Technical Aspects of Multidetector Computed Tomography
Paul Schoenhagen, Frank Dong in Cardiac CT Made Easy, 2023
Currently available contrast media are quickly diluted in the blood and distributed into the extracellular space, providing only a short time window for enhanced imaging. The transit time from the standard injection site (antecubital vein) to the heart is patient-dependent, and can vary between 20 and 40 s depending on the contrast flow rate and the patient's cardiac output. Therefore, determination of the scan delay (time between start of contrast agent injection and start of the scan) is critical to ensure optimal enhancement of the desired cardiovascular structures. This can be achieved with a small ‘timing bolus' or by monitoring the diagnostic bolus (‘bolus tracking’). With the use of a timing bolus, approximately 20 mL of contrast agent is injected, and a single image slice (typically at a level ∼2 cm below the carina for imaging of the coronaries) is repeatedly imaged. The transit time of contrast agent to the region of interest (ROI) is determined from a time enhancement curve (Figure 3.6) and used for timing of the diagnostic bolus (Figure 3.7). Alternatively, with bolus tracking techniques, the entire diagnostic bolus is injected, and contrast enhancement is monitored in the ROI by repeated imaging at a single level. Once a certain enhancement threshold is achieved, breath-hold instructions are given, and scanning is started.
Cardiovascular system
A Stewart Whitley, Jan Dodgeon, Angela Meadows, Jane Cullingworth, Ken Holmes, Marcus Jackson, Graham Hoadley, Randeep Kumar Kulshrestha in Clark’s Procedures in Diagnostic Imaging: A System-Based Approach, 2020
The bolus chase technique involves performing baseline, non-contrast, overlapping X-rays down the legs at pre-set positions for the mask images. A single, large volume contrast injection (100 ml at 6–8 ml/s) is then injected and angiograms are performed with the table moving sequentially to the pre-set positions. The table movement to the next pre-set position is triggered by the radiographer in order to ‘chase’ the contrast bolus down the leg. The advantage of bolus tracking is that it is potentially quicker and uses a smaller volume of contrast than the stepping table technique. In reality however, additional angiograms are often required if there is differential passage of contrast down each leg or if there has been movement of the limb between the single mask image and subsequent contrast images. Thus, in most departments the stepping table technique is preferred.
Cardiovascular PET-CT
Yi-Hwa Liu, Albert J. Sinusas in Hybrid Imaging in Cardiovascular Medicine, 2017
Test bolus vs. bolus tracking: Accurate timing of the scan to the arrival of the IV contrast to the structures of interest is crucial. Vascular enhancement has to be maintained for the duration of the scan. Due to the short duration of the CCTA scans, timing errors of 5–10 s can make a crucial difference in image quality. As a general rule, the scan delay should equal the contrast travel time from the injection vein to the ascending aorta, plus the addition of 2–3 s. Three strategies can be utilized to determine the vein-to-aorta time (called the delay time). (1) Easiest in implementation but with the least reliability is the 22–25 second best-guess rule. (2) Bolus tracking is preferred where there is automatic scan triggering; a region of interest (typically the ascending or descending aorta) is sampled every 2 s after the initiation of the contrast bolus infusion. Exceeding a preset imaging value (typically 100 HU) will start the diagnostic scan. (3) Test bolus is where a small bolus injection (typically 10–20 mL of contrast followed by saline push of 50 mL, both at a rate of 4–7 mL/s) is given with end-inspiration breath-holding; sampling at the level of the ascending aorta every 1–2 s, the delay time can be accurately determined. The benefits of this strategy include lower risk of false-starts or delays, identifying contrast dilution, IV line patency, and assessment of patient breath-holding ability prior to the actual diagnostic scan.
Optimization of radiation settings for angiography using 3D fluoroscopy for imaging of intracranial aneurysms
Published in Computer Assisted Surgery, 2021
Thomas Linsenmann, Alexander März, Vera Dufner, Christian Stetter, Judith Weiland, Thomas Westermaier
It should be emphasized that radiation settings do not alone determine the quality of vascular imaging. The flooding of the contrast agent is likely to play an equally important For CTA, this was extensively examined in the past. Maintaining stable blood pressure is certainly a prerequisite for good-quality contrast imaging but not in a proportional function [16]. To date, there is still no clear algorithm for the timing of contrast agents in CTA. For that reason, manufacturers added bolus tracking in the field of CTA [17]. In 3D-rotational fluoroscopy, this issue may be even more challenging because the arteriogram is – at least in peripheral vessels – hard to distinguish from the venogram. The latter, however, is not useful for the purpose of imaging cerebral imaging and may worsen image quality.
Toward PET/MRI as one-stop shop for radiotherapy planning in cervical cancer patients
Published in Acta Oncologica, 2021
Sahar Ahangari, Naja Liv Hansen, Anders Beck Olin, Trine Jakobi Nøttrup, Heidi Ryssel, Anne Kiil Berthelsen, Johan Löfgren, Annika Loft, Ivan Richter Vogelius, Tine Schnack, Bjoern Jakoby, Andreas Kjaer, Flemming Littrup Andersen, Barbara Malene Fischer, Adam Espe Hansen
The accuracy of the generated sCT relies on an ideal alignment between CT and MRI in training data. Given the different positioning between MRI and CT scans for pilot patients and GC patients, some residual registration error remains. In both PET/CT and PET/MRI scans, no positioning equipment has been used for GC patients, while for pilot patients, only PET/MRI was acquired in RT position. Another limitation of the present study is that the network was trained with both contrast and non-contrast CTs, which may cause errors in the predicted sCT map. However, another study [47] evaluated the effect of IV contrast on dose calculation and confirmed that the difference between plans based on contrast and non-contrast CT is clinically insignificant with a relative mean dose of 2%. The accuracy of the sCT is expected to improve with a larger dataset for training. However, it should be noted that it is an inherent property of deep learning-based methods that they can occasionally fail unpredictably in very few patients, despite performing well in the vast majority. We would therefore strongly recommend an independent verification of the sCT algorithm before clinical implementation. Finally, after inclusion and analysis of the 8 patients reported here, a subsequent patient was observed not to fit PET/MR coil holder. The patient had a weight of 137 kg. Hence, the requirements of the RT equipment combined the 60 cm bore size of current integrated PET/MRI systems can be a limitation for some patients.
The effect of hemodynamic parameters in patient-based coronary artery models with serial stenoses: normal and hypertension cases
Published in Computer Methods in Biomechanics and Biomedical Engineering, 2020
K. E. Hoque, M. Ferdows, S. Sawall, E. E. Tzirtzilakis
Coronary CT angiography was performed on a dual-source CT (DSCT) scanner (SOMATOM Force, Siemens Healthineers, Germany). The scanning parameters were as follows: (i) tube voltage 100 kV, (ii) tube current 450 mAs. Detector collimation 0.75 × 192 × 0.6 mm. Time per gantry revolution 0.25 ms resulting in a temporal resolution of 66 ms. The CTA was performed using prospective ECG-gating. A bolus tracking technique was used for CTA scans and the triggering threshold was set to a CT-value of 100–140 HU in the ascending aorta. The scan was obtained with intravenous injection of 40–60 ml IOHEXOL (350 mg I/mL, IOPAMEDOL 350) at a flow rate of 4–5 ml/s followed by 30 ml saline chaser at the same flow rate. The CTA scan was acquired from 2 cm below the level of the tracheal bifurcation to 1–2 cm below the level of the diaphragm. Image data were routinely automatically reconstructed in best diastolic and best systolic position in the R-R interval with a slice thickness of 0.75 mm, slice increment of 0.75 mm and a medium to smooth convolution kernel B26f. The Field of view (FOV) was 170 mm with a matrix size 256 × 256.
Related Knowledge Centers
- Aorta
- Cerebral Arteries
- Pulmonary Artery
- Cannula
- CT Scan
- Radiocontrast Agent
- Iodinated Contrast
- Bolus
- Injection
- Region of Interest