Advanced Topics in Neuro MRI Pt.2 Flashcards
What is MR Spectroscopy?
the sequence in which we suppress the water signal to look at the hydrogen signals of the other metabolites.
Signal amplitude depends on _
chemical concentration
the # of protons giving rise to the peak
What are metabolites?
chemical substances necessary for taking part in a particular metabolic process; chemicals present in body tissues to ensure its living and function
What are differences between Imaging and Spectroscopy?
Imaging: observe only the water & fat signal; water concentration is high through all the body tissues
Spectroscopy: suppress the water signal so other chemical resonances can be observed (trying to minimize the relaxation effects; metabolite concentrations are high in the brain, prostate & breast (we are limited to those body parts)
What are the major metabolites?
choline, creatine, NAA, Myo-Inositol, & Lactate
What is single-voxel Spectroscopy?
selection of a voxel and getting its signal
How can we reduce water signal by 1000 times to get the right dynamic range?
frequency selective pulses followed by dephasing gradients
Why is a short and long TE used in single-voxel spectroscopy?
Short TE: shows all metabolites; good for quantification
Long TE: smooth baseline; lactate inverts (distinction from fat)
What is multi-voxel spectroscopy also called?
Chemical Shift Imaging (CSI) or Spectroscopic Imaging (SI)
What is multi-voxel spectroscopy?
the sequence that acquires spectra of many voxels (or makes a color map of the whole slice) at the same time; phase encodes the spatial dimensions, but the readout gradient is replaced with step-wise encoding
Where is CSI used?
most commonly used on the brain and prostate
What is the magnetic property of deoxyhemoglobin?
paramagnetic with respect to the surrounding tissue
What is the magnetic property of oxyhemoglobin?
diamagnetic; isomagnetic with respect to the surrounding tissue
What are some clinical practices of functional MRI (fMRI)?
Motor functions, language assay, pre-surgery, speech, listening
What are the challenges in functional MRI?
- BOLD signal change is weak
- SNR of in single-shot EPI images is low
- Physiological pulsations (cardiac and respiratory)
- Head motion; instrumental instability
- susceptibility artifacts as EPI is very prone to that
Temporal resolution: limited by BOLD impule-response function , image sampling rate, and spin relaxation times
Spatial Resolution: limited by BOLD point-spread function, SNR, and image sampling rate
