====== Magnetic Resonance Imaging ======

Source:
https://www.radiology.wisc.edu/education/med_students/neuroradiology/NeuroRad/Intro/MRIintro.htm

  - protons magnetic field rf energy --> transmit radiofrequency(rf) electromagnetic waves. 
  - signal strength <-- electromagnetic microenvironment of the individual protons and their movement within this environment
  - pixel == amplitude of the radio frequency signal coming from the hydrogen nuclei (protons) in the water and fat within the voxel
  - The amplitude of signal from the voxel is determined by a sequence of radiofrequency pulses and applied magnetic gradients as well as the density of protons and their electromagnetic microenvironment. The timing of the rf pulses and gradients are altered in different sequences to change the relative weighting between the proton density and factors in the microenvironment.

Standard sequences
Proton Density - the pixel intensity is primarily dependent on the density of protons within the voxel.
T1 weighting - pixel brightness dependent on proton density and weighted towards those protons that quickly retransmit rf energy decaying to their baseline unexcited state.
T2 weighting - pixel brightness dependent on proton density and the behavior of neighboring protons.

Tissue contrast
        T1 weighting
            dense bone - dark (few hydrogen protons)
            air - dark (few hydrogen protons)
            fat - bright
            water (CSF) - dark
            brain - anatomical
                Gray matter - gray
                White matter - whiter
        T2 weighting
            dense bone - dark (few hydrogen protons)
            air - dark (few hydrogen protons)
            fat - dark
            water (CSF) - bright
            brain
                Gray matter - gray
                White matter - darker than gray
        Proton Density - intermediate between T1 and T2 signals
            Gray matter - gray
            White matter - darker than gray

Pathological processes
  MRI superior to CT in reproduction of anatomy allowing smaller pathological alterations in anatomy to be identified.As there are few protons to image in dense bone CT remains superior in the imaging of bony anatomy.
  Pathological processes typically increase the water content in tissues. The added water decreases signal on T1 weighted images and increases it on T2 weighted images. Consequently pathological processes are usually more visible T2 weighted images.
        The signal from blood evolves in a complex way over time. Significantly, acute hemorrhage can be invisible on MRI images (isointense to surrounding brain on all sequences) so that CT is superior in the evaluation of acute hemorrhage.
        Intravenous MRI contrast (gadolinium chelate) primarily increases signal on T1 weighted images. Due to disruption of the blood-brain barrier pathological processes in general brighten on T1 weighted images.

 
MRI of the wrist

Techniques
  - 1.5 Tesla
  - Wrist coil
  - Supine versus superman
  - Usual views - Coronal, Saggital, Axial
  - do STIR

Special sequences
  - Contrast
    - for additional information
  - 3D GRE
    - allows section less than 1mm -- especially for TFCC
    - sensitive to different magnetic susceptibilities --> increased sclerosis in osseous regions
  - Rapid GRE
    - enhancement dynamics in inflammatory and tumororous tissue
  - MR Arthrography
  - Cinematic exam

standard sequences:
  - coronal T1-weighted (T1W) spin-echo (SE) or proton density-weighted (PDW) fast spin-echo (FSE), for anatomical detail
  - coronal PDW/T2-weighted (T2W) FSE fat-suppressed (FS), for detection of bone marrow oedema and the triangular fibrocartilage (TFC) complex
  - coronal T2*-weighted (T2*W) gradient-echo (GE), for assessment of the TFC complex and carpal ligaments
  - sagittal T1W or PDW FSE, for assessment of carpal alignment
  - axial T1W or PDW FSE with T2W/PDW FSE FS, for assessment of tendons and the carpal tunnel
  - three-dimensional GE sequences can also be used:
  – provide very thin sections with the ability to reconstruct images
  – optimal imaging of carpal ligaments

Anatomy

Specific MR Anatomy

Conditions
  - AVN
  - Ulnar impaction
  - Carpal bone injuries
  - Intrinsic ligaments
  - Extrinsic ligaments
  - TFCC
  - Nerve compression
  - Tumors
  - Ganglion cysts
  - Disorders of the synovial membrane
  - Tendon

Pitfalls