M2-481 3D Ultrasound Computer Tomography: Data Acquisition

3D Ultrasound Computer Tomography: Data Acquisition Hardware Tim–Oliver Muller,¨ Rainer Stotzka, Member, IEEE, Nicole Valerie Ruiter, Klaus Schlote–Holubek and Hartmut Gemmeke, Member, IEEE,

Abstract— 3D ultrasound computer tomography (USCT) is an mammograms. Ultrasound imaging on the other hand qualifies imaging method capable of producing volume images with both for women at any age. It is non–invasive and may be applied high spatial and temporal resolution. The promising results of frequently. Additionally the ultrasound based image shows a 2D experimental setup of an ultrasound computer tomography system showing structures of 0.1 mm in diameter encouraged us to partly different structures than the X–ray based. In particular it build a new 3D demonstration system. The main problem of such is well suited for cyst identification. a demonstrator is the high burst data rate of up to 36 GBytes per second for parallel recording of 1500 A–scans sampled at 12 MHz B. Ultrasound computer tomography and 12 bit. We present a hardware setup for an experimental 3D demonstrator, which is capable of handling high data rates. 1) Drawbacks of Common Ultrasound Imaging: Despite its This demonstrator for the first time allows high resolution (sub advantages ultrasound imaging also has some major drawbacks: wavelength) 3D ultrasound imaging in almost realtime. usually it leads to images with low spatial resolution of about Index Terms— USCT, Ultrasound, Computertomography, To- 1 mm. It contains speckle noise and shadowing effects, which mography, Data Acquisition. significantly reduce image quality. The images are hard to in- terpret. The manually guided probe results in non–reproducible images. Until now imaging in most cases only in 2D is I. INTRODUCTION available. Registration with images from 3D modalities like LTRASOUND imaging is an imaging method, which has magnetic resonance tomography is not possible. U several advantages over e.g. X–ray imaging. It is non 2) Overcoming common ultrasound Imaging: Most disad- invasive and therefore repeatedly applicable without harm. It vantages of conventional ultrasound imaging can be conquered shows structures like cysts that may not be detected with other by a fixed 3D setup, where not just direct back–scattered imaging methods. Additionally it is inexpensive and can be signals are recorded but all reflected, scattered and transmitted applied easily. But until now ultrasound imaging has significant signals. As fixed setup a cylindrical container fully covered with low quality. Our long term goal is to provide 3D ultrasound ultrasound transducers is used. An arbitrary transducer emits an imaging with high spatial resolution for early breast cancer ultrasound signal and all other transducers record the trans- detection by overcoming the disadvantages of conventional mitted/reflected/scattered signals from within the container. ultrasound imaging. Afterwards a different transducer is used as emitter and so on. Images will become reproducible and reconstruction using all A. Background signals will lead to high spatial resolution. Furthermore speckle Breast cancer is the most common cancer in western hemi- noise should be strongly suppressed in such images. However sphere. Every 10th woman will be affected within her life. a full 3D setup requires the recording of all signals in parallel, Breast cancer is deadly due to metastases, i.e. spreading tumors which leads to high burst data rates of up to 36 GBytes per to vital organs. The most promising way to cure breast cancer second assuming a sampling rate of 12 MHz, sampling with is to detect it as early as possible when the probability of 12 bits and 1500 receivers recording. These high data rates metastases is low. The risk to develop breast cancer increases could not be handled until now at reasonable costs. 3) Proof of concept: 2D USCT demonstrator: significantly at the age of 40 to 50 years. From this age We built an on mammography screening is performed using mostly X–ray experimental 2D setup to prove the potential of ultrasound imaging, which is a proper imaging method for senior women computertomography [4]. It consists of two transducer arrays because of their reduced dense glandular tissue. It is mostly of 16 elements each, which both are mounted relocatable in a not applicable to younger women. X–rays interact with living cylindrical container. With this setup we were able to emulate tissue and therefore should not be applied to often. Additionally a virtual circular emitter/receiver array of 1600 transducer not all structures of the examined breast are visible on X–ray positions relocating the transducers sequentially. The sequential measurement reduces the burst data rate so that a conventional Manuscript received October 22, 2004. PC card could be used for recording. We measured several self– All authors are with the Forschungszentrum Karlsruhe, Institute of made ultrasound phantoms. Fig. 1(a) shows the cross–section Data Processing and Electronics, Hermann–von–Helmholtz–Platz 1, 76344 Eggenstein–Leopoldshafen, Germany. (email: {mueller, stotzka, ruiter, schlote, blueprint of one of these ultrasound phantoms containing dif- gemmeke}@ipe.fzk.de) ferent structures (straws, nylon threads) with minimum size of

Fig. 1. 1(a) Blueprint of the cross–section of an ultrasound phantom. Note the nylon threads of 0.45 mm in diameter. 1(b) Cross–section image derived from a medical gold standard scanner (Acuson Sequoia 512). The scanners transducer array is located at the bottom of the image. The image suffers from shadowing and speckle noise. 1(c) Cross–section image derived from the 2D USCT demonstrator. All receiver positions but only 2 percent of the emitter positions were used in this measurement. Neither speckle noise nor shadowing is visible and even the nylon threads can be detected (indicated by red pointers).

0.45 mm in diameter. Fig. 1(b) shows the phantom imaged with approximately 5 mm in width and 50 mm in length. It is glued a 3 MHz probe (Sonograph Acuson Sequoia 512). The image is onto a circuit board. Eight emitters and 32 receivers are cut full of speckle, distant structures are shadowed from closer ones from this piezo piece into still connected structures to operate and the nylon threads are not visible at all. Fig. 1(c) shows the at 3.0 MHz. The required read–out and driver electronics for reconstructed cross–section obtained with the 2D demonstrator. each transducer system is build symmetrically on two circuit Only 2 percent of all emitter positions were used leading to boards with the piezo ceramics in between. The structured piezo approximately 45000 A–scans. The structures of the phantom is connected with the electronics via wire bonds. The circuit are clearly visible in the image. Even the nylon threads can be boards are finally folded together and filled with polyurethane detected. No speckle and no shadowing effects are visible in to form the housing of the whole transducer array. Included in the image. Fig. 2 shows the blow-up of the reconstructed cross– the compact transducer systems are pre–amplifiers, eight times section of a different phantom. It contains nylon threads with a multiplexing and two microcontrollers. The amplification gain diameter of 0.1 mm in 0.5 mm distances. These structures are can be gradually set from 5 to 20 dB in steps of 5 dB. The far below the applied wavelength and are clearly visible. manufacturing process is partly automated and therefore leads The recording of all possible positions takes approximately to low–cost and highly reproducible transducer systems. Fig. 3 12 hours thus making it impossible to image living tissue. The shows a fully assembled transducer system. next major step therefore is to image cross–sections of living tissue by reducing the time needed for measuring. The container for the ultrasound computer tomograph is shaped cylindrically from stainless steel, which will be filled II. 3D ULTRASOUND COMPUTER TOMOGRAPH with water as coupling medium. It has a diameter of 20 cm Fig. 4 displays the three main components of the 3D ul- and a height of 23 cm. In its walls 48 transducer systems are trasound computer tomography demonstrator. It consists of a embedded in 3 stacked layers [7]. The setup results in 384 container fitted with transducer systems for data acquisition, a emitters and 1536 receivers overall. The size and positions digitization unit (the crate) for parallel data recording and a of the transducer systems are chosen in a way that the gaps high–end PC for control and reconstruction. The development between them can be covered by rotating the container in six arises two problems: transducer design and handling of high steps mechanically. The rotation is controlled by the PC using data rates. All transducers have to be very similar in their an internal PC card and a step motor. Thus the number of features like e.g. resonance frequency and transducer charac- transducers positions can be increased if needed. teristics. Additionally they have to be low–priced as several thousands of them are needed. At our institute we developed a Each transducer system has a hard–wired address. The micro- method to produce inexpensive transducer systems with similar controllers are responsible for enabling/disabling emitters and characteristics. [7]. for multiplexing receivers after obtaining a broadcasted address. The desired address is generated by the PC, looped through the data acquisition hardware and broadcasted to the transducer A. Transducer systems and container systems. The output of the transducer systems is transferred Ultrasound transducers are made from piezo ceramics. For through four signal repeaters and into the data acquisition a single transducer system we use a piezo piece of size of hardware for digitization.

0-7803-8701-5/04/$20.00 (C) 2004 IEEE Fig. 2. Reconstructed nylon threads of 0.1 mm in diameter and 0.5 mm distance Fig. 3. Front view of a transducer system with eight emitters and 32 receivers. recorded at 3 MHz with the 2D USCT demonstrator [4]. The threads’ diameters Every emitter is surrounded by four receivers. Included in the housing are pre– are below the applied wavelength. ampliﬁcation, eight times multiplexing and two microcontrollers. [7].

B. USCT crate: fast data acquisition hardware a second step adding more crates thus removing multiplexing towards the final goal of faster data recording. The experimental results of the 2D demonstrator showed that 3) Master board: The master board connects the PC to the the received signals need to be sampled at least at 12 MHz crate and controls the whole measuring process. It accepts with 12 bits for satisfactory accuracy. The analog signals from commands from the PC and delegates them to the slave the four signal repeaters have to be digitized in parallel. This boards. Reading recorded signals from the memory of the slave task is only feasible through the use of our digitization crate, a boards is only possible via the master board under use of the specifically designed hardware for processing high data rates. backplane. The emitters of the transducer systems will be driven Originally designed for the Auger project [1], [3] the approach by the master board using coded excitation to allow arbitrary was purposefully generic and modular structured. Therefore the waveforms. The master board also controls multiplexing of crate could be adapted for ultrasound computer tomography emitters and receivers by broadcasting addresses to the trans- with minimal efforts. Below the main components of the crate ducer systems. The master boards is fitted with an FPGA for are listed. control and 512 KBytes of memory to store transducer system 1) Backplane: The crate provides slots for different circuit address series and waveform samples for coded excitation. boards connected by a backplane. This backplane bases on 4) Controller board: The controller board is in charge for a single master VME–Bus with 32 bit width and a transfer minor functions like measuring the average temperature within rate of approximately 4 MBytes per second. Each slot provides the container from eight different locations and providing it connectors for the circuit boards to access the shared bus. The to the PC. The temperature is required for the correction of bus speed limits the maximum transfer speed from crate to PC. a possible temperature drift during measuring. During recon- However important for ultrasound computer tomography is the struction the run–length of the A–scans is adapted according parallel recording of the A–scans, which is performed by the to the temperature. digitization boards at a much higher rate 5) Controlling and reconstruction via PC: For data acqui- 2) Slave boards for digitization and preprocessing: The sition and reconstruction a high–end PC with the Windows crate is designed to hold up to 20 slave boards each capable to 2000 operating system is used. The PC is connected to the handle 24 different channels in parallel. Every board consists master board of the crate via a MicroEnable framegrabber card. of an analog part for digitization and a digital part with four The data acquisition is controlled via a commandline interface Altera FPGAs for data processing. The sampling for USCT is (USCT shell). The USCT shell is implemented in Java. It is performed at a maximum sample rate of 12 MHz and 12 bits object oriented designed and provides a basic functionality per second. Each FPGA processes six channels in parallel and like e.g. initializing the crate, starting the measurement, etc. has 64 KBytes of configurable ring buffer for each channel. For The access to the MicroEnable card is encapsulated by a Java the USCT demonstrator eight slave boards are sufficient due to native interface, which implements the card’s driver in C++. At multiplexing of the transducer systems. Each board connects this experimental stage the recorded data is stored as Matlab via four signal repeaters to 192 input channels of the crate, see structures as Matlab is used for prototypical development of Fig. 4(a) and Fig. 4(b). the reconstruction software. Several 2D and 3D reconstruc- The modular design of the USCT easily allows scaling, i.e. tion algorithms have been implemented so far to reconstruct in a first step adding more slave boards into the crate and in absorption, sound speed or scatter images. Reconstruction of

Fig. 4. System overview of the experimental 3D ultrasound computer tomography demonstrator. 4(a) The container holding 48 transducer systems, which consist of eight emitters and 32 receivers each. The outputs of the transducer systems is connected to four signal repeaters. 4(b) The fast data acquisition hardware with eight digitization boards possessing 24 channels and FPGA pre–processing each, a master board and an additional low–level controller board. 4(c) A high–end PC, which is connected via a fast data transfer card and handles both reconstruction and control of the data acquisition process. absorption and sound speed images requires the detection of or 200 µs (see below, 3D and 2D recording mode). After the transmission peak within the A–scans. For reconstruction each recording of A–scans 200 µs are waited for recovery to of scatter images two algorithms have been implemented both assure that all ultrasound signals within the container have using the analytical continuation of the A–scans [4]. A(t, e, r) been attenuated enough so that they do not interfere with the denotes the analytical continuation of the A–scan between next shot. Every A–scan is recorded n times and afterwards emitter e and receiver r (at location e and r) depending on the averaged to reduce the noise, n being a parameter set during discrete time index t. The first algorithm bases on the envelope initialization of the crate. Averaging is performed during the |A(t, e, r)| of an A–scan. An image E(x) is reconstructed by recovery time. If the memories of the slave boards are used to their full capacity, the recording process is interrupted and the E(x)= |A(τ,e,r)| recorded data is transferred to the PC. After this transfer the e,r recording resumes until the complete address series has been |e − x| + |x − r| processed. Due the required data transfer the recording process τ = is slowed significantly. Therefore we defined two different where c recording modes: assuming a homogenous sound speed c of water at 20◦ Celsius. The second algorithm includes phase information and 1) 3D recording mode: The first mode is used for full 3D thus generally leads to better images. An image P (x) is imaging. The recording duration for each A–scan is limited to reconstructed by 300 µs. Digitization is performed at 12 MHz and 12 bit. This P (x)=| A(τ,e,r)| leads to approximately 7 KBytes memory usage per A–scan, because the 12 bit samples are stored using 16 bit. Eight A– e,r scans will be stored in a channel’s memory before the data is The images of Fig. 1(c) and 2 have been reconstructed with transferred to the PC. Overall measuring of a volume in this the second algorithm. mode will take approximately one hour due to the slow transfer to the PC. C. Operational Sequence To perform a measurement the master board is initialized and 2) 2D recording mode: The second mode is used for fast the PC generates and transmits a series of transducer addresses, 2D cross–section imaging. The recording duration is limited to which will be processed sequentially by the master board. After 200 µs per A–scan, because the traveling paths of the ultrasound the crate is fully initialized and instructed the PC triggers the are shorter regarding only a 2D cross–section. Digitization measuring process. The following steps are then repeated until is performed at 12 MHz and 12 bit. This mode leads to ap- the address series has been fully processed: the current address proximately 4.5 KBytes per A–scan. Only one emitter and one of the series is broadcasted to all transducer systems. The receiver are used per transducer system. In this way 240 A– actual water temperature is read from the controller board. scans (16·15) are recorded filling the memories of the involved After that the waveform samples used for coded excitation channels to the maximum. Thus a complete cross–section can are transferred to the selected emitter at 40 MHz per second. be recorded in a single pass. The data transfer to the PC will At the same time the slave boards begin the recording of take several seconds but imaging an object’s cross–section will the A–scans. The recording of an A–scan takes either 300 µs take approximately 6.4 ms.

0-7803-8701-5/04/$20.00 (C) 2004 IEEE III. RESULTS The finished setup will allow us to perform many new The reconstruction algorithms have been implemented and experiments. We will measure 3D ultrasound phantoms to tested. The control software including a basic graphical user in- find out the technical limits of the design. Iteratively we will terface and hardware driver has been finished. The master, con- improve the transducer design and reconstruction algorithms. trol and slave boards of the crate have been developed/adapted Sharp images depend on accurate localization of the trans- and tested. At the time of writing the container is fitted with ducer positions. Calibration of the setup is crucial. We devel- 16 of 48 transducer systems, which are fully operational. The oped several methods to calibrate the 2D demonstrator, which remaining 32 are partly assembled and will be finished within cannot be applied to the 3D setup directly. For a start the theo- weeks. Therefore the 3D ultrasound computer tomograph will retical positions from the container blueprint will be sufficient. be ready for use very soon. We expect first images of 3D To achieve high–resolution images sophisticated methods have phantoms at the beginning of 2005 and images of living tissue to be developed onto the basis of the 2D approach. to be reconstructed by midyear of 2005. Optimization of the algorithms and native coding may not Data structures for measured A–scans and the geometrical be sufficient to increase the reconstruction process for real- locations of transducers have been developed and implemented. time 3D imaging. Therefore we plan to use GRID–computing Several reconstruction methods are available. First we imple- technologies to achieve a significant acceleration by order of mented a transmission tomography algorithm for fast recon- magnitudes. The necessary middleware, i.e. services, is under struction of cross–sections based on [2]. Second we imple- development. First steps for parallelizing the reconstruction mented algorithms for scatter reconstruction, which we used algorithms and distributing subtasks on several computers have for 2D reconstruction [4]–[6]. We modified the 2D algorithms already been undertaken. to allow not only stacked slices but real 3D reconstruction by The next major step towards our vision of detecting breast overlapping ellipsoids. The functionality of the algorithms was cancer with 3D ultrasound imaging is the examination of tested by reconstructing 3D volumes containing simulated point living tissue. Initially we will image cross–sections of human scatterers. However the reconstruction algorithms are prelimi- volunteers. If a satisfying imaging process is established clinical narily implemented in Matlab for developing reasons without studies on patients are necessary. any optimization. Reconstruction of a volume of 64x64x56 voxels takes therefore about 2 days and has to be accelerated REFERENCES significantly. [1] H. Gemmeke, D. Tcherniakhovski, V. Klinger (1998): A fast and sim- We developed a hardware setup for ultrasound computer ple trigger for high energy cosmics, LEB98, Rom, Sep.21–25 98, CERN/LHCC/98–36, 560–563 tomography, which is capable of handling very high burst data [2] A.C. Kak, and M.G. Slaney: Principles of computerized tomographic rates. By modifying an established hardware the USCT crate imaging, 1999, ISBN:0–89871–494–X is very reliable and cost effective. Imaging 3D volumes at [3] H. Gemmeke, A. Grindler, H. Keim, M. Kleifges, N. Kunka, Z. Szad- kowski, D. Tcherniakhovski (2000): Design of the trigger system for the high resolution will take approximately one hour. However a Auger fluorescence detector, IEEE Transaction on Nuclear Science, Vol. fast cross–section imaging mode has been implemented. Using 47, NO. 2, April 2000 this mode for the first time it will become possible to record [4] R. Stotzka, J. Wurfel,¨ T.O. Muller,¨ and H. Gemmeke: Medical imaging by ultrasound–computertomography, 2002, pages 110–119, SPIE Medical cross–sections of living tissue in real time. The presented Imaging, Volume 4687, Number 25, ISBN: 1605–7422 demonstrator is a major step for making ultrasound computer [5] T.O. Muller,¨ T.M. Deck, R. Stotzka, H. Gemmeke, D. Hopfel,¨ and M. tomography clinically possible. Li: Ultrasound computertomography: image reconstruction using local absorption and sound speed profiles, ESEM European Society for En- gineering and Medicine, 9 2003 IV. DISCUSSION AND FUTURE WORK [6] R. Stotzka, T.O. Muller,¨ K. Schlote–Holubek, and H. Gemmeke: Ultra- sound computertomography for breast cancer diagnosis, ESEM European One major drawback of the 3D demonstrator so far is the Society for Engineering and Medicine, 2003 bottleneck of the crate’s backplane bus. The transfer of recorded [7] R. Stotzka, H. Widmann, T.O. Muller,¨ and K. Schlote–Holubek: Prototype signals between USCT crate and PC takes far too much time. of a new 3D ultrasound computer tomography system: transducer design and data recording, SPIE Medical Imaging, February 2004 Our vision of realtime 3D imaging is only achievable if some kind of data reduction and compression is introduced. The use of the processing power of the built–in FPGAs (at Auger– Experiment 9 · 109 Operations per second) for part of the analysis and the compression algorithms is under development. Data reduction will also cause the reconstruction algorithms to process less data hence decreasing reconstruction time. The reconstruction algorithms itself have to be improved to accel- erate image/volume reconstruction. For clinical acceptance the reconstruction time has to be speed up by several magnitudes. It is planned to use native coding to improve the runtime. Integrated in this step will be a code optimization and redesign of algorithms to reduce complexity.