Magnetic resonance imaging (MRI) is widely used in hospitals and clinics for medical diagnosis. World­wide, there are approximately 36,000 MR machines. At pre­sent, about 2,500 MR imaging units are sold worldwide every year (1). How they work? Oversimplified, they identify the protons of hydrogen atoms in tissues containing water molecules. The energy of an oscillating magnetic field stimulates the molecules in briefly applied resonance frequencies to the patient. The stimulated hydrogen atoms emit an RF signal that is then measured by a receiving coil. Gradient coils vary the main magnetic field to determine positional data, which is translated by a computer. And by the way, those coils, briskly switching on and off, produce the loud and, as anyone who has received the procedure would tell you, obnoxiously noisy environment of an MRI scan. The rate at which the stimulated atoms return to stability, shows up as contrast between tissues, thus providing an image used for diagnosis.

An MRI system requires a strong and uniform magnetic field for the procedure to work. The basic physical theory behind a MRI system was independently discovered in 1946 by both Felix Bloch (2) and Edward Purcell (3). Inside the nucleus of every atom, individual protons and neutrons spin around an axis. Since atomic nuclei have charge, this spinning motion produces a magnetic moment, which can be understood as a vector having a magnitude and direction, along the spin axis. The relative strength of this magnetic moment is a unique property for each element, and therefore determines its suitability for magnetic resonance energy absorption and detection. The hydrogen nucleus with one proton is the nucleus of choice in MRI because it possesses the strongest magnetic moment and is abundant in organic tissues. In Figure 1, we see that in the absence of an external magnetic field, the individual magnetic moments have no preferred orientation.

Fig1
Figure 1. Magnetic dipoles in the absence of an applied magnetic field.

The field strength of the magnet is measured by Tesla (T). With the application of an external magnetic field B0 the magnetic moments will align with the external field with a preferred orientation to guarantee the lowest energy state. The spin angular momentum causes the magnetic moment to precess about the B0 axis as shown in Figure 2

Fig2
Figure 2. Precession of the hydrogen nucleus in the B0 field

The frequency of precession is governed by the Larmor equation as:

Eq1
Where f is the frequency of precession, γ is a characteristic constant that depends on the given nucleus, and B0 is the strength of the externally applied field. For hydrogen nuclei, B0  is given as 42.576 MHz/T. Thus, in a field strength of 3T, the hydrogen nucleus will precess with at a frequency of 127.728 MHz (B0=3T is about 60000 times the Earth's magnetic field strength).

Consider the application of RF radiation at the Larmor frequency to a sample of non-magnetic material in an applied static magnetic field. When the frequency of the RF radiation matches the Larmor frequency of the protons, we have resonance, the most efficient state to exchange energy with protons (absorption and emission).

The applied RF radiation is composed of coupled electric and magnetic field components. The magnetic field component is denoted by B1, and it resides in a plane perpendicular to B0 while rotating about B0 at the Larmor frequency as shown in Figure 3.

Fig3
Figure 3. Magnetization M rotating about B1 when the RF pulse is present

Applying B1 causes M (magnetic moment) to rotate by a certain angle away from the B0 axis.

Hence, if B1 of the RF pulses persists for the appropriate duration of time, M can be made to rotate onto the transverse plane.

While in the transverse plane and rotating at the Larmor frequency, M will induce an NMR (Nuclear Magnetic Resonance) signal in the RF receiver coil, which is oriented in the transverse plane as shown in Figure 4.

Fig4
Figure 4. M rotates on the transverse plane after the application of a 90◦ RF pulse

When the RF pulse vector is switched off, M rotates in the transverse plane at the Larmor frequency and gradually decays to zero with time constant T2. Similarly, we will have a decay to zero with the time constant T1 that returns the protons to the original direction of the field B1. Both time constants depend on the nature of the body tissues under investigation.

Fig5
Figure 5. MRI System configuration.

The strength of the magnetic field can be altered electronically from head to toe using a series of gradient electric coils, and, by altering the local magnetic field by these small increments, different slices of the body will resonate as different frequencies are applied.

In 3-Tesla (3T) MRI scanners, the required frequency should be around 127 MHz; some manufacturers still consider a 3T MRI to be achieved even when the Larmor frequency considered is slightly lower. In the design example presented below, the central frequency is 123 MHz.

RFPA architecture for MRI systems

For MRI systems, RF power amplifiers (RFPAs) are optimized for pulse operation to provide high RF power pulses within a short period of time. This amplifier uses two STAC4932B-100V N-channel MOSFETs from STMicroelectronics (4) in a push-pull configuration capable of 1.2 kW, each under pulse conditions, with each STAC244B (5) using a bolt-down air-cavity package (see Figure 6).

The design specifications for the 3T MRI project are:

Specs

In order to reach an RF peak-power level of 2 kW, it was decided to employ two STAC4932B MOSFETs in a double push-pull class AB configuration (see Figure 6).

Fig6
Figure 6. STAC® packaging.

Fig7
Figure 7. Push-pull topology.

The STAC4932B is an N-channel MOS field-effect RF power transistor. It is intended for 100 V pulse applications up to 250 MHz. This device is suitable for use in industrial, scientific, and medical applications (ISM). The STAC4932B benefits from the STMicroelectronics air-cavity STAC® packaging technology. In the double push-pull schematic, each of the STAC4932B high- and low-side devices contains two dice. From the input RF point of view, each STAC4932B is seen as a single gate. The design must ensure each gate has an equal and independent signal level (about 20 W shared by four gates), and at the same time prevent any additional gate interaction that may cause dangerous instability issues. The PCB's layout was especially designed to minimize any electrical asymmetry at the input of the devices.

Biasing and Stability Control

Several circuit topologies were taken into consideration for synthesizing the double push-pull amplifier (PPA). To create a compact design, our conclusive idea was to realize one single RF output balun transformer to serve the four drains (T2 in Figure 8). For the input, we used an input balun transformer (T1) in combination with two in-phase power splitters (L4, L8, C16, C18, C20 and L12, L16, C36, C41, C45) in order to guarantee proper isolation while the RF signal drives each gate. The balun transformer T1 and the input power splitter sections were carefully designed to respect the electrical symmetry.

Fig8
Figure 8. 2-kW PPA electric circuit.

The RF LC decoupling filters fed through the VG1 and VG2 connectors (see Figure 8) are needed to bias each of the STAC4932B gates. The output biasing network includes the center tap of the primary winding of T2 and several multilayer ceramic capacitors. The capacitors charged at 100 V deliver the high DC current to the device’s drains and also dampens the voltage overshoot generated by the pulsed RF modulation, with the electrical transients controlled by L10, R13, C29, C30 and C33. Two test points can be inserted between two calibrated Rm resistors for current / voltage monitoring and an LED D1 for safety purposes during testing.

Design and Implementation

The main features of the 2-kW PPA are the planar implementation of the balun transformers (T1 and T2 embedded in PCB) and the SMT components as well as the lack of the ferrite elements in either. The PCB is embedded on a robust base plate (100 x 150 mm) to form a compact board that can withstand high RF power and high voltage (Vcc = 100 V). The 2-kW PPA amplifier layout is built on the Roger 4350B substrate with separate input-output PCB cards. The input PCB (see Figure 9) integrates the RF balun transformer T1 and the RF decoupling networks.

Fig9
Figure 9. RF balun transformer T1 (Top and Bottom view).

The balun transformer T1 is λ/4 - 25 Ohm transmission line @ 123 MHz made using a strip-line technique on a 3-layer substrate (Roger 4350B, with a thickness of 20 +20 mils, see Figure 9) and is fed by a suspended 50 ohm microstrip line ('line bridge' in Figure 8).

In summary, the transformer T1 plays several important roles:

  1. Balun function and lowering the impedance;
  2. A quasi-one-dimensional RF structure (the λ/4 - 25 Ohm transmission line) on the PCB surface that allows the electrical symmetry to feed the RF to all gates.

The PCB shown in Figure 10 is the layout of the output RF transformer T2 with the remaining input power splitter and the biasing/filtering networks for the STAC4932B’s gates and drains.

Fig10
Figure 10. RF balun transformer T2 (Top, left, and bottom, right).

The transformer T2 (ratio 4:1) is designed on the top/bottom layers (Figure 10) using a substrate Roger 4350B of 60 mils thickness in suspended broadside coupled strips and acts as a composite transmission line transformer from balanced to unbalanced mode. The RF output (type N-female connector) is directly connected to the T2 winding output strip (top view, Figure 10) through an air-suspended strip-line (50 Ohm). This allows the differential current flowing on the primary winding strip (PCB top layer) to move to the unbalanced RF output without power losses.

In this way, transformer T2:

  1. Provides the optimal impedances to the DMOS drains;
  2. Feeds high DC current filtered at Vcc = 100 V through the center tap of the winding top strip of T2;
  3. Carries out a ground current path to the RF output load in order to circumscribe the RF grounding issues.

The transformer T2 has been designed using EDA Software (ADS Keysight), and the refinement between electromagnetic (HFSS) and circuit simulation (see Figure 11). T2 uses C25, C26, and C23 on the winding top strip, and C37 and C42 on the output secondary strip, which makes it possible to tune drain impedance levels. On the copper carrier, two deep cavities (7 mm) accommodate the transformers T1 and T2; additionally they minimize the parasitic impedance (leakage) to ground.

Fig11
Figure 11. Architecture of T2 and its essential transmission line view.

Figure 12 shows the assembled board and Table 1 lists the parts.

Fig12
Figure
12. 2-kW MRI final assembled board.

 

Component ID

Value

Manufacturer

Part code

C12, C13, C6

1000 µF, 100 V

Panasonic

ECA2AM102

C10, C11, C52, C53

100 nF

Murata

GCM188R71E104KA57D

C28

4.7 µF, 100 V

TDK

CKG57NX7R1E226M

C29

15 µF, 100 V

Murata

KRM552R72A156M

C15, C51

10 µF, 35 V

KEMET

T494D106K035AT

C3,C47

100 µF, 20 V

KEMET

T491X107K020AT

C4, C7, C48, C49

22 µF, 25 V

Murata

GRM32ER61E226ME15

C18, C41

300 pF

ATC

ATC100B301FWN200XC

C16, C20, C36, C45

68 pF

ATC

ATC800A680JTN250X

C30

1000 pF

ATC

ATC100B102FWN300XC

C57, C58

3.3 pF

ATC

ATC100B3R3BW1500XT

C8, C31, C32, C50

470 pF

ATC

ATC 100B 471FWN200XC

C2, C17, C24, C34, C40, C55, C59, C60, C61, C62, C63, C64

2000 pF

ATC

ATC 200A202KTN50C

C9, C14

1 µF, 100 V

AVX

22201C105KAT2A

C33

1 µF, 100 V

AVX

12101C105K4Z2A

C21

18 pF

ATC

ATC100B180FWN1500XT

C25

75 pF

ATC

ATC100B750FWN1500XC

C26, C23

100 pF

ATC

ATC100B101FWN1500XC

C37, C42

56 pF

ATC

ATC100B560FWN1500XC

L10

700 nH

Coilcraft

CP-K0376-A

L2, L5, L13, L15

82 nH

Coilcraft

1515SQ-82NJEB

L3, L14

110 nH

Coilcraft

132-10SMJ

L4, L8, L12, L16

5.4 nH

Coilcraft

0906-5JLB

R1, R5, R28, R30

50 Ω, 100 W

Anaren

C100N50Z4

R9

5600 Ω

Tyco Electronics

SMF25K6JT

R13

22 Ω

Tyco Electronics

SMW222RJT

R7, R29

100 Ω

Panasonic

ERJP14J101U

R11, R22

4.7 Ω

Vishay

4.7 Ohm -1206

R3, R24

43 Ω

Panasonic

ERJP14J430U

R32, R33, R34, R35

27 Ω

Panasonic

ERJP14J270U

R4, R16, R17, R31

20 Ω

Panasonic

ERJP14J200U

R6, R8, R10, R12, R14, R15, R19, R20, R21,R25,R26, R27

1 Ω

Phycomp

232271111108

Rm x 2

0.001 mΩ

Tyco Electronics

TL3A R001 1%

1P_J3

1 double pole

Wieland

25.700.0153.0

Spacer_J3

Spacer

Wieland

07.300.2753.0

3P_J1,J2

3 poles

PHOENIX CONTACT

1725669

P2

N_Female

Telegartner

J01021A1084

P1

SMA_Female

RADIALL

R124.510.000W

(Q1-Q2)/(Q3-Q4)

STAC4932B

STMicroelectronics

STAC4932B

D1

LED

Kingbright

KP-1608SURC

Line bridge

Roger 4350B, three layers, 20+20 mils, 1 OZ Cu on top-mid-bottom layers, Finit. metal HAL LF; total Tk=1.2 mm max., top screen printing component, tin chemical surface deposition.

Board input

Roger 4350B, three layers, 20+20 mils, 1 OZ Cu on top-mid-bottom layers, Finit. metal HAL LF; total Tk=1.2 mm max., top screen printing comp., tin chemical surface deposition.

Fin fixing

Roger 4350B, two layers, Tk=60 mils, 1 OZ Cu on top- bottom layers, Finit. metal HAL LF; total Tk=1.6 mm max., top screen printing comp., tin chemical surface deposition.

Board output

Roger 4350B, two layers, Tk=60 mils, 1 OZ Cu on top-bottom layers, Finit. metal HAL LF; total Tk=1.6 mm max., top screen printing comp., tin chemical surface deposition.

Mechanical plate

PPAMRI_002-Rev B

         

Table 1. Bill of materials.

Thermal Features of Air Cavity packages

The ST air cavity (STAC®) technology allows very low thermal impedance: Ztjc = 0.075 K/W (with t = 1 ms pulsed RF, Duty Cycle = 10%). Combining it with an appropriate heatsink (Rth <0.2 K / W) lowers the junction temperature below the device’s maximum rating (Tjmax = 200 degC).

The ability of STAC® to dissipate a high power pulse makes it possible to reduce board dimensions and external heat-sinks; so that, by using the flangeless package STAC244F shown in Figure 13, one can design a new board with the necessary electrical characteristics but with a dimension target of 80 x 100 mm or less.

Fig13
Figure 13. Air cavity package.

Prototype RF Performance Measurements

Figure 14 and Figure 15 show measurements taken in pulsed condition (tpulsed = 1 ms and period 10 ms) with the power amplifier operated in class AB (100 V x 200 mA). In Figure 14, the power gain and the input return loss is depicted versus frequency @ Pin = 20 W. In Figure 15, we see the power gain and the drain efficiency versus the output power @ f = 123 MHz. The drain efficiency of 60% is reached at 2.2 kW RF power.

Fig14
Figure 14. Power gain and input return loss over frequency.

Fig15
Figure 15. Gain compression and Eff (%) Vs Pout @ f = 123 MHz.

Summary

A pulsed RF high power amplifier (> 2 kW) has been described as a guideline-design, oriented to new high-voltage DMOS devices STAC4932B in an air-cavity package (STAC®) at Vcc = 100 V. In particular, the amplifier combines excellent high frequency response with an efficient use of DC power. Due to the combination of SMT components and the fully planar technology of the RF transformers, a very compact and robust design was produced. This amplifier is the single unit of a high-power RF chain, and by combining some of them, we can achieve the very high power level required for an RF pulse generator in 3T-MRI systems.

Bios

Author1Roberto Cammarata received his Master’s degree in Electrical Engineering from the University of Catania, Italy. He began his career in 1985 working in the microwave field on hearth satellite ground stations (Selenia - Marconi Comm., Catania). Afterwards, he worked on Space Satellite Projects (Alenia Aerospace , Rome) before joining STMicroelectronics (Catania) to design power amplifiers for Communications and ISM RF applications.

 

Author2Alfio Scuto received his Master’s degree in Microelectronics Engineering from the University of Catania, Italy. In 1999, he joined ST's RF Power Design Center in Montgomeryville (PA) USA. As an RF Application Engineer, he was involved in the characterization of high-power and high-frequency transistors. In 2002, he moved to STMicroelectronics in Catania (Italy) to support the RF products marketing group developing power amplifiers in order to evaluate and verify product performance in relation to customer specifications. Today he works as a High-Power Application Engineer supporting customers and exploring new applications for high-power MOSFET devices.

References

1. Magnetic Resonance, a critical peer-reviewed introduction. http://www.magnetic-resonance.org/contents.html. [Online] 17 November 2014.

2. Nuclear induction. Bloch, F. s.l. : Physical Review, 1946, Vols. vol. 70, pp. 460-473.

3. Resonance Absorption by Nuclear Magnetic Moments in a Solid. E. Purcell, H. Torrey, and R. Pound,. s.l. : Physical Review,, 1946, Vols. vol. 69, pp. 37-38.

4. STMicroelectronics. STAC4932B.

5. Mounting recommendatios for STAC and STAP boltdown packages. AN3232.

6. Grebennikov, Andrei. RF and Microwave Power Amplifier Design. s.l. : Mc Graw Hill, 2005.

7. Holzman, Eric. Essentials of RF and Microwave Grounding. s.l. : Artech House, 2006.

8. Magnetic Resonance Imaging. Wright, C. A. s.l. : IEEE SIgnal Processing Magazine, January 1997.

9. BRUSH UP THE THEORY TO DESIGN AN HIGH POWER CLASSE-E AMPLIFIER. Alfio Scuto, Roberto Cammarata. s.l. : MICROWAVE - Engineering Europe, June 29, 2016.