The Design and Test of a Pulsed 250 MHz EPR Spectrometer

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The Design and Test of a Pulsed 250 MHz EPR
Spectrometer
10th International Workshop on Bio-Medical ESR
Spectroscopy and Imaging Fukuoka, Japan April
1-3, 2003
Gareth R. Eaton, Richard W. Quine, George A.
Rinard, and Sandra S. Eaton Department of
Chemistry and Biochemistry and Department of
Engineering, University of Denver Howard Halpern
and Colin Mailer, University of Chicago
NIH-funded Center for Electron Paramagnetic
Resonance Imaging for in Vivo Physiology. H.
Halpern, University of Chicago, G. Eaton,
University of Denver, and G. Rosen, University of
Maryland
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Low Frequency EPR

• Low EPR frequencies being used include
• 200, 220, 250, 280, 300, 600, 700, 1000, 1200 MHz
•  
• RF penetration as a function of frequency depends
  on
• dielectric properties
• conductivity
• sample shape
• homogeneity
•  
• RF penetration increases
• the lower the RF frequency
• the more heterogeneous the sample
• ?deeper penetration in animals than in spheres or
  cylinders of water
•  

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Pulsed 250 MHz EPR  

• Is being developed to
• Exploit the high sensitivity of electron spin
  relaxation times to O2
• Gate data acquisition to physiologic motion
• Detect transient phenomena

• Electron spin relaxation times are at least
  1000-fold faster than the nuclear spin relaxation
  times important in MRI. Consequently, we need to
  develop several crucial enabling technologies
• Resonators with sub-microsecond dead time
  following the high-power pulse
• High-power pulsed RF amplifiers
• Methodology for fast data acquisition

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Comparison of CW and Pulsed EPR - 1

• For a sample for which
• all of the spins in the sample form the echo
• the CW spectrum is not saturated
• the CW magnetic field modulation is approximately
  equal to the line width
•  
• The relative CW and spin echo signal intensities
  are given by the ratio

If the CW spectrum saturates, then the B1 has to
be reduced below the maximum available in the
spectrometer, reducing the CW intensity. If the
magnetic field modulation is reduced to avoid
distorting the CW line shape, then the CW
intensity if reduced roughly proportionately.
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Comparison of CW and pulsed EPR - 2
If noise is thermal noise, then the noise in the
signal is proportional to the bandwidth of the
spectrometer. In a CW spectrometer the bandwidth
is usually limited by a filter at a late stage of
the detection system. Commonly a time constant
of a few ms to 1 s is used for CW EPR,
corresponding to bandwidths of 160 Hz to 0.16
Hz.   The -3dB bandwidth of a single pole filter
with time constant TC is
In a pulsed spectrometer, the filter is wider to
pass all relevant frequencies, and commonly is
several MHz.   If the bandwidths were, e.g., 10
Hz (time constant 0.016 s) for the CW spectrum
and 10 MHz for the pulse spectrum, the noise in
the pulse spectrum would be 103 greater.
However, the pulse spectrum might be acquired at
a repetition of 104 per second, so in 100 seconds
of signal averaging, the noise would be about the
same in the pulse spectrum as in a CW spectrum
acquired as quickly as the line width
permitted.   from Rinard, Quine, Song, Eaton and
Eaton, JMR 140, 69-83 (1999)
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Relative Benefits of Pulsed and CW EPR

• CW
• can be performed on many radicals
• including radicals that have wide spectra
• can be performed for large gradients, to achieve
  high spatial resolution
• Pulse
• can be more sensitive per unit time than CW for
  very-narrow-line spin probes such as the trityl
  radicals
• is limited by dead time following the pulse
• FID detection is limited by the effective T2
  caused by the gradient field
•  
• see experimental comparison at 300 MHz in
• K.-I. Yamada, R. Murugesan, N. Devasahayam, J. A.
  Cook, J. B. Mitchell, S. Subramanian, and M. C.
  Krishna. Evaluation and Comparison of Pulsed and
  Continuous Wave Radiofrequency Electron
  Paramagnetic Resonance Techniques for in Vivo
  Detection and Imaging of Free Radicals. J. Magn.
  Reson. 154, 287-297 (2002).

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Selection of Experiment - 1

• The goal of the measurement dictates the
  conditions for spectroscopy
•  
• Optimum detection occurs when the line is
  saturated and over-modulated
• The maximum CW signal for a single Lorentzian
  line occurs when ?2B12T1T2 1
• The line broadens when saturated. It is about
  1.2 times broader at the power that gives the
  maximum in the signal amplitude.
• The maximum intensity occurs when the modulation
  amplitude is approximately equal to the line
  width, at which point the width is increased by
  about a factor of 2 (depends on details of line
  shape).

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Selection of Experiment - 2

• One might use less saturation and lower
  modulation if the goal is to
• identify the radical
• discriminate between radicals - also use
  multi-frequency, differential relaxation
• measure viscosity
• measure pH

• However, if the goal is to measure O2
  concentration, the spectroscopy becomes more
  demanding.
• Small changes in line width measure O2
  concentration
• Over-modulation by a factor of 1 to 2x to improve
  S/N is permitted if Colin Mailers program is
  used to deconvolute the excess modulation.
  (Robinson, Mailer, Reese, J. Magn. Reson. 138,
  199, 210 (1999)).
• Relaxation times are sensitive to changes in O2
  collision rate, so pulsed EPR may be valuable.

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MRI Comparison
Concentrations 1H in vivo is about 100 M, but
electron spin concentrations for EPR typically
are less than 1 mM Relaxation Times about 1000
times longer for NMR than EPR Echo vs. FID MRI
uses spin echos but most pulsed EPR uses
FID's Maximum Frequency MRI at 8T, 340 MHz
(Robataille et al. J. Computer Assisted
Tomography 23821-831 (1999).
1.5 T 4.7 T 8 T
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MRI Field Distributions
Robataille et al., J. Computer Assisted
Tomography 23821-831 (1999).
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C. M. Collins, S. Li, M. B. Smith, Magn. Reson.
Med. 40 847-856 (1998).
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Absolute Signal-to-Noise
The EPR signal voltage, Vs, is given by

• C"(w) is the imaginary component of the effective
  RF susceptibility
• is the filling factor
• Q is the loaded quality factor of the resonator
• Zo is the characteristic impedance of the
  transmission line
• P is the RF microwave power to the resonator

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Resonator B1

• d is the diameter of the LGR
• z is the length of the LGR
• ?0 is the permeability in a vacuum
• is the conductivity of the surface of the
  resonator
• ? is the resonance frequency
• P is the power to the resonator

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Frequency Dependence of EPR Parameters
P is RF power, and B1 is the RF magnetic field
intensity.
G. A. Rinard, R. W. Quine, S. S. Eaton, and G. R.
Eaton J. Magn. Reson. 156, 113 (2002).
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Frequency Dependence of Signal Intensity
Experimental Results

• VHF vs. L-band FID of trityl normalized signals
  agreed with prediction within 5
• VHF, L-band, X-band resonator dimensions scaled
  by factors of 6
• For each pair of frequencies the CW signal at
  lower frequency was predicted to be larger by
  factor of 1.57.
• 250 MHz / 1.5 GHz was 1.52
• 1.5 GHz / 9 GHz was 1.14

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Frequency Dependence of EPR S/N when sample loss
dominates the noise
r is sample resistivity
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CW operation with Bruker console
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Pulse operation with DU hardware
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Pulse operation with Bruker console
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Cross-Loop Resonator and Pre-Amplifier Modules
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Magnet and Gradient Coil Specifications
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Location of gradient coils
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Hall-probe/current control adapter
a Two six-turn spiral coils of no. 12 copper
magnet wire with 2.2 cm mean diameter spaced
1.25 cm apart b Aluminum coil forms c 5 cm
aluminum channel d Aluminum spacers e
Fiberglass insulating spacers f insulated
Plexiglas mount for Hall probe
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Magnet and cross-loop resonator
The 250 MHz crossed loop resonator is shown in
the very homogeneous (lt 40 ppm variation over 15
cm diameter) 4-coil resistive magnet designed for
EPR imaging. The outer diameter of the large
coil is ca. 1 meter. The picture also shows the
z-gradient coils.
The x, y gradient coils have been removed to show
the other coils more clearly. The crossed loop
resonator can isolate the time-domain EPR signal
from the high power RF pulse by 60 dB (typical).
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Cross-Loop Resonator
a Sample tube resonator b Cross resonator
c,d Brackets to adjust orthogonality e
Isolation adjustment screw f Sample tube g
Sample tube holder h Coupling screw for
resonator a i Coupling screw for resonator
b j Frequency tuning for resonator b
Resonators machined from tellurium-copper alloy
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Cross-Loop Resonator

• Sample tube
• Sample tube holder
• Loop (inductor) of sample tube resonator
• gap (capacitor) or sample tube resonator
• Re-entrant loop for sample tube resonator
• Loop for cross resonator
• Re-entrant loop for cross resonator
• Sample access hole

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Phase Noise Comparison
Blue CLR Red - LGR
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Isolation of cross-loop resonator
Loop gap resonator
Cross-loop resonator
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Q-switching circuit
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Q-switching sequence
The Q of the excitation resonator is held high
and the detection resonator Q is spoiled during
the RF pulses. The Q states are reversed during
the time when the EPR signal is recorded.
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Active Q-spoiling
Without Q-spoiling Q1 (excitation) 74 Q2
(detection) 469
With Q-spoiling
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High-power amplifier
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High-power amplifier
There was no RF amplifier commercially available
that would meet the needs of pulsed 250 MHz EPR
imaging.   Two companies, Communications Power
Corporation (CPC) and Tomco, have contracted to
develop and produce amplifiers with       
Output power gt 400 W        lt 30 ns rise and
fall times        blanking of output noise to
lt10dB above thermal within 80 ns   The
Tomco amplifier (top) meets the speed
specifications and was tested at 550 W.   A
400 W prototype of the CPC amplifier is shown in
the bottom figure.
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Design Philosophy of Pulsed VHF Bridge for
University of Chicago

• Use of a crossed-loop resonator (CLR)
• Stand-alone testing of resonators
• Stand-alone FID
• Electron spin echo (ESE) when controlled by a
  Bruker PatternJet
• 1 kW maximum RF pulse power
• 2 kW maximum RF power
• To be used with Bruker E540 console
• Data collection via Bruker SpecJet

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Time-domain data acquisition methodology at 250
MHz
       The isolation of the crossed loop
resonator reduces the dead time of the system
following the high-power pulse, because the pulse
power reaching the detection system is reduced by
the amount of the isolation.        We have
decreased dead time even more by active
Q-switching of the excitation and detection
resonators in synchrony with the RF pulses and
detection times.        The signal on the
following slide is the free induction decay (FID)
of a sample of Nycomed triarylmethyl (trityl)
radical, whose electron spin relaxation time is a
measure of oxygen concentration in vivo.  Full
FIDs were digitized with a repetition time of 100
?s
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Pulse Sequences

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FID, no gradient
FID for two 0.3 mm i.d. tubes of 0.2 mM
trityl-CD3 separated by 1.8 cm 4096 data points,
199,680 averages Decay time constant is 2.88 ms.
Zero-filled to 16k, FFT Full-width half-height
35 mG DBpp 21 mG
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FID-detected projection
Two 0.3 mm i.d. tubes of 0.2 mM trityl-CD3
separated by 1.8 cm. Gradient of 0.34
G/cm 99,328 averages 4096 points, zero-filled to
16,384 Phase determined by starting point in the
FID. Signals are sharper in the real component of
the absorption spectrum than in the absolute
value spectrum.
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Comparison of spin echo and FID
Same two 3 mm id tubes of 0.2 mM sym-trityl
separated by 1.8 cm. Gradient of 0.55 G/cm and
99,328 acquisitions At this higher gradient the
FID decays so quickly that the resolution in the
Fourier transform is poorer than for the
transform of the spin echo data.
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Block Diagram of DU Bridge