Cone beam computed tomography

Cone beam vs. multidetector CT: Experts discuss pros and cons

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Radiology practitioners have highlighted the benefits of cone beam CT in delivering high resolution at a low dose. Delegates at ECR 2023 in Vienna heard how cone beam CT (CBCT) could replace multidetector CT (MDCT) in some areas and is already showing cost-effectiveness benefits.

Report: Mark Nicholls

The aim of the session, entitled “High resolution at low dose: where and why cone-beam CT will replace multidetector CT”, aimed to offer an understanding of the technical principles, advantages and limitations of CBCT and where it outperforms MDCT. Current clinical applications were presented with a specific focus on the utility of CBCT in daily practice.

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Expanding applications

portrait of Mika Kortesniemi

Medical physicist Mika Kortesniemi outlined the technical principles, dose and artefacts of CBCT, looking at advantages and disadvantages. ‘There are many applications and they are expanding in certain clinical indications,’ he said. ‘That includes dental and ear, nose and throat (ENT) radiology, and also for radiotherapy on-board imaging for verification of treatments.’ 

The technique is also seeing applications in interventional radiology, rotational angiography, and physical extremities in musculoskeletal (MSK) imaging, with the emerging value of the 3D beam becoming clearer. ‘CBCT can also be used in various orientations,’ Kortesniemi continued. ‘We can do weight bearing imaging for extremities, bringing added value on how the tissue structures behave under pressure of the gravitational force.’

Meaningful data and other benefits

CBCT technology is smaller and easier to install in a clinical setting with associated cost benefits as compared to MDCT, the expert pointed out. In addition, when compared to traditional 2D imaging, CBCT can offer ‘supplementary and often much more diagnostically meaningful data.’ On the other hand, he also acknowledged technical and physical shortcomings, with limited field of view and issues of heterogenous radiation dose distribution within the area that is imaged. 

Kortesniemi, who is Adjunct Professor and Chief Physicist in the Department of Medical Imaging at the University of Helsinki, Finland, highlighted the higher resolution with lower dose, pointing to Voxel sizes down to 0.1 mm and with a scalable field of view from 2-26 cm and the potential to scan the whole head, with scanners being developed capable of covering larger body regions. With the flat panel detector technology, he noted that gantry weights vary from about 60kg to more than 600kg.

One of the key optimisation strategies in CBCT is to optimise the field of view to the minimum needed for a diagnostic question. A range of different field of views have direct impact on the patient doseMika Kortesniemi

He said there are a limited number of projections for the raw data CBCT acquires, especially compared to multi-slice CT, which has much more projections to a rotation. The lower number of projections, in combination with limited field-of-view, means a decrease in low or soft tissue contrast and there is a longer scan time of 10-30 seconds, during which patients may move. 

Most CBCT scanners use short radiation pulses rather than continually exposing patients during gantry rotation, and reconstruction time is getting shorter due to more efficient reconstructions, with a move towards AI-based image reconstruction. ‘One of the key optimisation strategies in CBCT is to optimise the field of view to the minimum needed for a diagnostic question,’ he said. ‘A range of different field of views have direct impact on the patient dose.’

Will technology make artifacts a thing of the past?

Artifacts – caused from metals in dental implants, for example – are an issue to consider with CBCT, Kortesniemi added. These can cause additional scatter and beam hardening. He believes with evolving image calculation and reconstruction techniques there will be better means to correct these artifacts, which may also be caused through patient movements, to improve the overall image quality. 

The expert said DAP (dose area product) provides a simple and robust dosimetry unit for CBCT and avoids scatter problems in measurement. Comparing CBCT to MDCT, he said CBCT had lower performance in terms of scan time, soft tissue contrast and clinical applications, but had positives in terms of cost, required area, patient dose – to some extent – and spatial resolution.

‘The method of choice’ for head and neck region

portrait of bert de foer

Following up, Professor Bert De Foer, from the GZA Hospitals in Antwerp, Belgium, spoke about current clinical indications for CBCT in the head and neck, discussing its use in sinonasal, dental and temporal bone imaging. He said with CBCT, most patients are sitting upright and the X-ray source is turning around the patient and scanning on flat panel. Disadvantages of CBCT include poor soft tissue resolution, limited scan range, motion artefacts, and the process is time consuming. 

However, Professor De Foer, who is also President of the European Society of Head and Neck Radiology (ESHNR) and chaired the session, added: ‘I have replaced MDCT by CBCT in the head and neck area for temporal bone, sinonasal and dental imaging since 2011 in my department and it is definitely the method of choice to do CT examinations in the head and neck region, due to its very high spatial resolution, low dose and equal quality of images in all planes.’

‘A choice for quality and lower radiation’

Sana Boudabbous from the Faculty of Medicine at the University of Geneva, Switzerland, focussed on CBCT in morphologic and functional MSK imaging. She said: ‘It gives 3D imaging with high quality, high resolution, weight bearing position is first advantage of this technique for many diseases for lower limbs and lower dose than conventional CT. The future will be reduced contrast, use of dual energy and bone quantification for example in osteoporosis.’ 

In closing the session, De Foer said: ‘I hope we have been successful in proving that CBCT is something completely different from MDCT and there is definitely a place for CBCT in modern radiology departments. It is a choice for quality and lower radiation.’ 

Profiles: 

Dr Mika Kortesniemi works as the Chief Medical Physicist and Adjunct Professor at the HUS Diagnostic Center, University of Helsinki and Helsinki University Hospital, Finland. His professional, teaching and research focus is on the quality assurance, radiation dosimetry, optimisation, and radiation protection in x-ray imaging, especially related to the computed tomography and utilisation of artificial intelligence. Dr Kortesniemi is also actively involved in international collaboration with IAEA, ICRP, EFOMP and ESR. 

Professor Bert De Foer is a consultant radiologist at the department of Radiology of the St Augustinus Hospital, GZA Hospitals in Antwerp, Belgium, and responsible for Head and Neck and Neuroradiology. He is president of the ESHNR and has a current scientific interest in the field of the value of MRI in the diagnosis of Menière’s disease.

AN INTRODUCTION TO ADAPTIVE RADIOTHERAPY USING IMPT

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Adaptive radiotherapy describes the process by which a patient’s treatment plan can be changed or “adapted” as necessary during the course of their treatment. This can mitigate changes in the patient’s anatomy due to events like weight loss, tumor motion or shrinkage, and differential organ motion. The need for, and benefits of, adaptive techniques has been well-known and exhaustively researched in radiation therapy, but to date the implementation of adaptive techniques has been relatively limited, largely due to technology and workflow challenges.

The integration of adaptive techniques into treatment with intensity-modulated proton therapy (IMPT) affords clinicians the opportunity to change the delivery of the proton beam treatment plan to create a personalized, time sensitive, treatment approach for every patient. Additionally, employing adaptive radiotherapy allows for the treatment of a substantially greater range of cancer diagnoses and presentations with proton therapy, giving physicians more treatment options when determining their treatment. It is particularly useful in treating those anatomic regions which are subjected to substantial tumor and normal organ motion such as the lungs and upper abdomen.

Recent advancements in high quality volumetric imaging using cone-beam computed tomography (CBCT), deformable registration, and dose tracking have greatly advanced adaptive proton therapy implementation. Clinicians can now track interfractional changes both visually from high quality CBCTs acquired prior to treatment and by tracking the real dose delivered that day. With the combined understanding of the geometrical changes along with calculated day-to-day dose delivered, clinicians can calculate, for example, regions of overdose or underdose in the tumor volume, bringing about for the first time plan adaptations based on what was historically delivered to meet the initially intended prescription.

Improved Technology

Adaptive radiotherapy for proton treatment has been around in one form or another for decades; however, recent technological advances such as in-room CBCT, integrated software workflows between information systems and treatment planning systems, and pencil-beam scanning(PBS) have greatly improved adaptive radiotherapy for IMPT.

On April 18, 2016, Scripps Proton Therapy Center (SPTC) became the first facility to use the fully-integrated CBCT on the ProBeam System to image patients in the treatment room on a daily basis to see if there were any changes in a patient’s anatomy or tumor that might “trigger” a diagnostic CT scan. Without in-room CBCT, proton therapy centers have to move a patient from the treatment room to a CT scanner in another room or building to check for changes in the anatomy or tumor.

CBCT not only serves as a “trigger” for replanning, but also for daily positioning. Unlike orthogonal X-rays, CBCT images give clinicians additional data regarding soft tissue, which is helpful in determining the optimal position for patients and for observing changes in tumor and normal tissue anatomy. This is especially critical in proton therapy where positioning that’s off by 4 or 5 millimeters can have a profound effect on treatment.

Along with CBCT, PBS helps us quickly design a more precise treatment plan as a patient’s clinical condition changes. Prior to PBS, passive scatter systems had been used for about 50 years. If the treatment plan needed to be changed, any change in the treatment plan which was desired by the clinicians had to rely upon physically manufactured blocks to shape the proton beam to match the shape of the target, a process which could, and often did, take days. With PBS, once the treatment plan is updated and approved, the proton beam can be delivered immediately.

Cone-beam CT of the retromolar nerve.

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The ‘retromolar’ nerve is a collateral branch of the inferior alveolar nerve. Cone-beam computed tomography (CBCT) provides higher resolution images. This CBCT study reports the frequency of the retromolar nerve. Materials and methods. From 2007-2010 the CBCT study of 233 hemi-mandibles have…

Abstract

Objective The ‘retromolar’ nerve is a collateral branch of the inferior alveolar nerve. Cone-beam computed tomography (CBCT) provides higher resolution images. This CBCT study reports the frequency of the retromolar nerve. Materials and methods. From 2007-2010 the CBCT study of 233 hemi-mandibles have been examined. The CBCT study was obtained from an investigation of the posterior mandibular region in 187 patients suffering from different pathologies and it was aimed at detecting in patients the presence of a retromolar canal and foramen.
Results Thirty-four retromolar canals with a foramen were detected on 233 CBCT (14.6%) in 30 out of 187 patients (16%). In the 46 patients who underwent CBCT bilaterally, the retromolar canal was found in nine subjects (19.6%) and was present bilaterally in four subjects, for an incidence of 8.7%.
Conclusions The results suggest that the radiological frequency of the retromolar nerve is notable, with a possible relevance in the surgical approach of the mandibular retromolar area. The presence of a retromolar canal, well detected with CBCT, may warn clinicians about the possibility of inadequate pre-surgical anaesthesia, local intra-operative bleeding and post-operative alterations of the sensation in the third molar area.

Giuseppe Lizio , Gian Andrea Pelliccioni , Gino Ghigi , Alessandro Fanelli & Claudio Marchetti

Department of Oral and Dental Sciences, University of Bologna, Italy

Correspondence: Dr Giuseppe Lizio, Department of Oral and Dental Sciences, University of Bologna, Via San Vitale 59, 40125, Bologna, Italy. +011-39-051-2088155. +011-39-051-225208. giuseppelizio@libero.it

Source: www. getinsidehealth.com