Diffusion-weighted imaging (DWI) has revolutionised stroke imaging since its introduction in the mid-1980s, and it has also become a pillar of current neuroimaging.
The number of k-space lines (echoes) collected in a single shot is called the "Echo Train Length (ETL)" by GE and Canon; the "EPI factor" by SIemens and Philips; and the "shot factor" by Hitachi. The regions of k-space divided up by each shot of a multi-shot sequence are called segments.
Functional magnetic resonance imaging (fMRI) measures the small changes in blood flow that occur with brain activity.
Most scanners used in medical imaging have a main magnetic field strength of 1.5 Tesla, which is approximately 30,000 times the earth's magnetic pull.
MR spectroscopy is conducted on the same machine as conventional MRI. The MRI scan uses a powerful magnet, radio waves, and a computer to create detailed images. Spectroscopy is a series of tests that are added to the MRI scan of your brain or spine to measure the chemical metabolism of a suspected tumor.
The spin echo sequence is made up of a series of events : 90° pulse – 180° rephasing pulse at TE/2 – signal reading at TE. This series is repeated at each time interval TR (Repetition time). With each repetition, a k-space line is filled, thanks to a different phase encoding.
High signal seen on these images indicates a pathological process such as infection, tumour, or areas of demyelination – as in this patient with multiple sclerosis.
The best way to tell the two apart is to look at the grey-white matter. T1 sequences will have grey matter being darker than white matter. T2 weighted sequences, whether fluid attenuated or not, will have white matter being darker than grey matter. Read more about FLAIR sequence.
White matter hyperintensities (WMHs) are lesions in the brain that show up as areas of increased brightness when visualised by T2-weighted magnetic resonance imaging (MRI). WMH's are also referred to as Leukoaraiosis and are often found in CT or MRI's of older patients.
Both MRIs and CT scans can view internal body structures. However, a CT scan is faster and can provide pictures of tissues, organs, and skeletal structure. An MRI is highly adept at capturing images that help doctors determine if there are abnormal tissues within the body. MRIs are more detailed in their images.
Fast spin echo (FAISE or FSE, ref 65bis), also called turbo spin echo (TSE) is a sequence that results in fast scan times. In this sequence, several 180 refocusing radio-frequency pulses are delivered during each echo time (TR) interval, and the phase-encoding gradient is briefly switched on between echoes.
MRI can be used to detect brain tumors, traumatic brain injury, developmental anomalies, multiple sclerosis, stroke, dementia, infection, and the causes of headache.
MRI protocols are a combination of various MRI sequences, designed to optimally assess a particular region of the body and/or pathological process.
On a T2-weighted scan compartments filled with water (such as CSF compartments) appear bright and tissues with high fat content (such as white matter) appear dark.
Conclusions: High signal changes on T2 weighted images on MRI are common across degenerative (AD and FTD) and vascular dementias. Although lacunes and grade III DWMH are specific for VaD, the low sensitivities (sensitivities: for lacunes, 0.32; for grade III DWMH, 0.16) limit their use as diagnostic markers for VaD.
Overall, SPECT imaging yielded a higher sensitivity (93% vs 82% for SPECT and planar imaging respectively, p < 0.05), especially in the inferior and anterior regions. SPECT was also more sensitive for the detection of a single-vessel disease (90% vs 74% for SPECT and planar imaging respectively, p < 0.01).
Planar imaging is the acquisition of 2D nuclear images, similar to plain films in x-ray imaging.
Parallel imaging is a widely used technique where the known placement and sensitivities of receiver coils are used to assist spatial localization of the MR signal. Having this additional information about the coils allows reduction in number of phase-encoding steps during image acquisition.
1. n. [Formation Evaluation] The time between each echo in a nuclear magnetic resonance (NMR) measurement. The time is also the time to the first echo and is therefore an important parameter in defining the fastest relaxation time that can be measured.
The k-space represents the spatial frequency information in two or three dimensions of an object. The k-space is defined by the space covered by the phase and frequency encoding data. The relationship between k-space data and image data is the Fourier transformation.
Nuclear magnetic resonance spectroscopy is widely used to determine the structure of organic molecules in solution and study molecular physics and crystals as well as non-crystalline materials. NMR is also routinely used in advanced medical imaging techniques, such as in magnetic resonance imaging (MRI).
What are TR and TE? TR and TE are basic pulse sequence parameters and stand for repetition time and echo time respectively. They are typically measured in milliseconds (ms). The echo time (TE) represents the time from the center of the RF-pulse to the center of the echo.
Fluid-attenuated inversion recovery (FLAIR) is an MRI technique that shows areas of tissue T2 prolongation as bright while suppressing (darkening) cerebrospinal fluid (CSF) signal, thus clearly revealing lesions in proximity to CSF, such as cerebral cortical lesions.
T2* can be considered an "observed" or "effective" T2, whereas the first T2 can be considered the "natural" or "true" T2 of the tissue being imaged. T2* is always less than or equal to T2. T2* results principally from inhomogeneities in the main magnetic field.
The spin echo (SE) sequence uses an additional, 180-degree pulse, to flip protons and generate an echo as they rephase. This negates T2* effects from magnetic field inhomogeneity. It also provides better signal but is slower. Fast spin echo (FSE) uses many successive 180-degree pulses to speed acquisition.
Echo time is the time at which the electrical signal induced by the spinning protons is measured. A long echo time results in reduced signal in tissues like white matter and gray matter since the protons are more likely to become out of phase.
In fact, spin echoes are formed when two successive RF-pulses of any flip angle are employed! Hahn, in his original paper, used two 90° pulses. When flip angles other than 90° and 180° are employed, the resultant spin echo is sometimes referred to as a Hahn echo.
A spin echo (SE) is produced by pairs of radiofrequency (RF) pulses, whereas a gradient echo (GRE) is produced by a single RF pulse in conjunction with a gradient reversal. The formation of a GRE is illustrated schematically.
Spin-Echo. S = k Ï (1-exp(-TR/T1)) exp(-TE/T2) Inversion Recovery (180-90)
