Magnetic Resonance Imaging

• When protons are placed in a magnetic field they become capable of receiving and transmitting radiofrequency(rf) electromagnetic waves. After receiving rf energy the protons retransmit rf energy proportional to the density of protons. The strength of the signal is also dependent upon the electromagnetic microenvironment of the individual protons and their movement within this environment.
• A pixel within an MRI image represents the 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.