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Spikes
Page 3. Introduction Page 4. Tribo-electric sequence Page 5. Spike behavior, Recognize spikes Page 6. JDB QPI Page 7. Spike localization, localization tool Page 8. STD_HB spike test Page 9. STD_LB spike test Page 10. Sweep spike test Page 11. Spike localization test Page 12. Locating spike sources
Page 13. Flow-chart
Page 15. Examples Page 29. Typical spike sources, Spike suppression
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Introduction: A spike is a sudden, sharp increase in the received signal. NOT caused by the MR response of the object. Gyroscan systems are sensitive for RF disturbances caused by electrical and electrostatic discharges in the examination room. Electrical discharges: -Interrupt current - like bad contact in a connector or switch. -Spark caused by High voltage Electrostatic discharges: Static electricity is a well-known phenomenon. It affects many industries in diverse environments. Static charge buildup can result in potentially dangerous electrical shocks, which can cause fires, explosions and severe damage to sensitive electronic components. Static charge buildup can be caused by friction between two surfaces. This is called tribo-electrification. Electrons migrate from the surface of one material to the surface of the other. Upon separation of the two surfaces, one surface loses electrons and becomes positively charged. The other surface gains electrons and becomes negatively charged.
As the pressure and the speed of contact and separation increase, the amount of the static charge buildup increases. Rapidly moving materials can quickly develop charges, which produce a potential of more than 25,000 volts.
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Tribo-electric sequence: The items on top are less attractive to electrons and become positively charged, while the items on the bottom are more attractive to electrons and become negatively charged.
Human Hands (very positive) Rabbit Fur Glass Human Hair Nylon Wool Fur Lead Silk Aluminum Paper Cotton Steel (neutral) Wood Amber Hard Rubber Nickel, Copper Brass, Silver Gold, Platinum Polyester Styrene (Styrofoam) Saran Wrap Polyurethane Polyethylene (like scotch tape) Polypropylene Vinyl (PVC) Silicon Teflon (very negative)
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Spike behavior: Spike signals can travel along each cable making it very difficult to pinpoint the exact location where the spikes are generated. Recognize spikes:
• “Spikes” are visible as white dots in the raw data • Spikes cause a bad SNR • A few spikes may result in a pattern of interference lines in the image. Depending on the position
of spikes in the raw data, horizontal, vertical or oblique lines are the result. • RF interferences can also cause line artefacts in the images. (Check raw data)
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Check JDB_QPI (Quality Performance Indicator) / threshold (search: QPI, JDB) REC_JDB: -nr responses expected 25892 REC_JDB: -nr responses analysed 25892 REC_JDB: -factor responses detected 100.0 percent REC_JDB: -nr suppressed 0 REC_JDB: -noise power time sample 26.403 REC_JDB: -threshold 1: 40 REC_JDB: -threshold 2: 300 REC_JDB: -max spec jdb power: (threshold 3) 39604 REC_JDB: -nr above threshold 1 8 REC_JDB: -nr above threshold 2 4 REC_JDB: -nr above threshold 3 4 REC_JDB: -power above threshold 1 4807716 REC_JDB: -power above threshold 2 4685396 REC_JDB: -power above threshold 3 4685396 REC_JDB: -nr expected 0.5 REC_JDB: -noise fraction 0.012 REC_JDB: -QPI 29.576
REC_JDB: -threshold 1: 40 REC_JDB: -threshold 2: 300 REC_JDB: -max spec jdb power: (threshold 3) 333825 REC_JDB: -nr above threshold 1 2 REC_JDB: -nr above threshold 2 0 REC_JDB: -nr above threshold 3 2 REC_JDB: -power above threshold 1 55664408 REC_JDB: -power above threshold 2 0 REC_JDB: -power above threshold 3 55664408 REC_JDB: -nr expected 0.1 REC_JDB: -noise fraction 0.088 REC_JDB: -QPI 83.374
Energy of the sample Threshold ratio = Average energy of neighboring samples
Spike localization: Check raw data. (Save RAW data = YES, delayed reconstruction, make raw data visible (matrix)) Check receive coils (also body coil)
• Eliminating receive coils • Check RF receive chain (coil input connector / cable / cable trap)
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Principle spike localization tool: Real-time evaluation / localization protocols (example: dual-mode gradient system)
• STD_LB (standard 0-100Hz) • STD_HB (fast standard 0-300Hz) • SWEEP (sweep 1000Hz => 300Hz)
Sweep single-mode 555Hz => 275Hz Sweep air-cooled grad 415Hz => 225Hz Select Spike determination / localization tool: STT Application Diagnostic procedures Spike determination Spike Localization “Proceed” 2x
“Load” button
Select spike test: (sweep / std_lb / std_hb) For adapting parameters see page 11
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(STD-LB spike test)
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STD-LB spike test
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The Y gradient has the opposite waveform (compare with X) in the default sweep measurement. The Z gradient is switch off. (Grad. Orientation. [x, y, z] => 1, -1, 0)
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Safety: Always mind the high voltage present on the gradient system. Use hearing protection inside the examination room. Spike localization test: Run spike evaluation / localization protocol. Check if spikes are visible with this protocol. Noise QPI should be less than 2 Visibility QPI should be less than 1 Start localization protocol – first use parameters which show the largest / most spikes. => Then try to optimize the localization protocol When none of the protocols show spikes => adapt parameters in localization protocol. Measurement type: Localization Scan type Sweep Start / Stop frequency values for fine tuning Nr repetitions / freq. Between 2 and 5 (for quick searching) Nr Grad startup cycles (start value default) spikes => so low as possible (15) Receive coil connector / coil / channel RF switching enabled / disabled =>(switching tune/detune, transmit/receive) Gradient slope [ms] so low as possible Grad. strength [mT/m] try higher value if no spikes visible. (also try 0 mT/m)
(Combination with gradient orientation, repetition time, startup cycles) Grad. orient. [x,y,z] default [1,-1, 0] try: 1,-1, 0 1, 1, 0 0, 1, 1 0, 1,-1 1, 0, 1 1, 0,-1 1, 1, 1 1,-1, 1 -1, 1,-1 Try also x, y, z separately with maximum gradient strength Repetition time [ms] 600 - for localization mode if possible. Acq. duration [ms] default or longer to check spike behavior after gradient pulse (Loose parts) For Scan type = Standard => check x, y, z separately with maximum gradient strength When none of the adapted protocols show spikes, then use a scan with the spike artefact.
• Save RAW data = yes (make raw data visible) • Manual start = yes for scanning without RF or bottle (don’t use a phantom) • Switching off the preparation phases for disabling the body-coil (check raw data /
JDB nr above threshold / QPI) • Check different coils / connectors
After repair – check the complete spike test
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Locating spike sources:
• Usually vibrations in the examination room can cause spikes. • Measure with the Quad body coil => RF-switching enabled and disabled => enabled spikes –
disabled no spikes then there is probably a bad hybrid box or T/R switch (disabled =>small spikes possible) / (For SW release. 10 and 11 there can also be a bad (de)tune / (de)tune-matching board in the QBC)
• Measure with a (quad- / sense-) head coil positioned in the iso-centre. Measure with RF-switching
enabled and disabled => enabled spikes – disabled no spikes then probably there is a bad (de)tune / (de)tune-matching board in the QBC. The receive coil is defect or there is a bad T/R switch
• Measure with a (quad/sense) head coil positioned in the iso-centre, to the front and rear of the
bore. Compare the spike level.
• Stop vibrations locally to find the spike source. o Dismount the vibrating part or o Press with a piece of wood (for instance in case of QBC spikes))
• Fix or Insulate loose vibrating material (see Tribo-electric sequence) (use anti-static foam, paper or
cardboard) • Use a surface coil (e.g. C1) and search for the spike-source. (Spike signals can travel along each cable
making it very difficult to pinpoint the exact location where the spikes are generated.)
• Spike behavior: Single (small loose part) Burst (gradient coil, bad connector) Resonance (Airflow, Helium pressure switch) Erratic (loose / vibrating parts)
See examples Surface coil positioning when checking a cable for spikes
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Examples:
Defect detune board 1.5T system (RF switching enabled => spikes) (RF switching disabled => No spikes)
Trimmer-spikes
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Defect detune board 3T system (RF switching enabled)
Disconnect “detune cable” X1 => spikes disappear
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Gradient coil spikes (try X, Y, Z separately)
TR switch 1.5T sweep 370Hz C1 receive coil (measured above hybrid box) RF switching enabled => spikes RF switching disabled => no spikes QBC receives: RF switching enabled => spikes (No gradient pulse is needed) RF switching disabled => -small spikes
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Erratic spikes => vibrating gradient cable (std-lb test)
Erratic spikes => loose parts in QBC booster (sweep test QBC receive)
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Resonance spikes (sweep spike test) => Airflow or Helium pressure switch (Also spikes after the gradient puls) Airflow pressure switch
Helium pressure switch (DPS)
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Erratic spikes => QBC booster (sweep test (640-600Hz) QBC / Flex M - receive) Spikes between detune cable / connector semi rigid
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RF shield QBC booster (example: sweep test 1000Hz QBC receive) Spikes between RF screen and joint-strip
(Reverse side)
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No spikes => distortion from gradient chain received by C1 coil (STD-LB spike test)
No spikes => distortion from gradient chain received by C1 coil (STD-LB only X grad) Suppress distortion with filter (Channel 1 => MRX x4 1.5T)
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STD LB Spike-test Pleiades (Y gradient) measured with C1 coil
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SWEEP Spike-test Z only => loose gradient bracket gradient strips measured with C1 coil close to gradient coil
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(STD-LB spike test)
1 T Pleiades system: (after February 2006 the trunk is put to earth potential) Sometimes spikes in consequence of discharge between gradient coil, trunk and magnet during the edge of a gradient pulse. The spikes stay away when the isolated trunk is put to earth potential.
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ODU connector (sweep 700 - 600Hz), => Check coax connection
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Measurement type: Localization Scan type: Sweep Grad. Strength: 0 mT/m Repetition time: 600 ms Acq. Duration: 100 ms Receive coil: QHC outside the bore Spikes from the examination room light (DC light – 135 W bulbs) High temperature of the 135 W bulbs was causing electrostatic discharges. (100 W bulbs replaced the 135 W bulbs)
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Spikes / RF interference Switch-off kit for the cold head motor fails every now and then and makes the cold head run for a few seconds occasionally and a bad cold head motor filter in the system filter box.
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Typical spike sources: Vibrating (gradient) cables Loose closing cap on magnet Helium lines touching Magnet or false ceiling Airflow or Helium pressure switch Loose supply and ground cables Loose bolts / nuts / fixation-brackets gradient strips Detune boards QBC RF-screen 3T QBC-booster Bad contact in SMD / BNC connector PICU / connector / ODU connector Hybrid-box PU coils Loose vibrating parts (see tribo-electric sequence) Emergency light Empty shim rails in magnet Click-sons in gradient coil Finger protection switch Disconnect the mains cable to the RF room (Other equipment in RF room) (RF leak => Spikes from “switch off kit” compressor cold head and leak RF filter in system filter box) (Reconstruction error => SKY reconstructor check TSW RECSIM test // BCP check TSW TBDAS –BCP test) SPIKE SUPPRESSION The application software offers the possibility to automatically suppress spikes from the acquired raw data. This mechanism is enabled by means of a software key. This software key can be obtained via the MR Helpdesk in Best. The following considerations must be taken into account: - Policy of the Customer Services is to fix spike sources rather than to suppress the effect. - Not all raw data is checked for spikes and therefore not all spikes will be suppressed. - The only good permanent resolution remains elimination of the spike source. After installing the "spike suppression" software key (obtain the software key via the MR-Helpdesk), the following changes must be made to enable the automatic spike suppression. Procedure: - Login under the application software. - Select: <Scan Control> - Select: <Scan Utilities> - Select: Enter Service mode - Select: Control Parameters - Select: Reconstruction Control Parameters - Modify: Enable_jdb_suppression yes - Select: <Store> To permanently change the value of the "enable_jdb_suppression" parameter. The software key to enable "spike suppression" will always be a temporary key. The ultimate goal must always be to repair the system and remove the "spike source".
