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Gated Myocardial SPECT

Gated Myocardial SPECT. Technical overview and our experiences. J O’Brien & WH Thomson, City Hospital, Dudley Road, Birmingham. Introduction. Began gating SPECT MPI studies in 2002. Gating has proved useful to patient management. However , some technical pitfalls came up along the way!.

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Gated Myocardial SPECT

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  1. Gated Myocardial SPECT Technical overview and our experiences J O’Brien & WH Thomson, City Hospital, Dudley Road, Birmingham

  2. Introduction • Began gating SPECT MPI studies in 2002. • Gating has proved useful to patient management. • However , some technical pitfalls came up along the way! • MPI protocol adjusted to minimise such problems • Would like to share these with PNUG, with discussion (hopefully!) to help us optimise our protocol.

  3. Introduction to Gating • During a myocardial perfusion SPECT acquisition, the gamma camera records the photons at multiple projections around the patient. • In an UNGATED acquisition, at each projection one static image is acquired • In an ECG GATED acquisition, at each projection several dynamic images spanning the length of the cardiac cycle are acquired at equal time intervals • Gating enables us to track the motion of the heart.

  4. Introduction to Gating (cont) • Gated SPECT data can be used by special software packages to give extra information on the heart such as: • Ejection Fraction (EF) • End Diastolic Volume (EDV) • End Systolic Volume (ESV) • Regional wall motion

  5. What about our Ungated data used for stress/rest perfusion images? • Its not lost (hopefully!). Simply sum up gated data Gated data Ungated data

  6. Patient Setup • Connect patient to ECG trigger unit with electrodes at left leg (LL) , right arm (RA) , and left arm (LA) (MCL1 - modified central lead one) • Connect trigger unit to R-wave input on camera.

  7. General theory of gated acquisition • The acquisition starts with the R-wave on the ECG, corresponding to End Diastole. • One cardiac cycle, represented by the R-R interval is divided into multiple frames of equal duration. • Image data for each of the frames are acquired repeatedly, over many cardiac cycles and stored separately on computer.

  8. BEAT 3 BEAT 2 BEAT 1 BEAT 4 BEAT n A gated acquisition ECG Trace e.g. R-R interval = 800msec 8 dynamic time frames each equal to 800 / 8 = 100msec 8 dynamic time frames produced at each beat To achieve adequate counts per frame, must acquire gated data over several cardiac cycles Images courtesy of: Gated Myocardial Perfusion SPECT: Basic Principles, Technical Aspects, and Clinical Applications. J Nucl Med Technol 2004; 32:179–187

  9. Heartbeat Variation and Tolerance • Several cardiac cycles per projection are needed. • If patient R-R interval varies significantly during these cycles, then ‘mixing’ of counts from adjacent time frames occurs degrading the gated data. • We can combat this by rejecting beats that are significantly shorter and longer than mean R-R interval • Achieve this by setting ‘limits’ around the Mean R-R interval • Typical limits ± 20% of the Mean R-R interval

  10. Height of peak indicates how many beats fell into that interval Accepted Rejected Rejected R-R Interval in milliseconds How do we determine the Mean R-R and its limits? • Philips Odyssey computers have the ability to plot a histogram illustrating the range of R-R intervals • By acquiring the histogram over 30-60 seconds, the user may visually find mean R-R and set limits before acquisition starts. • This also determines time per frame

  11. R-R Interval in milliseconds R-R Interval in milliseconds Tuning

  12. Other parameters to set.. • How many frames.. 8 or 16 ?

  13. How many frames? • Try to balance ‘temporal resolution’ against counts per frame • Too many frames and counts per frame will be too low , leading to ‘noisy’ images. • Too few frames will result in ‘temporal blurring’ – resulting in the inability to correctly visualise cardiac motion • Routinely 8 frames are sufficient

  14. Other parameters to set.. • How many frames.. 8 or 16 ? • Set length of each projection to be based on Time or Gated beats..?

  15. Time or Gated beats? • Time per projection ensures consistent acquisition time per patient. But in Arrhythmia, a loss of counts will occur (see later) • Gatedbeats per projection ensure adequate and consistent count density in the presence of Arrhythmia. But acquisition time will increase (very bad for patient). • Locally we use Time becauseextra time is already required to do attenuation correction.

  16. End results For the majority everything is all OK!

  17. Experiences to share! • Heart rate can slow down during scan as patient relaxes. • Make use of R-R Tracking option on Philips Axis. This moves R-R limits at each projection, over the average R-R interval from previous projection. AUTOTUNE!

  18. Projection 12 of 68 Projection 13 of 68 Tracking

  19. Experiences to share! • Heart rate can slow down during scan as patient relaxes. • Make use of R-R Tracking option on Philips Axis. This moves R-R limits at each projection, over the average R-R interval from previous projection. AUTOTUNE! • Also, we display the R-R Histogram on the odyssey acquisition screen during the scan.

  20. R-R Histogram during scan But histogram is NOT stored for later viewing (as far as we’re aware)

  21. Experiences to share! • Watch out for loose ECG lead or poor electrode-skin contact • A poor R-R signal (or even no R-R signal!) results in reduced / zero counts. This adversely affects gated and ungated data to various degrees ….

  22. Experiences to share! • Cine looks OK • Sinogram and linogram look odd • streaks are present in many projections • Why? Poor electrode connection during the scan • New electrodes fitted

  23. Repeated Study Experiences to share! • Cine looks OK • Sinogram and linogram look odd • streaks are present in many projections • Why? Poor electrode connection during the scan • New electrodes fitted • Repeat scan shows no streaks

  24. Shorts Poor Good V L As Poor Good

  25. Experiences to share! • To improve gating technique, we.. • Use the sinogram to check quality of gating • Do not trust cine for checking • Try to get good electrode-skin contact • Position leads so that they can’t be moved or pulled.

  26. Experiences to share! • You can get away with a few poor projections..! • Example with a single poor projection on both heads • Use ‘average of two nearest neighbours’ function available in Image Algebra program to help out.

  27. Make new frame based on average of two neighbours

  28. Experiences to share! • How many poor projections can we get away with?! • Use ‘replace counts’ function to artificially degrade good ungated data to understand effect. • Removed 1, 2, 3, 4, and 5 pairs of projections!

  29. Normal 1 pair removed 2 pairs.. 3 pairs.. 4 pairs.. 5 pairs..

  30. Experiences to share! • How many poor projections can we get away with?! • Use ‘replace counts’ function to artificially degrade good ungated data to understand effect. • Removed 1, 2 , 3 , 4, and 5 pairs of projections • Hard to spot the difference! Slight degradation in inferior wall. • Quite robust probably because AXIS acquires 204 degrees worth of data in cardiac mode

  31. Experiences to share! • Check edge detection has worked with ungated and gated data in Software analysis package • Bowel activity near inferior wall can play havoc with gated data, severely affecting results:

  32. Good detection

  33. Bad detection

  34. Contours look OK Reasonable results Typical results Typical shape

  35. Contours look wrong Poor results strange results Weird shape!

  36. Experiences to share! • Check edge detection has worked with ungated and gated data in Software analysis package • Bowel activity can play havoc with gated data, severely affecting results: • Always check edge detection is correct in the packages for all data that is inserted in to them.

  37. Amount of rejected beats not displayed during acquisition setup. However it is displayed during acquisition! Experiences to share! • What to do with Arrhythmia patients? • How many bad beats to classify patient as having Arrhythmia? • How many can we get away with? Don’t know! • The general consensus is to avoid gating Arrhythmic patients! Principle of test is to detect reversible ischemia. This ‘could be’ severely compromised. • Often Arrhythmia is not marked on refferal card and is picked up at first visit by stress-room staff, who forewarn imaging room staff. • Technicians understand that if they’re unsure about amount of Arrhythmia, call second opinion. Its OK not to gate patient.

  38. Summary of local protocol changes • Always check sinogram for motion AND gating errors • Avoid patients with Arrhythmia. Sometimes not mentioned on referral so imaging staff tend to be forewarned by stress-room staff. • Ok to lose some frames (but not too many!) • Improve ECG connection technique • Always check edge detection in gated analysis software • Display the R-R Histogram during aquisition • Make use of R-R Tracking option • Ask for help if unsure about gating – its OK not to gate patient

  39. - Future optimisations - QC of a Gated System • Few recommendations in literature. None in IPEM 86 • Our EBME dept placed test input with known R-R interval (bpm) into trigger unit. Check R-R histogram for correlation. • Looking to custom designed phantoms, such as those being developed at Coventry Walsgrave in partnership with Durex™.

  40. - Future optimisations - Motion correct gated data Tricky to motion correct gated data using sinogram due to poor countsSolution already done by PhilipsBatch correct ungated and gated data togetherAvailable in Odyssey LX System

  41. - Future optimisations - Check professional body advice Techniques Covered – slide nicked from M Smith All downloadable free of charge off net.  = Covered in detail  = Mentioned

  42. - Future optimisations - 3D post filtering Currently filter the gated data using ungated settingsGated data has more noise, so it seems reasonable to change cutoff to smooth data more.Bill’s the expert on this!

  43. - Future optimisations - More Choice of Acquisition Parameters • Beat rejection options: • None (what we use already) • Single bin • Percentage • Interval • - end up with three sets of projection cardiac data. Short , normal, long. • Single F/B • - Real time gated list mode

  44. Single F/B – gated list mode • This menu choice is only offered when you set the SiteEnv variable FB_GATED to use gated list mode (why?). • Data is acquired in list mode and then re-framed in real time into images of equal duration. • Two separate gated sequences are created. • One is created by gating forward from the R-wave that marks the beginning of the cardiac cycle. • The other is created by gating backward from the R-wave that marks the end of the cardiac cycle. • The first 2/3 of the forward sequence is added to the last 1/3 of the backward sequence. The composite sequence is then added to the summed gated sequence. • Sounds good.

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