A high precision, high speed X-ray detector for the noninvasive coronary angiography with synchrotron radiation

NUCLEAR INSTRIJUENTS & MBTHOOS

Nuclear Instruments and Methods in Physics Research A3 I11(It)t~l) 44f~-448 North-tloUtmd

IN PHYSICS RESEARCH

A high precision, high speed X-ray detector for the noninvasive coronary angiography with synchrotron radiation * H.J. Besch, E.J. Bode, R.H. Menk, H.W. Schenk, U. Tafelmeier, A.H. Walenta and H.Z. Xu U~lit'vrsily Of Siegt'n, Germany

A pt~sition sensitive I-dimensional X-ray deteclor with high dynamic range ( > 45(141:)) for a high photon flux with fast image recording sequence (in principle up to I Mtlz) has been dcveloped for medical applications. A detective quantum efficicney (DOE) of at least 311% and a position resolution of 4311 ~.m (FWIIM) could be achieved for 33 kcV phottms.

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I. Introduction Noninvasivc coronary angiography [1] requires a line detector scanning the object (heart) in at least 256 lines with a time resolution of I ms per line. At the same time thc precision of the intensity measurement in each pixel must be precise enough (a few 10 -a) to extract the weak signal after subtraction of two pictures taken simultaneously at slightly different energies.

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2. Detector and readout electronics A gaseous dctcctor based on thc principle of an ionization chamber has been developed for this application. An aluminium pressure vessel contains two line detectors (fig. 1) which are separated by a common drift electrode. Each line detector consists of 256 readout strips with a cell width of 400 ttm and a length of 5.5 cm connected to charge sensitive preamplifiers. At a distance of 800 txm Frisch grids above the strip planes provide fast signals. A xenon-CO~ mixture (80: 20) at a pressure of 10 bar is used as activc medium. Preamplifiers and shaping amplifiers are built in ceramic hybrid technology to achievc a high packing density. The bunch structure of the synchrotron radiation / I .~ offers the possibility of "pulsc integration" 'scc fig. ~' The photons of each bunch generate frcc electrons by photoionization in the gas. Due to the electric ficld electrons drift towards the readout electrode. Fig. 3 shows the readout system. The induced chargc on the readout strips is collected by charge sensitive

* Work sup~rled by the Deutsche Forschungsgemeinschaft.

Fig. I. Setup of the detector.

preamplifiers. The output signal of the preamplifiers is formcd and amplified by shaping amplifiers. Becausc of the AC-coupling in thc electronic chain, "baseline" and "maximum" have to be measured by FADCs scpararely. The difference of maximum and baseline corresponds to the number of converted photons. Parallel to serial converters (PSC) are used to add the FADC contents to sealers. With a clock frcquency of 70 MHz the readout of a 6-bit FADC roughly takes l p.s. For

I1168-9002/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved

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each image line two measurements havc been performed successively: one line containing the information of the baseline (t~,,,.,,~. = 0.25 ms) and a second where the maximum is digitized (/maximum= | ms). After the total time for each line the scaler contents is transferred to the imagc memory.. Ideally a pulse by pulse subtraction mcthod should be used for optimum

results. Unfl~rtunatcly subtraction circuits were not available at the time of measurement, and therefl)rc the results contain noncorrectable beam fluctuations as will be discussed bclow. The de~ribcd procedure is done in parallel for each channel and repeated for 2 x 256 lines. At the end the image mcmory delivers a "'baseline image" and a "'maximum image" for each photon energy via CAMAC to a personal computcr.

3. Results

In May lqt)O the imaging system was tested at the synchrotron of HASYLAB at DESY. Hamburg. A spaTRIGGER

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Fig. 6• Radiography of the heart of a pig. tial resolution of 430 p,m FWHM was measured (fig, 4). Further analysis showed the following contributions: photoelectron range: strip width: charge diffusion:

o"r = 130 Ixm, o-.~,, 115 p.m, cO = 61 p.m.

Capacitive coupling between strips is negligible because of the high dynamic input capacitance of the preamplifiers of 500 pF. In fig. 5 the logarithm of the output signal is shown as a function of the absorber thickness. The maximum beam intensity resulted in a value corresponding to bin 45 of the FADC, which translates to 900 photons per p,s and readout strip. Within the error bars ( < 10 -'a) no deviation from linearity could bc noticed. At low rates (high thickness of the absorber) the nonlinear behaviour is due to the fraction of the third harmonics (99 keV). A fit yields a content of (1 + 0.5)% in the beam. The analysis of fluctuations in a structureless part of a "maximum image" results in a D Q E of 30% that has to be considered as a lower limit because of not completely corrected local beam fluctuations. The dynamic range found in this measurement was limited by the beam intensity. For the full beam a value of 6400: 1 will be achieved. Fig. 6 shows a subtraction image of a pig's heart where coronary arteries arc filled with iodine at the concentration of 40 m g / m l . The smallest vessels visible have a diameter of less than 500 p.m. Every pixel of the two original images contains about 270000 photons. Quantitativc analysis of the upper artery with a diamcter of 2 mm yields a value SNR = 30; this measured value is lower than expected with a DQE of 30%. which is duc to the additional noise of

the "baseline image" which has to be subtracted from

the "maximum image".

4. Conclusion

It could be shown that a Xc-filled ion chamber is an adequate detector for noninvasive coronary angiographic application. The advantage of a pulse structure of the machine can be used to build a very stable "'pulsc integrator"•

Acknowledgement

We would like to thank the members of the HASYLAB -~ngiography group [2] for their help with the mea~,urements.

References

[I] E. Rubinstein. E.B, Hughes, L.E. Campbell, R. Hofstadter. R.L. Kirk, T,J. Krolicki, J.P. Stone, S. Wilson, ll.D. Zeman, W.R. Brody, A. Macovski and A.C. Thompson, SPIE 314 (1981) 42: W.R. Dix, W. ,ol,c,." ,,,,u ..... W. Kuppcrs, Nichtinvasivc Koronarangiographie mit Synchrotronstrahlung, Physikalische Bl~itter 44 (5) (1988). [2] W.R. Dix, K. Engelke, J. Heuer, W. Graeff, W. Kupper, M. Lohmann, T. M6chel and R. Reumann, HASYLAB, DESY and UKE. Hamburg. Germany.