cardiac MRI + principle of ECG gating Flashcards
FSE - GE, picker
TURBO spin echo - philips, siemens
og name - rapid acquisition with relaxation enhancement (RARE)
How long does a regular spin echo sequence take compared to FSE/turbo
regular = 8 - 12 mins (t2 weighted dual echo)
fast = reduced by 2-3 mins
why is flow sensitivity for a balanced SSFP sequence low where as for a spoiled GRE, its high
bSSFP uses balanced gradients (it refocus’s all signals for the signal sampling so differentiation is bad)
Spoiled GRE can visualise flow jets
what is the gating used to control motion artefact due to a) the heart continually beating
b) the heart continually moving up and down due to diaphragmatic contraction when breathing
a) ECG gating (best) or PEG (peripheral pulse gating)
b) Respiratory gating or breath hold
how many phases of the heart cycle should be scanned in one breath hold, ideally
30
note: high temporal resolution and high spatial resolution required but with short acquisition times
what is ECG gating
measures the hearts QRS complex and synchronises the MRI scan to trigger at reproducible phases of the heart
what part of the ECG cycle (QRST) is MRI triggered
Time of repitition (TR) = from R-R
(the point of ventricular diastole/systole TR happens and ends at the next ventricular diastole/systole etc)
note: with conventional GRE, theres only one line/trace formed per R-R interval
with fast/turbe GRE, theres a no of lines (depending on the turbo factor) per R-R interval
compare retrospective and prospective gating
- Prospective gating (triggering)(single phase single slice imaging) acquires images continually in one breath hold till all data needed by k-space has been acquired) (done in breath hold). (segmented imaging) (images taken mid-end diastole)
Retrospective gating collects data continuously (throughout whole cardiac cycle) then groups and fills k-space according to the phase of the cardiac cycle. (data is sorted into time points retrospectively) (records temporal position of acquired data relative to r- wave)
ECG synchronization is done by means of prospective or retrospective ECG triggering (ECG-T).
Prospective ECG-T performs measurements only at a certain phase of the cardiac cycle (e.g., ventricular systole) chosen to meet the application requirements [6] and triggered by the desired physiological event (e.g., R-wave).
In retrospective ECG-T, measurements are continuously performed at several phases of the cardiac cycle and afterwards correlated to the ECG phase and sorted accordingly. The optimal method depends on the application to be used
what is a sequence used to identify cardiac function and flow? how does it work
CINE
- is a movie and uses SSFP
- images are acquired continuously with retrospective ECG gating filling in k-space (done within a breath hold 6-10s) however know that people with cardiac issues tend to struggle to hold breath
how many times roughly is breath held in respiratory gating
70+
give some examples of patents unsuitable for CMR
- HIGH BMI
- claustrophobia
- dementia
- severe learning difficulties
note 95% patients are given contrast for CMR
what is the main issue of ECG gating
both forms of gating rely on R-R interval remaining same distance apart and patient HR (heart rate) remaining stable
what 2 things can be done to resolve issue of unstable heart rate/unequal R-R interval
- arrhythmia rejection (reject abnormal heart beats)
- select correct HR (adapt HR used for gating)
as heart moves up and down with diaphragm for each breath hold, respiratory gating and ECG gating must both be done
what are 2 types of respiratory gating
- breath holding
- on expiration
- 1-22secs
- 80+ times - respiratory gating with navigator echo
- continuous breathing
how does respiratory navigator work
- navigator box over dome of liver
- scanner tracks rise and fall of diaphragm and movement of liver
- accepted data within a respiratory gated window (when dome is within that window, the data is accepted)
true axial slices of the heart are taken from AORTIC ARCH TO BASE OF HEART
compare 2 chamber (vertical long axis /VLA) and 4 chamber (horizontal long axis/HLA) plane during cardiac mri
2 chamber is a ‘sagittal’ view e.g you see L atrium and l ventricle and mitral valve
4 camber is a ‘coronal’ view e.g you see all 4 chambers and both mitral and bicuspid valves
what is a 3 chamber / left ventricular outflow tract plane view of heart
sagittal view but cutting through the aortic trunk as well so u see ‘3’ chambers
’ can see L atrium/ventricle/mitral valve and aortic trunk(outflow tract)
what is a short axis view/plane of heart
axial view of ventricles only
