Bernd O. Knoop

Recovery correction for quantitation in emission tomography: a feasibility study.

Abstract.In emission tomography, the spread of regional tracer uptake to surrounding areas caused by limited spatial resolution of the tomograph must be taken into account when quantitating activity concentrations in vivo. Assuming linearity and stationarity, the relationship between imaged activity concentration and true activity concentration is only dependent on the geometric relationship between the limited spatial resolution of the tomograph in all three dimensions and the three-dimensional size and shape of the object. In particular it is independent of the type of object studied. This concept is characterized by the term ”recovery coefficient”. Recovery effects can be corrected for by recovery coefficients determined in a calibration measurement for lesions of simple geometrical shape. This method works on anatomical structures that can be approximated to simple geometrical objects. The aim of this study was to investigate whether recovery correction of appropriate structures is feasible in a clinical setting. Measurements were done on a positron emission tomography (PET) scanner in the 2D and 3D acquisition mode and on an analogue and digital single-photon emission tomography (SPET) system using commercially available software for image reconstruction and correction of absorption and scatter effects. The results of hot spot and cold spot phantom measurements were compared to validate the assumed conditions of linearity and stationarity. It can be concluded that a recovery correction is feasible for PET scanners down to lesions measuring about 1.5×FWHM in size, whereas with simple correction schemes, which are widely available, an object-independent recovery correction for SPET cannot be performed. This result can be attributed to imperfections in the commercially available methods for attenuation and scatter correction in SPET, which are only approximate.

Use of recovery coefficients as a test of system linearity of response in positron emission tomography (PET).

Quantitative determination of activity concentration has always been regarded as a goal of PET scanner design. Therefore, a linear response with respect to activity concentration is essential. To prove system linearity of response a test is proposed based on the determination of recovery coefficients (RC) for different object geometries. If all degrading effects, especially scatter and attenuation, are corrected to a sufficient degree of accuracy, the system behaves linearly resulting in a theoretical relationship between hot (HSRC) and cold spot recovery coefficients (CSRC). Therefore this relationship, applied to appropriate phantom data containing hot or cold spheres, combined with the constraints on asymptotic behaviour of RC can be used as a test of system linearity of response. After applying the systems standard corrections RC were determined using both 2D and 3D data acquisition modes. For both 2D and 3D data, HSRC directly measured showed an excellent agreement with the HSRC calculated from the CSRC. The analysis was extended to radionuclides of different positron ranges, to the effect of data smoothing, and to that of iterative reconstruction. The agreement between the RC determined under varying border conditions demonstrates the effectiveness of the corrections used in constituting system linearity. Therefore, the method proposed is used as a test of system linearity of response.

Physical aspects of scintigraphy-based dosimetry for nuclear medicine therapy.

In nuclear medicine therapy the treatment of tumours by radiation exposure from internally deposited labelled antibodies or labelled peptides is currently an active field of investigation. To permit the efficient delivery of high amounts of radiation dose to tumours while limiting the radiation dose to critical organs dosimetry calculations have to be performed. These are relying on scintigraphic data being input to the well known MIRD formalism. This paper focuses on the methods and the difficulties associated with the scintigraphic determination of organ kinetics. The physical properties of the well-known scintigraphic imaging modalities, PET, SPECT and planar scintigraphy, are discussed thereby taking into account the properties of the appropriate radionuclides currently being available for therapy and dosimetry. Several arguments are given and disputed for the limited clinical use of PET and SPECT in dosimetry and the ongoing preference of planar whole-body imaging as the method of choice. The quantitative restrictions still inherent to this method are also discussed in detail. Procedural recommendations are proposed covering all processes related to data acquisition, data correction and data analysis which finally lead to reliable estimations of organ dose.

Calibration Test of PET Scanners in a Multi-Centre Clinical Trial on Breast Cancer Therapy Monitoring Using 18F-FLT

A multi-centre trial using PET requires the analysis of images acquired on different systems We designed a multi-centre trial to estimate the value of 18F-FLT-PET to predict response to neoadjuvant chemotherapy in patients with newly diagnosed breast cancer. A calibration check of each PET-CT and of its peripheral devices was performed to evaluate the reliability of the results. Material and Methods 11 centres were investigated. Dose calibrators were assessed by repeated measurements of a 68Ge certified source. The differences between the clocks associated with the dose calibrators and inherent to the PET systems were registered. The calibration of PET-CT was assessed with an homogeneous cylindrical phantom by comparing the activities per unit of volume calculated from the dose calibrator measurements with that measured on 15 Regions of Interest (ROIs) drawn on 15 consecutive slices of reconstructed filtered back-projection (FBP) images. Both repeatability of activity concentration based upon the 15 ROIs (ANOVA-test) and its accuracy were evaluated. Results There was no significant difference for dose calibrator measurements (median of difference −0.04%; min = −4.65%; max = +5.63%). Mismatches between the clocks were less than 2 min in all sites and thus did not require any correction, regarding the half life of 18F. For all the PET systems, ANOVA revealed no significant difference between the activity concentrations estimated from the 15 ROIs (median of difference −0.69%; min = −9.97%; max = +9.60%). Conclusion No major difference between the 11 centres with respect to calibration and cross-calibration was observed. The reliability of our 18F-FLT multi-centre clinical trial was therefore confirmed from the physical point of view. This type of procedure may be useful for any clinical trial involving different PET systems.

Radioembolisation mit 90Y-markierten Mikrosphären: Posttherapeutische Therapievalidierung mit Bremsstrahlungs-SPECT

During the last years angiographic Selective Internal Radiotherapy (SIRT) with (90)Y-labelled microspheres has become a common technique for the local-ablative treatment of cancer patients. SIRT is a palliative therapy concept for the treatment of liver malignancies. As a result of (90)Y-decay as β(-)-emitter without a concomitant gamma radiation, Bremsstrahlung imaging is needed to validate the distribution achieved by radioembolisation. This article demonstrates the method of imaging through phantom measurement and shows the advantages of post-therapeutic tomography by means of a patient study. Approaches for further optimization of Bremsstrahlung imaging are discussed.

Estimation of glomerular filtration rate in liver‐transplanted children:Comparison of simplified procedures using 51Cr‐EDTA and endogenous markers with Sapirstein’s method as a reference standard

Berding G, Geisler S, Melter M, Marquardt P, Lühr A, Scheller F, Knoop BO, Pfister E‐D, Pape L, Bischoff L, Knapp WH, Ehrich JHH. Estimation of glomerular filtration rate in liver‐transplanted children: Comparison of simplified procedures using 51Cr‐EDTA and endogenous markers with Sapirstein’s method as a reference standard.
Pediatr Transplantation 2010: 14:786–795.

Evaluation of PET count rate performance

A proposal is made for the test conditions to evaluate PET count rate performance. This performance depends in a complex manner on the spatial distribution of activity and scattering material. Therefore, a combined body phantom is proposed, which is as simple as possible but which adequately simulates the range of clinical application of a whole body tomograph. Taking into account the special properties of the new block detector design, a comprehensive test procedure is developed. This includes not only the common count rate characteristic, but also checks for the accuracy of randoms estimation and count loss correction schemes, and for the occurence of pulse pile up. This is done for different source and scatter configurations, simulating brain, cardiac, and abdominal imaging, respectively. Examples are given, based on measurements of the latest generation PET scanners, namely the CTI PT 931/08-12 and the Scanditronix PC 2048-07WB.

Äußere Strahlenexposition und effektive Halbwertszeit bei Therapie mit Lu-177-Dota-Tate

The aim of the study was to estimate the external radiation exposure emitted by the patient to his surroundings after discharge. Being in compliance with legal requirements is especially important when doing multiple therapies. To estimate the effective half-life to be used quite realistically, the individual effective half-lives for 41 patients with 52 therapies were calculated. From the resulting histogram the maximum value was determined to be 100 h. Substituting the physical half-life by this maximum effective half-life results in dose estimates, which are lower but still conservative. In addition, the analysis of dose related parameters for patients who underwent multiple therapies demonstrates that the parameters estimated for the first therapy cannot be transferred to the subsequent ones.

Überprüfung der Kreuzkalibrierung von Positronen-Emissions-Tomographen zu ihren peripheren Geräten

The aim of this study was to evaluate the cross-calibration of positron emission tomographs and their peripheral devices prior to a multicenter study. The instrumentation of 22 sites (including 24 dedicated PET scanners), including 21 that could be operated in 2D mode and 20 in 3D mode, was investigated according to standardized acquisition and reconstruction protocols, and the cross-calibration was checked against the dose calibrators (22 instruments) and the sample changer (20 instruments). The deviation of the cross-calibration to the dose calibrators was below 5% for 10 of 21 PET scanners (2D mode) and 6 of 20 (3D mode). For 6 (2D mode) and 6 (3D mode) scanners, the corresponding error was up to 10%. In turn, the deviation between dose calibrator and standard was less than 8% for 19 devices, and even less than 5% for 14 devices. In most of the well counters evaluated, the cross-calibration error was less than 10%. Although required in general, the cross-calibration between positron emission tomographs and their peripheral devices becomes even more critical when pooling data in the framework of quantitative clinical multicenter studies.

Qualitätskontrolle an PET/CT-Systemen: Erfahrungen und Erfordernisse

Today, in most cases PET examinations are performed using PET/CT hybrid systems. While acceptance testing and routine control of the basic modalities PET and CT, respectively, are described by appropriate regulations, corresponding instructions with regard to the interface connecting both are still missing. This interface includes the adjustment of gantries and patient bed to each other as well as the energy scaling of attenuation coefficients from CT energy to 511 keV. Measurements checking the mechanical adjustment (determination of off-set parameters) are performed following manufacturers recommendation, with a typical frequency twice a year. On a Biograph 16 (Siemens, Inc.), these measurements were systematically extended to a weekly frequency over an observation period of 10 months, supplemented by measurements with additional load to the patient bed (up to 135 kg), and different vertical bed positions. The results show, that for the construction tested additional off-set measurements for routine control extending well beyond manufacturers recommendation are not necessary. The energy scaling of attenuation coefficients is depending on methodological aspects and software implementation, and therefore is not part of routine control. On the contrary, the development of appropriate methods for acceptance testing to assess and to determine the mechanical adjustment in all its degrees of freedom as well as the accuracy of attenuation corrected emission data is urgently needed.