OBJECT OF

INTEREST, COMPRISING A TRANSMIT/RECEIVE SWITCH

Field of the Invention

5 The present invention generally relates to an apparatus for magnetic resonance imaging (MRI) or spectroscopy of an object of interest, comprising a transmit/receive switch. The invention further relates to a method of magnetic resonance imaging (MRI) or spectros copy by means of such an apparatus.

10 Background of the Invention

For imaging compounds with very short coherence life times, it is of great importance that the excitation pulse is short. This is particularly true for the zero-echo time (ZTE) technique, in which the readout gradient is already on during excitation. In this case, the pulse duration determines a k-space gap to be minimized [1-5]. Further, in ZTE the exci- 15 tation bandwidth must be at least as broad as the acquisition bandwidth, limiting the pulse duration when block pulses are used. Finally, rapid excitation also minimizes re laxation during the pulse.

As excitation pulses get shorter, ever-higher RF power is required to still reach optimal 0 flip angles. This imposes steep requirements on the transmit-receive (T/R) switches in volved, which must not only handle increasing power, but also switch very fast, equally in the order of microseconds. T/R switches are typically based on PIN diodes, which re quire strong current pulses to change state rapidly and might overload the RF receive chain by switching transients. To-date, the fastest T/R switching in the kilowatt range 5 has been achieved by symmetrically biasing PIN diodes [6]. With this design, switching in less than 1 ps has been accomplished at 2kW peak power. Flowever, emerging modes of ZTE imaging with large bandwidth and flip angle call for still higher RF power [7-8].

Summary of the Invention 0 It is the principal object of the present invention to overcome the limitations and disad vantages of currently known setups. According to one aspect of the invention, an apparatus for magnetic resonance imaging (MRI) or spectroscopy of an object of interest at a predefined Larmor frequency, com prises: a) a coil arrangement (TR) for a1) transmitting an excitation signal into the object, thereby exciting nuclear or electron spins within the object, and a2) receiving a response signal emitted by excited nuclear or electron spins from the object, b) a high-power high-frequency amplifier (TX) for generating the excitation signal to be transmitted into the object, c) a low-noise amplifier (LNA) for receiving and amplifying the response signal from the object, d) a transmit/receive switch (T/R) having a first port (P1), a second port (P2) and a third port (P3), i) the first port (P1 ) being connected to said high-power high-frequency am plifier (TX), ii) the second port (P2) being connected to said low-noise amplifier (LNA), and iii) the third port (P3) being connected to said coil arrangement (TR), the transmit/receive switch (T/R) being alternatively operable in d1 ) a transmit mode, in which the excitation signal from the high-power high- frequency amplifier (TX) is allowed to pass from said first port (P1) along a transmit path to said third port (P3) connected to the coil arrangement (TR) while being blocked from passing along said receive path to said second port (P2) connected to the low-noise amplifier (LNA), and d2) a receive mode, in which the response signal incoming from said third port (P3) connected to the coil arrangement (TR) is directed along a re ceive path to the said second port (P2) connected to said low-noise am plifier, e) a network arranged in the receive path of the transmit/receive switch between said third port (P3) and said second port (P2), said network comprising a plu rality of at least two serially arranged network segments (N1 , N2), and f) each network segment (N1 , N2) comprising a proximal end (E1 P, E2P) and a distal end (E1 D, E2D), each network segment comprising an electronic mod ule operable to form a low impedance path from a respective proximal end to ground at said Larmor frequency, thereby creating a high impedance at said distal end and thus preventing the outgoing excitation signal from reaching the second port P2 connected to the low-noise amplifier.

According to the invention, said electronic modules are configured in such manner that the serially connected network segments form a timed cascade of switching stages, said electronic modules being selected from MEMS, semiconductor devices and relays.

The term "object of interest" shall be understood to include any objects amenable to magnetic resonance imaging or spectroscopy. In particular, this term shall also include any human or animal subject.

According to another aspect of the invention, there is provided a method of carrying out magnetic resonance imaging (MRI) or spectroscopy of an object of interest by means of an apparatus according to one of the preceding claims, the method comprising repeti tively applying the steps of: a1 ) transmitting a pulsed excitation signal into the object, thereby exciting nuclear or electron spins within the object, and a2) receiving a response signal emitted by excited nuclear or electron spins from the object.

Advantageous embodiments are defined in the dependent claims.

In general, the principles of the present invention are applicable to any type of magnetic resonance. It will be understood that, depending on the type of magnetic resonance and the settings employed, the relevant Larmor frequency will have a specific value in a very broad range.

It will also be understood that the method of the present invention is based on applying so-called sequences, such a sequence comprising a plurality of excitation pulses ap plied to the object, each excitation pulse being followed by a signal acquisition step. These repetitive pairs of pulse and acquisition events are typically carried out at a pre determined repetition frequency. The latter repetition frequency depends on the specific type of MR process being carried out. For example, in the case of short-T2 MRI, the rel evant T2 ^* relaxation times are in the range of 10 to 500ps, thus enabling to use repeti tion frequencies in the range of 500Hz to 5kHz, typically of about 1 kHz.

In particular, applying nuclear magnetic resonance (NMR), which comprises nuclear MR imaging but also NMR spectroscopy, the Larmor frequency will be in a frequency range from 1 to T000 MHz (claim 2). For example, the Larmor frequency for ¹ H nuclei in a 3 T main magnetic field is about 127.7 MHz.

When applying electron spin resonance (ESR,) the Larmor frequency (ESR) will be in a range from 10 to 10Ό00 MHz (claim 3).

According to an advantageous embodiment (claim 4), each network segment comprises an impedance member acting as a quarter wavelength phase shifter at said Larmor fre quency.

According to a particularly advantageous embodiment (claim 5), each one of said elec tronic modules is configured as a pair of antiparallel PIN diodes.

According to one embodiment, each successive stage in the cascade injects less transi ent power into the RF or microwave circuitry until the transient power is small enough that it can be handled by the LNA without driving the LNA into its non-linear range.

According to a further embodiment, the timing of the cascade is such that the driving cir cuit detects when the previous stage has completed its transient power injection and thereafter starts changing its state in a self-triggered way.

According to yet another embodiment, the timing of the stages is fixed and done in such a manner that each stage changes its stage when it is known by design that the previ ous transient is over. According to a further embodiment, the timing is adjusted over time compensating for thermal effects by either incorporating a thermal model or measuring the temperature, enabling tighter timing resulting in decreased overall switching time. According to another embodiment, the overall rise time from the start of the state change until completion is less than 2ps, most particularly less than 1ps, enabling effi cient use for methods as ZTE and the like.

According to a further embodiment, all stages are set into the receive configuration be- fore all switch modules are fully risen, thereafter the attenuated signal can be compen sated by a multiplication with 1/(normalizedGain(time)) thereby decreasing the effective switch time until useful data can be acquired.

According to yet another embodiment, the timing is modified such that the process of changing its state starts prior completion of the previous stage but late enough such that the transient of the previous stage is still suppressed enough since it takes a finite amount of time until the current stage does not attenuate power.

According to a further embodiment, there are at least three stages in the cascade and the stages are distributed in at least two groups with at least one group with more than one stage in it, then the timed cascaded is executed with simultaneous state change in each group.

According to one embodiment of the method (claim 7), the pulsed excitation signal has a peak power exceeding 1 kW, particularly exceeding 5 kW, more particularly exceeding 10 kW.

According to a further embodiment (claim 8), the pulsed excitation signal has a duration not exceeding 10ps, particularly not exceeding 5ps, more particularly not exceeding 2ps, most particularly not exceeding 1 ps.

According to another embodiment (claim 9), the pulsed excitation signal has an average power not exceeding 20W, particularly not exceeding 10W. These threshold values de- pend on the particular application, e.g. 10 W should not be exceeded in MRI measure ments of the human brain. Other body parts allow somewhat higher average powers. Even higher average power levels can be used for non-living objects of interest. Brief description of the drawings

The above mentioned and other features and objects of this invention and the manner of achieving them will become more apparent and this invention itself will be better under stood by reference to the following description of various embodiments of this invention taken in conjunction with the accompanying drawings, which show the following:

Fig. 1 RF topology of the high-power T/R switch. The antiparallel PIN diode pairs are driven in a cascaded way. Towards the RX port, PIN diodes with shorter carrier lifetime are deployed. Fig. 2 Snippet out of the passive self-triggered PIN diode driver. As long as a PIN diode is still in its low impedance state, it has a forward voltage present, despite bulling a reverse current out of it. When the PIN diode changes to its high impedance state, current is drawn out of the next stage.

Fig. 3 Transient switch behavior when switching from the TX to the RX state. The reverse bias is built up in a cascade, which reduces the transient voltage peaks (a) measurement taken with RF signal; (b) measure ment taken without RF signal.

Fig. 4 3D ZTE images of phantoms with short-T2 made of rubber (left and middle: Ampelmannchen Shop, Berlin, right: giveaway ISMRM 2015, Toronto) (a) photographic pictures and (b) acquired with high-power RF pulses. Imaging parameters were: field of view 100mm, resolution 0.62mm, bandwidth 500kHz, TR 1 ms, block pulse with 1 ps and 13kW, dead time 3.6ps, and scan time 13m43s. Fig. 5 In-vivo 3D ZTE images with 500kHz bandwidth and block pulses of

1ps duration used for spin excitation, at two different power levels: (a) Low power, 1 kW peak; (b) High power, 13 kW peak. Detailed description of the invention

An apparatus for magnetic resonance imaging (MRI) or spectroscopy of an object of in terest at a predefined Larmor frequency is shown in Fig. 1.

The apparatus comprises a coil arrangement (TR), such as e.g. a head coil for MRI, which is configured for transmitting an excitation signal into an object of interest, thereby exciting nuclear or electron spins within the object, and for receiving a response signal emitted by excited nuclear or electron spins from the object. Moreover, the apparatus comprises a high-power high-frequency amplifier (TX) for generating the excitation sig nal to be transmitted into the object. Furthermore, the apparatus comprises a low-noise amplifier (LNA) for receiving and amplifying the response signal received from the ob ject. As shown in Fig. 1 , the amplified signal produced by the LNA is fed into a receiver RX for further processing into an MR image or spectrum.

Still further, the apparatus comprises a transmit/receive switch (T/R) having a first port (P1 ), a second port (P2) and a third port (P3), wherein: i) the first port (P1 ) is connected to the high-power high-frequency amplifier (TX), ii) the second port (P2) is connected to the low-noise amplifier (LNA), and iii) the third port (P3) is connected to the coil arrangement (TR). The transmit/receive switch (T/R) is alternatively operable in d1 ) a transmit mode, in which the excitation signal from the high-power high-frequency amplifier (TX) is allowed to pass from the first port (P1) along a transmit path to the third port (P3) connected to the coil arrangement (TR) while being blocked from passing along a receive path to the second port (P2) connected to the low-noise amplifier (LNA), and d2) a receive mode, in which the response signal incoming from said the third port (P3) connected to the coil arrangement (TR) is directed along the receive path to the second port (P2) connected to the low-noise amplifier. A network is arranged in the receive path of the transmit/receive switch between the third port (P3) and the second port (P2), the network comprising, in the example shown, a plurality of three serially arranged network segments (N1 , N2, N3). Each network seg ment (N1 , N2, N3) comprises a proximal end (E1 P, E2P) and a distal end (E1 D, E2D), and each network segment comprises an electronic module operable to form a low im pedance path from a respective proximal end to ground at the Larmor frequency, thereby creating a high impedance at the distal end and thus preventing the outgoing excitation signal from reaching the second port P2 connected to the low-noise amplifier. The electronic modules are configured in such manner that the serially connected net work segments form a timed cascade of switching stages. The electronic modules are selected from MEMS, semiconductor devices and relays.

Example The proposed RF topology is shown in Fig. 1. During RF transmission, all PIN diodes are forward-biased. Towards the receive port RX, the PIN diodes are successively se lected with shorter carrier lifetime and less charge is injected during forward biasing. During reception, the PIN diodes are reverse-biased. The anti-parallel arrangement of each diode pair ensures rough cancelation of any transient voltages. The PIN diode pairs in the receive path are reverse biased in a timed cascade, starting with the one nearest to the TR port. In this way, transient voltage peaks are absorbed by the subse quent PIN diodes, which are not yet reverse-biased when the previous PIN diode stage peaks. Because later stages require less current to reverse-bias rapidly, the final transi ent peak is sufficiently small to be within the linear range of the utilized low noise ampli- tier.

The cascaded PIN diodes which act as switching element are separated by lumped ele ment quarter wave networks. In general, any passive network between the switching el ements can be utilized as long as the present a high impedance compared to the low impedance of the switch element when they are in their conductive state. Further said passive network need to be able be matched to the system characteristic impedance in the receive state and be of low loss such that low insertion losses can be achieved dur ing reception. A quarter wave element is optimal since in the ideal case it presents an open circuit if it is shorted to ground by the active element. Further in the receive state it does not need any additional matching circuit since it just shifts the phase of the signal but does not change the impedance. Such quarter wave network could be implemented by lumped elements or a transmission line or cable with required length. As active switching elements PIN diodes, MOSFETs, MEMS switches or relays could be used. Further combination of different active elements can also be beneficial for such TR switches. An example would be a PIN diode pair in the first stage and subsequent stages with GaN MOSFETs. The PIN diode driver as shown in Fig. 2 is made such that the cascading is passively self-triggered. A later PIN diode stage starts pulling out charge carriers from the intrinsic layer as soon as the previous stage crosses zero Volt biasing. At zero Volts, the majority of charge carriers have been extracted and the PIN diodes change to a high impedance state. This ensures that when a PIN diode stage peaks, the subsequent stage is still conducting RF and thereby catching such voltage peak.

The required delay for the subsequent stages can be made with analog or digital circuits or combination of both. Measurements of the transient response were made with an Agilent Technologies

MSO7054A oscilloscope, a R&S SMB 100A signal generator, an Agilent Technologies 81104A pulse-pattern generator and a National Instruments PXIe-5622 16-bit digitizer. Insertion loss and isolation were measured with an Agilent Technologies E5071C net work analyzer. To validate the electrical and thermal integrity an Analogic AN8134 18kW amplifier, a R&S FSL spectrum and a Fluke TiS20 were utilized.

Finally, high-bandwidth ZTE imaging with algebraic reconstruction was performed on phantoms and in-vivo using a Philips Achieva 3T scanner with a high-performance insert gradient [9].

At the operation frequency of 128MHz, the insertion loss from the TX to the TR port in the transmit state is -0.25dB and the insulation to the RX port is -76.3dB. In the receive state the insertion loss from the TR port to the RX port is -0.5dB and the insulation to wards the TX port is -62.5dB. Fig. 3 shows the cascaded reverse biasing and the switching process. Based on this data, the rise time was assessed at 855ns and the maximum transient voltage is - 25.5dBm.

Bench measurements verified that full peak output power of the 18kW amplifier could be applied without losing the integrity of the pulse. It was found that 100W average power with 15kW peak power could be applied without issues. Thermal images (not shown here) of the T/R switch board were taken in the steady state after ~1 min of heating. It was found that the RF inductors get the hottest with a peak temperature of 99.7°C. The hottest PIN diode appears to have a temperature of 63.2°C, while the background tem perature was 26.3°C.

Imaging experiments revealed that ZTE images of short-T2 phantoms, as shown in Fig. 4, exhibit no visible artifacts related to the switching process.

As can be seen from Fig. 5, by applying higher power of e.g., 13 kW a better SNR and higher contrast could be achieved in in-vivo ZTE as compared with the more common power level of 1kW. Imaging parameters were: field of view 180mm, resolution 1.1mm, TR 2ms, dead time 3.6ps, and scan time 1m42 s.

In the above example, we have proposed a timed cascade of PIN diodes to overcome current power limits. Using this approach, it is shown that sub-microsecond switching can be reconciled with 18kW RF transmission.

Conclusions

The presented T/R switch was shown to be able to handle high peak power of 18kW, while maintaining a switching time below 1ps. These capabilities present will facilitate imaging of materials with ever shorter T2 samples such as myelin or collagen [8,10].

Rapid switching was accomplished with a timed cascade. The passive self-triggered re verse bias sequence makes the design not only simple to implement but also robust since it inherently adapts to alterations in voltage dynamics. To further reduce the switching time of the current implementation, higher reverse volt age could be applied and the related increase in the transient voltage peak could be re solved by an additional PIN diode stage in the cascade.

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