gated myocardial spect

Gated Myocardial SPECT

Technical overview and our experiences

J O’Brien & WH Thomson, City Hospital, Dudley Road, Birmingham

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Introduction

• Began gating SPECT MPI studies in 2002.

• Gating has proved useful to patient management.

• However , some technical pitfalls came up along the way!

• Would like to share these with PNUG, with discussion (hopefully!) to help us optimise our protocol.

• MPI protocol adjusted to minimise such problems

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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.

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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

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What about our Ungated data used for stress/rest perfusion images?

• Its not lost (hopefully!). Simply sum up gated data

Gated data Ungated data

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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.

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General theory of gated acquisition

• The acquisition starts with the R-wave on the ECG, corresponding to End Diastole.

• Image data for each of the frames are acquired repeatedly, over many cardiac cycles and stored separately on computer.

• One cardiac cycle, represented by the R-R interval is divided into multiple frames of equal duration.

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BEAT 2

BEAT 3

BEAT 1

BEAT 4

BEAT n

A gated acquisition

Images courtesy of: Gated Myocardial Perfusion SPECT: Basic Principles, Technical Aspects, and Clinical Applications. J Nucl Med Technol 2004; 32:179–187

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

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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

• Typical limits ± 20% of the Mean R-R interval

• Achieve this by setting ‘limits’ around the Mean R-R interval

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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

R-R Interval in milliseconds

• Height of peak indicates how many beats fell into that intervalAccepted

RejectedRejected

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Tuning

R-R Interval in milliseconds R-R Interval in milliseconds

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Other parameters to set..

• How many frames.. 8 or 16 ?

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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.

• Routinely 8 frames are sufficient

• Too few frames will result in ‘temporal blurring’ – resulting in the inability to correctly visualise cardiac motion

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Other parameters to set..

• How many frames.. 8 or 16 ?

• Set length of each projection to be based on Time or Gated beats..?

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Time or Gated beats?

• Time per projection ensures consistent acquisition time per patient. But in Arrhythmia, a loss of counts will occur (see later)

• Gated beats 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 because extra time is already required to do attenuation correction.

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End results

For the majority everything is all OK!

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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!

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Tracking

Projection 12 of 68 Projection 13 of 68

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• 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.

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R-R Histogram during scan

But histogram is NOT stored for later viewing (as far as we’re aware)

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• 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 ….

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• Cine looks OK

• Sinogram and linogram look odd

• streaks are present in many projections

• Why? Poor electrode connection during the scan

• New electrodes fitted

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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

Repeated Study

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Shorts

V L As

Poor

Poor

Good

Good

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• 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.

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• 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.

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Make new frame based on average of two neighbours

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• 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!

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1 pair removed

5 pairs..

4 pairs..

2 pairs..

3 pairs..

Normal

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• 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

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• 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:

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Good detection

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Bad detection

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Reasonable results

Typical results

Typical shape

Contours look OK

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Poor results

strange results

Weird shape!

Contours look wrong

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• 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.

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• 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.

Amount of rejected beats not displayed during acquisition setup. However it is displayed during acquisition!

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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

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- Future optimisations - QC of a Gated System

• Few recommendations in literature. None in IPEM 86

• Looking to custom designed phantoms, such as those being developed at Coventry Walsgrave in partnership with Durex™.

• Our EBME dept placed test input with known R-R interval (bpm) into trigger unit. Check R-R histogram for correlation.

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- Future optimisations -

Motion correct gated data

Tricky to motion correct gated data using sinogram due to poor counts

Solution already done by Philips

Batch correct ungated and gated data together

Available in Odyssey LX System

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- Future optimisations - Check professional body advice

Techniques Covered – slide nicked from M Smith

BNCS SNM ASNC

MPI -Planar

MPI - SPECT

MPI - Gated MUGA

First Pass

= Covered in detail = Mentioned

All downloadable free of charge off net.

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- Future optimisations - 3D post filtering

Currently filter the gated data using ungated settings

Gated data has more noise, so it seems reasonable to change cutoff to smooth data more.

Bill’s the expert on this!

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- 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

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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.

gated myocardial spect

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