Spiral scanning

The system shown in Fig. 26.47 has a vertical z movement of 1(W mm to facilitate specimen mounting and spiral scanning and a lateral x movement of —40 mm for specimen alignment. It has... 

Parameter that characterizes a spiral scan, defined as table feed per rotation divided by the total width of the collimated beam. In the early days of four-section CT, the term detector pitch had been additionally introduced, which accounts for the width of a single section in the denominator. In this way two different pitch can be defined relative to total X-ray collimation (pitchy) or to individual detector width (pitchj). Consequently pitch =pitsingle-section CT and multi-detec-tor row CT the pitch (p) is given by p=TF/w, where TF is table feed per rotation, and w is total width of the collimated beam, according to International Electrotechnical Commission specifications. For pdata acquisition occurs with overlap in the z-axis direction for p>l, data acquisition occurs with gaps. 

For ECG-synchronized examinations of the cardio-thoracic anatomy, either ECG-triggered axial scanning or ECG-gated spiral scanning can be used. A technical overview on ECG-controlled CT scanning can be found in Flohr et al. (2003b). 

With retrospective ECG gating, the heart volume is covered continuously by a spiral scan. The patients ECG signal is recorded simultaneously to data acqui-... 

Fig. 1.15. Principle of retrospectively ECG-gated spiral scanning with single-segment reconstruction. The patient s ECG signal is indicated as a function of time on the horizontal axis, and the position of the detector slices relative to the patient is shown on the vertical axis (in this example for a four-slice CT system). The table moves continuously, and continuous spiral scan data of the heart volrnne are acquired. Only scan data acquired in a pre-defined cardiac phase, usually the diastolic phase, are used for image reconstruction (indicated as red boxes). The spiral interpolation is illustrated for some representative projection angles...

Fig. 1.20. Since the introduction of multislice detectors, it is a known issue that at the start and the end of each spiral scan, a region is irradiated for which no images can be reconstructed (red). That portion depends on the width of the detector and becomes more severe the wider the detector becomes and the shorter the scan region is. That problem can be overcome, but introducing a tube side collimator that continuously opens at the start of the scan and closes at the end of the scan. The Siemens SOMATOM Definition AS-i- is the first scanner offering that technology, which saves 10-25 % dose depending on the application. Typical dose savings using this technology are 10% for abdominal, 15% for thorax, 20% for head and 25% for cardiac examinations...

Various adult effective doses for CT scans have been reported in the literature. For example, Becker et al. (1998) obtained an effective dose of 6 mSv for a spiral scan of the abdomen. In a comprehensive review, McCollough and Schueler (2000) provide an effective dose between 1.9 mSv and 2.6 mSv for a (male) CT head scan and 7.3 mSvto 7.8 mSv for a (male) CT abdo-... 

Strategies for Reducing Dose Exposition in Cardiac CT 77 Scan Coverage 77 ECG-Controlled Tube Current Modulation 77 Low-Dose Protocol and Adaptation to Patient Morphology 77 Non-Spiral Scanning and Prospective Gating 78 Adaptive Pitch 79... 

If a purely intracranial CT is to be performed, it should be kept in mind that most of the latest MDCT-systems do not allow the gantry to be tilted for spiral scanning. Therefore, the only option to keep the radiation-sensitive eye-lenses out of the radiation beam is by correcting the patient positioning (Fig. 10.1). 

With modern MDCT scanners, the indications for sequential CT scanning—as opposed to spiral scanning—are usually limited to dynamic protocols such as perfusion CT (PCT) and to sequential scans of the infratentorial brain or skull base. Sequential scanning can reduce beam hardening artefacts and overbeaming however, 3D-refomation options are limited. 

A 3D model of tibia was created with the real geometry of one human male and mid-sized left leg using spiral scan images. It was materialized by MIMIX software (version 10.01), then the model was developed in ABAQUS software (version 6-7.1) for the purpose of modeling of some other in-vivo conditions (see Fig. 1). 

Fig. 1 The 3D model of the human left tibia used in this research with the exact geometry of real bone using spiral scan images and MIMIX software...

Liang Y, Kruger RA (1996) Dual-shce spiral versus single-slice spiral scanning comparison of the physical performance of two computed tomography scanners. Med Phj 23 205-220... 

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