A. Donath

The High-Density Avalanche Chamber for Positron Emission Tomography

The development of wire chambers for positron emission tomography continues at CERN. In its present form, the basic detector is called a HIgh Density Avalanche Chamber (HIDAC). Owing to the Penning effect, electron avalanche multiplication is obtained in the gamma-ray converter: coincidence time resolution is reduced to 20 ns and stable chamber operation is achieved with a safe, non-polymerizing gas mixture. Spatial resolution remains at 2-3 mm FWHM. A rotating camera consisting of two 20 × 20 cm chambers has now been under evaluation at Geneva Hospital for one year. Multilayer printed-circuit techniques are now used to construct chambers with multiple converters, thus raising detection efficiency from 7.5 to 20%. Read-out electronics and back-projecting memory are being developed to handle the high data rate from these chambers. A four-chamber positron camera designed to achieve 100,000 coincidences s-1 is nearing completion.

In vivo imaging of the human thyroid with a positron camera using 124I

A high-density avalanche chamber (HIDAC) positron camera was used for tomographic imaging of the human thyroid in vivo. Images were made 7 and 24 h after the oral administration of the positron-emitting radionuclide, sodium iodide 124I (with activites varying between 0.3 and 1 mCi), to patients scheduled for either partial thyroidectomy or radioiodine treatment. The results of thyroid imaging performed on 38 patients and their clinical relevance are discussed; as an illustration, three typical cases are presented. In Graves disease, it was found that, whereas standard 131I and 124I scintigrams showed a diffuse goitre, positron images indicated a marked heterogeneity of the activity distribution, with “cold” areas in 8 out of the 11 cases studied. In conventional scintigrams, multinodular goitre showed a non-uniform radioiodine distribution, while positron images revealed considerable regional differences of activity uptake, with hot and cold areas in all of the 13 cases studied. As a consequence of the high spatial resolution of the camera [2.5 mm full width at half maximum (FWHM)], the functional volume of the thyroid may be estimated from 2 mm-thick transverse tomographic sections to within about 13%. This estimate may be compared with the measured volume after partial thyroidectomy, and in a follow-up scan, a further estimate can be made of the residual thyroid tissue remaining within the patients body. In the case of radioiodine treatment in Graves disease and multinodular goitre, the appropriate therapeutic dose of 131I can be calculated according to the functional volume of the thyroid estimated from 124I tomographic images.

A proportional chamber positron camera for medical imaging

Abstract A proportional chamber positron camera based on the principle of the high-density drift space has been constructed for medical imaging. Each of the two chambers has a 1 cm thick converter, 20 × 20 cm 2 in size, with a photon detection efficiency of 8.5% at 0.5 MeV and a time resolution of 100 ns. With a new mode of operation, using neon gas, less than 20 ns has now been achieved. Spatial resolution for a 1 mm line source in plastic is 2 mm fwhmfor 22 Na and 3.5 mm fwhm for 68 Ga. A source of 300 μCi activity, surrounded by a 20 cm diameter plastic scattering bolus, provides a count rate of 4000 coincidences per second, of which 50% are random. This will improve to 20% with the 20 ns time gate. A three-dimensional image comprising 16 slices may be obtained in 20 min. Bone and organ imaging is presented.

Image Reconstruction for a Rotating Positron Tomograph

A high-resolution positron tomograph consisting of two high-density avalanche chambers mounted on a rotating gantry has been installed in the Nuclear Medicine Department of the Cantonal Hospital in Geneva. Positron annihilation data are collected from six detector positions and back-projected in real time to form a single three-dimensional image. Up to 16 transverse or longitudinal sections may be imaged simultaneously. The derivation of the appropriate deconvolution filter is described in this paper, and the first images taken in the transverse mode are presented. It is shown that quantitative information can be obtained from such images.

Tomographic Imaging of the Human Thyroid with a Positron Camera before and after Partial Thyroidectomy

A high-density avalanche chamber positron camera was used for tomographic imaging of the human thyroid before and after partial thyroidectomy. Images were made between 6 and 24 h after oral administration of the positron-emitting radionuclide, Na-124I with activities varying between 0.1 and 0.3 mCi before the surgical intervention and with activities between 0.03 and 0.05 mCi following partial thyroidectomy. The results of thyroid imaging performed on 50 patients and their surgical relevance are discussed; as an illustration, one typical case is presented. As a consequence of the high spatial resolution of the camera (2.5 mm full width at half maximum), the functional volume of the thyroid may be estimated from the transaxial tomographic sections before and following partial thyroidectomy, correct to about 10%. The thyroid surface, defined by the contours from each transaxial section, may be displayed using three-dimensional shaded-graphics techniques. This new imaging technique makes possible a fully three-dimensional description of the thyroid in vivo and contributes significantly to the surgical follow-up.

Cobalt-57 bleomycin scanning for lung cancer detection: a prospective study in thoracic surgery

Patients displaying an abnormal chest X-ray, in some cases, cause a difficult diagnostic problem. A differential diagnosis between benign and malignant lesions is important to determine the choice of treatment i.e. whether or not to perform a thoracotomy. In a prospective study, we have examined the role of 57Co-bleomycin scanning for prethoracotomy assessment of 60 patients with a high clinical probability of lung cancer. For these patients, a sensitivity of 89%, a specificity of 84% and an accuracy of 88% were found. However, as a consequence of the six false-negative scans (two in-situ carcinomas and four stage I carcinomas), bleomycin scanning cannot be regarded as adequate for obviating thoracotomy in patients with a high clinical probability of lung cancer but a negative scan. Nevertheless, the technique is useful for the assessment of tumour size and for the detection of hilar, mediastinal and extra-thoracic metastases, with consequences for TNM staging. It has been found that the tumour dimension correlates well with the actual anatomo-pathologic size determined after surgical examination (r2 = 0.65 and p < 0.01). Therefore, with an accuracy around 90% for the diagnosis of lung cancer, 57Co-bleomycin scanning offers a major tool for use in clinical investigation.

Clinical applications with the HIDAC positron camera

Abstract A high density avalanche chamber (HIDAC) positron camera has been used for positron emission tomographic (PET) imaging in three different human studies, including patients presenting with: (I) thyroid diseases (124 cases); (II) clinically suspected malignant tumours of the pharynx or larynx (ENT) region (23 cases); and (III) clinically suspected primary malignant and metastatic tumours of the liver (9 cases, 19 PET scans). The positron emitting radiopharmaceuticals used for the three studies were Na 124 I (4.2 d half-life) for the thyroid, 55 Co-bleomycin (17.5 h half-life) for the ENT-region and 68 Ga-colloid (68 min half-life) for the liver. Tomographic imaging was performed: (I) 24 h after oral Na 124 I administration to the thyroid patients, (II) 18 h after intraveneous administration of 55 Co-bleomycin to the ENT patients and (III) 20 min following the intraveneous injection of 68 Ga-colloid to the liver tumour patients. Three different imaging protocols were used with the HIDAC positron camera to perform appropriate tomographic imaging in each patient study. Promising results were obtained in all three studies, particularly in tomographic thyroid imaging, where a significant clinical contribution is made possible for diagnosis and therapy planning by the PET technique. In the other two PET studies encouraging results were obtained for the detection and precise localisation of malignant tumour disease including an estimate of the functional liver volume based on the reticulo-endothelial-system (RES) of the liver, obtained in vivo, and the three-dimensional display of liver PET data using shaded graphics techniques. The clinical significance of the overall results obtained in both the ENT and the liver PET study, however, is still uncertain and the respective role of PET as a new imaging modality in these applications is not yet clearly established. To appreciate the clinical impact made by PET in liver and ENT malignant tumour staging needs further investigation, and more detailed data on a larger number of clinical and experimental PET scans will be necessary for definitive evaluation. Nevertheless, the HIDAC positron camera may be used for clinical PET imaging in well-defined patient cases, particularly in situations where both high spatial resolution is desired in the reconstructed image of the examined pathological condition and at the same time “static” PET imaging may be adequate, as is the case in thyroid-, ENT- and liver tomographic imaging using the HIDAC positron camera.

Analysis and Display of Medical Images Obtained with the HIDAC Positron Camera

The role of positron emission tomography (PET) in medical research, particularly in neurology [1] and cardiology [2], is now well-established, whereas its application in routine clinical diagnosis is less obvious. In recent years, major technological achievements include the availability of both low-priced, compact medical cyclotrons and ring-type positron cameras with greatly improved performance. Significant progress in radiopharmaceutical labelling and tracer-kinetic modelling has greatly extended the range of potential applications and the contribution of PET to basic clinical sciences such as neurophysiology.