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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