From the Guidelines
Hounsfield units (HU) are accurate in scans without contrast, as they represent the radiodensity of tissues based on their natural X-ray attenuation properties. Non-contrast CT scans provide reliable baseline HU measurements for various tissues, with water calibrated at 0 HU, air at -1000 HU, and bone typically above +400 HU. These measurements remain consistent and reproducible in non-contrast scans, making them valuable for evaluating certain conditions like urinary stones, lung nodules, or intracranial hemorrhage.
Key Points
- HU measurements are reliable in non-contrast scans for certain diagnoses, such as distinguishing between different types of kidney stones or characterizing adrenal lesions 1.
- A threshold value of 10 HU is generally accepted as a cutoff value for a region of interest obtained over the lesion to differentiate adenomas from malignant lesions 1.
- Histographic analysis can further improve the sensitivity of non-enhanced CT for identifying adenomas, by analyzing the distribution of pixel values within the lesion 1.
- The use of HU measurements in non-contrast scans is supported by studies that demonstrate their accuracy and reliability in various clinical applications 1.
Clinical Applications
- Non-contrast CT scans are useful for evaluating incidentally discovered adrenal masses, as they can help differentiate benign from malignant lesions based on HU measurements 1.
- HU measurements can be used to characterize adrenal lesions, with a threshold value of 10 HU generally accepted as a cutoff value for a region of interest obtained over the lesion 1.
From the Research
Accuracy of Hounsfield Units in Scans without Contrast
- The accuracy of Hounsfield units (HUs) in scans without contrast is a topic of interest in medical imaging, with various studies investigating its reliability 2, 3, 4, 5.
- A study published in 2014 found that HU measurements of soft tissues on unenhanced abdominal CT scans varied between two different manufacturers' MDCT scanners, with statistically significant differences in all anatomic sites 5.
- Another study from 2005 demonstrated the clinical utility of noncontrast computed tomography attenuation value (Hounsfield units) to differentiate adrenal adenomas/hyperplasias from nonadenomas, with a sensitivity and specificity of 40.5% and 100% for a 10-HU cutoff value, respectively 4.
- However, a 2024 study evaluating the HU assignment and dose differences between CT-based standard and deep learning-based synthetic CT images for MRI-only radiation therapy of the head and neck found that HU differences did not translate into a reduction in gamma pass rates or differences in average PTV/OAR dose metrics greater than 3% 3.
- The use of HUs in MRI-based radiotherapy treatment planning has also been explored, with a 2014 study demonstrating the feasibility of constructing high-quality pseudo-CT images by converting MRI intensity values into HUs in the male pelvis 2.
Factors Affecting HU Accuracy
- The accuracy of HUs can be affected by various factors, including the type of scanner used 5, the presence of contrast agents 4, and the specific tissue being imaged 3, 4.
- The use of different image processing techniques, such as threshold-based segmentation methods, can also impact the accuracy of HU measurements 6.
- Additionally, the size and position of the tissue or organ being imaged can influence the accuracy of HU measurements, as demonstrated in a study on adrenal tumors 4.