Quantitative imaging of the cervix: setting the bar

Information assessment is fundamentally either qualitative or quantitative. The former is subjective, based on apparent qualities, and the latter is objective, involving measurable quantities and numerical descriptors. Medicine is primarily quantitative; routinely, we use biomarkers to make diagnoses, distinguish normal from abnormal physiological conditions and monitor treatments. According to Wikipedia, a biomarker is: ‘anything that can be used as an indicator of a particular disease state or some other physiological state of an organism’. In order for a biomarker to be clinically useful, it must be validated, and its assessment standardized and reproducible between different laboratories and equipment, and the information provided by the biomarker should be relevant to the physiological condition1. A simple example is body temperature: fever is a biomarker of infection or inflammation. Medical imaging, in contrast, is primarily qualitative. The details of the physical interactions that occur between an object and an imaging modality hold a wealth of information but, historically, we have ignored these details in favor of qualitative interpretation. In other words, we rely on pattern recognition of the spatially varying image brightness to identify a structure and distinguish normal from abnormal. This is changing, however, as the value of image quantification is being recognized increasingly. Quantitative imaging is: ‘the extraction of quantifiable features from medical images for the assessment of normal or the severity, degree of change, or status of a disease, injury or chronic condition’1. The process for evaluating a biomarker can be simple – most people keep thermometers at home to test for fever – or it can be fairly sophisticated, involving tracking of radiolabeled substances introduced into the body to identify sites of interest. For instance, positron emission tomography (PET) using radiolabeled 18 F-fluorodeoxyglucose provides information about sites of metabolic activity (glucose uptake). Similarly, gadolinium-based magnetic resonance imaging (MRI) contrast agents can be tagged to improve target specificity and provide analogous information about tumor volume or pharmacokinetics. These complex imaging techniques are presently receiving a great deal of attention as the field of quantitative imaging is exploding. However, one of the earliest successful examples of quantitative imaging is simple and is quietly being used daily in clinical practice: fetal biometry. The metrics (biparietal diameter, head circumference, abdominal circumference and femur length) have been validated and standardized, they are applicable across different imaging systems and the information is relevant to the fetal condition and clearly clinically useful. We in obstetrics and gynecology continue to push the boundaries of quantitative imaging. Evaluation of the fetal heart is an excellent example2–5. Recently, the Journal published a summary of the state of quantitative fetal heart imaging5, in which Hornberger described how relating individual fetal cardiovascular measurements to normative data improves neonatal prognosis, because it allows interventions to be based on a more sophisticated understanding of individual cardiac pathophysiology. In other words, validation and standardization of specific measures which are relevant to the fetal condition has resulted in clinically useful interventions. This is precisely the role of an effective biomarker. There are many other examples of quantitative imaging in the fetus, including of bones6, craniofacial structures7, lungs8 and brain9. Quantitative ultrasound techniques are also being explored actively for assessment of the placenta10 and especially the cervix11–17. The first reference to cervical quantitative imaging17 was in this Journal in 2006, and the current issue holds a compelling paper by Hernandez-Andrade et al.18, which is likely the most recent word.