RC 1313 - Motion management in medical imaging
1. To learn about the origins of motion management in medical imaging.
2. To understand image motion in medical imaging.
3. To learn about solutions and work-arounds.
Motion management in medical imaging is very important for diagnostic and therapeutic application. Its quantification and control can increase image quality in diagnostic imaging and optimize the final results in radiotherapy for delivering ablative doses to tumours with limited normal tissue toxicity. The methods for quantification and control are modality and clinical application dependent. Respiratory induces motion artefacts, particularly those occurring in the abdomen and lung. It poses a hefty problem in diagnostic imaging and cardiac motion leads to blurred images with reduction of spatial and contrast resolution. In diagnostic application many methods can be applied in MRI, US, CT, CBCT, SPECT, PET where different image modalities should be registered in the same reference coordinates and a lot of methods have to be applied for different clinical applications. A modern radiotherapy tenet is to accurately identify the treatment target to follow and secure high-dose treatment target volume. Respiratory motion can induce errors in target volume delineation and dose delivery in radiation therapy for thoracic and abdominal cancers. 4D computed tomography and 4D magnetic resonance imaging are the most common contemporary imaging technique for identifying tumour motion. Radiotherapy application is not directly discussed during the section but imaging corrected for motion will support also this application.
A. Managing respiratory motion with CT and CBCT: conventional approaches and motion compensating techniques
1. To learn about conventional techniques for respiratory motion compensation.
2. To learn about new methods for respiratory motion compensation.
3. To contrast the available and up-and-coming cardiac motion compensation methods.
Motion in X-ray imaging is a common problem. It generally leads to blurred images and loss of spatial resolution. In this talk, various strategies for motion compensation will be discussed, on acquisition as well as tomographic image reconstruction level. In addition, it will be shown that motion blur in the acquired x-ray projections can be beneficial instead of disadvantageous with respect to CT image quality.
B. Managing cardiac motion with CT and CBCT: conventional approaches and motion compensating techniques
1. To learn about conventional techniques for cardiac motion compensation.
2. To learn about new methods for cardiac motion compensation.
3. To contrast the available and up-and-coming respiratory motion compensation methods.
Diagnostic CT is the workhorse of the radiologist: highly quantitative images with very high spatial resolutions (0.3 mm isotropic) for complete anatomical areas are acquired in scan times as short as about 1 s. CT further routinely achieves an unprecedented temporal resolution of 63 ms, which is one quarter of the rotation time, for complete anatomical regions. For moving objects, however, even 63 ms may introduce motion blurring. Given the spatial resolution of 0.3 mm, objects moving at a speed of about 4 mm/s or faster will introduce blurring. Such velocities occur in the thorax due to breathing and, even more dominantly, in the heart, where velocities of 70 mm/s are typical. In CBCT, a slowly scanning non-diagnostic modality, the situation becomes even worse because not only the temporal duration of the scan is long but also because respiratory motion cannot be ignored anymore and will be superimposed with the cardiac motion. The simplest methods to manage cardiac motion are cardiac-correlated (prospectively or retrospectively gated) scans performed during a single breath-hold. Gating may include scanning during a single heart beat (either in circle, in spiral, or in high-pitch spiral mode) or during multiple heart beats. Recently, new algorithms that have the ability to compensate for the motion during image reconstruction are being developed: they have the potential to significantly increase the temporal resolution or to improve the dose usage to 100%. The lecture discusses the methods in use, and those that are being developed.
1. To learn about motion measurement and compensation in MR.
2. To understand how patient motion affects PET images.
3. To learn about how MR data can be used to motion-compensate PET images, in PET/MR.
MR image quality and diagnostic accuracy can be considerably impaired by physiological organ motion (e.g. breathing, heartbeat or swallowing). Advances in MR hardware, plus novel image acquisition and reconstruction approaches, now allow for faster MR imaging; nevertheless, up to 20% of MR scans experience severe motion artefacts and require repeated data acquisition. A wide range of techniques to minimise motion artefacts has been developed for different MR applications. In clinical practice, this is commonly motion prevention (e.g. asking a patient to hold their breath) and motion gating (i.e. restricting data acquisition to a certain motion state). More advanced approaches measure displacement of organs due to physiological motion and utilise this information to correct motion artefacts. PET images are affected by motion twofold. Movement of organs can lead to blurring of the imaged structures and impair detectability, particularly of small features. In addition, quantitative PET requires correction for different tissue densities using so-called attenuation correction maps. Motion can lead to a mismatch between these and PET image data, resulting in severe errors in PET quantification. The recent introduction of simultaneous PET-MR now offers us the possibility to improve PET image quality and accuracy by utilising MR-based motion information.