Dynamic MRI

Taneli Hautaniemi

dMRI in general

• Concerns the imaging of moving objects, i.e. the respiratory and the circulatory organs, including blood

• Demanding in terms of hardware and software requirements, solutions: breatholding, partial k-space encoding, gating (yielding pseudo real-time cine representations )

• A tradeoff between the spatial and temporal resolutions• Produces a set of images that – when arranged

according to the phases of a biological cycle, produces a looping movie of it called the cine sequence

Time-of-Flight imaging and velocity encoding

• Methods for visualizing blood flow include TOF imaging, phase-contrast (PC) and contrast-enhanced MRA techniques (for shortening T1)

• TOF effect can be used to give a rough estimation of the blood velocity in the slice or used in a morphological image to either hide or reveal the moving blood

• Phase shift effect caused by the movement of protons is used by PC for velocity encoding (cine PC velocity mapping)

• Velocities can be encoded in all 3 perpendicular directions giving in essence a set of 7D data

Respiratory and cardiac motions in dMRI

• Today cardiac dMRI is fast enough to be completed while the patient is holding his breath (to avoid motion artefacts)

• Gated or ungated, image reconstruction either prospectively or retrospectively

• Imaging can be made faster by fractional k-space sampling, faster sequences (shorter TR shorter image acquisition times) or faster methods altogether (from segmented data acquisition to FLASH and EPI) that allow multiple shots per cycle even up to real-time speeds

Real-time MRI

• Future hardware & software (algorithm) developments will allow 4D MRI

• No need for gating or breatholding• Further development will only enhance the quality of the

movies• Hardware advances have led the researchers to do 4D MRI

with a frame rate of around and above 70 fps in rats• The process requires high computing power: a real-time MRI

rendering of a beating heart for one minute will produce >2000 images that must each be rendered and stitched together into a seamless video, requiring up 2 gigabytes of memory and taking around 30 minutes to complete