

Functional MRI (fMRI) uses deoxygenated hemoglobin as an endogenous contrast agent. Functional activation of specific areas of the brain causes an increase in blood flow and, in turn, increases oxygen usage. Using rapid scan techniques such as echo planar imaging, a series of images can be analyzed to identify areas of the brain changing in unison with some defined task. fMRI is useful for preoperative evaluation of brain surgery to estimate risks of post-operative deficits in various areas of the brain such as the motor strip, sensory strip, Broca and Wernicke areas, and visual cortex.

MR Spectroscopy alters the specific molecular environment of hydrogen nuclei by manipulating the magnetic field strength and measuring the change in precessional frequency. This shift in frequency can be used to evaluate specific molecular concentrations and can be useful in identifying tumor vs. scar or other conditions. Common MR spectroscopy markers are N-acetylaspartate (NAA) for neuron damage, choline (Cho) for membrane disruption and malignancy, myo-inisitol (mI) for gliosis, and lactate for hypoxia.