Managing CT dose while imaging the paranasal sinuses

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The case that was submitted by Teck Chong is about managing radiation dose exposure during a CT examination to image paranasal sinuses and its surrounding soft tissue.

Background information
Paranasal sinuses imaging has intensively progress over the past 30 years. Various modalities have emerged with new imaging techniques to depict the paranasal sinuses and its surrounding soft tissue structures. Computed Tomography (CT) of the paranasal sinuses has gradually replaced conventional radiographs due to its high resolution in visualizing the anatomical structures and osseous detail. Although techniques such as the application of orbital bismuth have been adopted to manage radiation dose for CT examination, it does degrade the image quality. Radiographers should remember the principle of medical imaging technique with application of ALARA (As Low As Reasonably Achievable). Therefore, decreasing the exposure parameters will play a great role in managing patient radiation dose. Consideration should be given to milliampere-second (mAs), a tube current time product which plays a big role in patient absorbed dose.   
  
Case objective
The study served the purpose to manage patient receive dose. A precise planning with an extreme low mA (10mA) was applied. The actual achieved mAs should then be reduced. The image quality may be degraded as compared with standard mA applied. Therefore, the CTDI and DLP will eventually reduce resulting in enhanced dose management.

Teck Chong

 

Macau Yinkui Hospital

 

Macau

Teck Chong

 

Macau Yinkui Hospital

 

Macau

The case that was submitted by Teck Chong is about managing radiation dose exposure with regards to a CT examination to image paranasal sinuses and its surrounding soft tissue.

Background information
Paranasal sinuses imaging has intensively progress over the past 30 years. Multi-modalities have emerged with new imaging techniques to depict the paranasal sinuses and its surrounding soft tissue structures. Computed Tomography (CT) of the paranasal sinuses has gradually replaced conventional radiographs due to its high resolution in visualized the anatomical structures and osseous detail. Although afford such as application of orbital bismuth has been adopted to manage radiation dose for CT examination, however; it does degrade the image quality. Radiographer show remember the principle of medical imaging technique with application of ALARA (As Low As Reasonably Achievable). Therefore, decrease the exposure parameter shall plays a great role in managing patient radiation dose. A greater consideration should be pay on milliampere-second (mAs), a tube current time product which play a greater role in patient absorb dose.   

Case objective
The study served the purpose to manage patient receive dose. A precise planning with an extreme low mA (10mA) was applied. The actual achieve mAs should be reduced. The image quality will be slightly degraded as compare with standard mA applied. Therefore, the CTDI and DLP will eventually reduce resulting in enhanced dose management.

 

This study suggests low mA, with iterative software application, which can decidedly help manage radiation dose to the patient.”

––  Teck Chong – Radiology Manager

Macau Yinkui Hospital, Macau

Clinical and patient background

The patient was presented with a headache and a mild runny nose. She had visited an ENT surgeon who then referred her for CT PNS to further evaluate the possibility of sinusitis. The possible imaging challenge is more likely due to beam hardening artefact from skull and facial bones, as well as metal artefact cause by dental filling or dental implant. The soft tissue image quality may be reduced, as a low mA was used. Therefore, this type of technique only applies to purely a sinusitis patient. Those suspected mass or tumor cases which required optimal soft tissue delineation may require a standard exposure factor.

Dose management methods and techniques that were used

A standard volumetric scan requested by a radiologist still preserved. The kVp remained constant, which was 120 kVp. The standard mA was reduced from 150 mA to 10 mA. Half rotation was applied to achieve 5mAs. The scanned length was specific from hard palate to frontal sinuses only, which was 11.6 cm. The CTDI (mGy)and DLP (mGy.cm) was measured. The DLP was used and multiplied with tissue weighting factor based on neck region. The conversion (k) factors for adults at head region was 0.0021, based on ICRP 103 weighting factors.


Conclusions and results

Grainy and noisy images were obtained, but the visualized paranasal sinuses were well depicted. The radiologist was able to provide a diagnostic report based on this series of dose managed images. The CTDIvol was 1.00 mGy and the DLP was 11.60 mGy.cm. Based on the ICRP 103 weighting factor for head region, the final estimated effective dose was 0.002436 mSv (11.6 X 0.0021). A comparison was done with an similar case for which the exposure factors remained at the factory settings. The final result was 0.28mSv.

Discussion of case outcome(s) and future implications

Volumetric scans have been widely applied to all CT imaging examinations. The factory setting exposure factor may vary from vendor to vendor. Most CT scanner providers have good iterative software reconstructions. The final estimated effective dose for this study was 0.0024mSv. It was lowered by 115 fold. This low mA setting is able to provide image quality for paranasal sinuses imaging. With the application of current iterative software technology, the presented images were acceptable, although a mild degree of noise was observed on the images. The paranasal sinuses and osteomeatal complexes (OMC) were, however, well depicted. Mucosal thickening and retention cysts were well delineated. The obtained effective dose is more likely lower than a standard radiography projection. Alternatively, it may replace standard radiography techniques which provide limited and 2D access to the paranasal sinuses. Ultimately, it may become the gold standard to access acquired paranasal sinuses diseases such as sinusitis.

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