Computed Tomography of Bone Tumours
• Computed tomography (CT) is a high-radiation-dose examination, which should therefore be both justified and tailored to the clinical need.
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Contents
K ey P oi n t s 2.1
Introduction 17
2.2
Developments in Computed Tomography 18 CT Technology 18 Dual-Energy Scanning 19 Single Gantry Rotation Volume Scanning 19 CT “Fluoroscopy” 19 Data Manipulation 19
2.2.1 2.2.2 2.2.3 2.2.4 2.2.5
2.3 Scan Image Quality 20 2.3.1 Internal Metalwork from Fixation Devices 21 2.3.2 CT Number, Hounsfield Units, Window Width and Levels 22 2.3.3 Radiation Dose Reduction 23 2.4 2.4.1 2.4.2
CT of Bone Tumours 24 Anatomy 24 Patient Positioning 24
2.5 2.5.1 2.5.2
Indications 24 Thoracic Staging 25 Whole-Body CT Scanning 26
2.6
CT-Guided Interventions 26
2.7
Conclusion 28
•• •• •• •• •• ••
Computed tomography (CT) is a high-radiation-dose examination, which should therefore be both justified and tailored to the clinical need. CT of solitary bone lesions may provide information on tumour mineralization difficult to identify on plain film or MR. Non-contrast-enhanced CT of the thorax is appropriate for staging of metastatic bone sarcoma. Whole-body CT in older patients should be considered where the “index” bone lesion may be a metastasis. CT with CT fluoroscopy is ideal for guiding bone biopsy and interventional procedures. Steps to minimize radiation dose are important for both the patient and operator. Ingenuity in patient positioning can produce high-quality scans of limb lesions (by removing unnecessary parts of the patient from the scan plane).
References 28
2.1
Introduction
R. W. Whitehouse, MD Department of Clinical Radiology, Manchester Royal Infirmary, Oxford Road, Manchester, M13 9WL, UK
Although magnetic resonance (MR) imaging has become the primary imaging modality for local staging of bone tumours, computed tomography (CT) has complementary roles in the diagnosis and local staging of bone tumours, as well as operation planning, custom prosthesis production, biopsy and percutaneous treatment guidance. Scanning the chest for detection of pulmonary metastases is also a primary role for CT. Computed tomography remains essential for the assessment of patients in whom MR is contraindicated (e. g.
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18
R. W. Whitehouse
due to intracranial aneurysm clips or cardiac pacemakers). By comparison with histological measurement of metastatic tumour size in resected spinal lesions, CT underestimates size, whereas MR overestimates it (Fujita et al. 2000), an observation probably also true for other tumours and in other locations.
2.2
Developments in Computed Tomography A CT image is a map of normalized X-ray attenuation coefficients, generated by computer, from filtered back projection of X-ray transmission measurements in multiple directions through the object in question. Each pixel in the image represents the averaged attenuation of the material that occupies the corresponding voxel in the subject. Recent developments in CT include helical scanning, multislice acquisition, single rotation volume scanning, simultaneous dual-energy scanning and real-time CT “fluoroscopy” (Dawson and Lees 2001). The number of slices acquired in a
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