"Perhaps it is for all these reasons, as well as on account of the heterogeneity of Crohn's disease and the unpredictability of the human condition that our quest for the Holy Grail of one 'best measure of disease activity' still remains today, in Korelitz's words of 15 years ago, 'a need unfulfilled.'"¹

Crohn's disease is an inflammatory disorder of the gastrointestinal tract that is characterized by multiform and, as yet, not well-established pathogenesis.² It is a chronic condition and has a relapsing course. Initial assessment of inflammatory activity may help determine which patients will benefit from prompt and tailored treatment, and this is consequently of paramount importance.³ Further periodic assessments of inflammatory activity are crucial for monitoring the effects of drugs and for surgical therapy planning.⁴

Many instruments measuring disease-specific activity have been evaluated over the past 30 years. To date, however, no diagnostic technique has emerged as a standard reference tool for the reliable and reproducible quantification of inflammatory activity in Crohn's disease. Gastroenterologists typically make this evaluation on the basis of clinical symptoms, physical findings, laboratory parameters, endoscopy, and imaging tests.^5,6

The Crohn's disease activity index (CDAI) is the most widely used clinical scoring tool for evaluations of inflammatory activity specific to Crohn's disease. The CDAI is used in everyday clinical practice and in most therapeutic trials, despite being based partially on subjective criteria that represent patients' perception of their disease rather than objective assessment.^7,8

Biochemical markers, including white blood count, erythrocyte sedimentation rate, C-reactive protein, and orosomucoids, have been used as references for activity in imaging studies.⁴

The "scoring system for histologic abnormalities in Crohn's disease mucosal biopsy specimens” has been used to quantify the histological activity of the disease.⁹ This assessment is not generally recommended as a treatment end point due to the poorly investigated correlation between histological improvement and other assessments of disease activity. Other major drawbacks include the potential for sampling errors and inherent variability in biopsy techniques.⁸ Histology nonetheless shows a closer and less equivocal correlation with morphologic and postcontrast MRI findings than with clinical aspects.^5,10

Barium examination, either by enteroclysis or followthrough technique, has traditionally been the gold standard tool for imaging the small bowel. Invasiveness, exposure to ionizing radiation, and the need for extensive bowel preparation, however, limit the use of this technique for the follow-up of disease.¹¹ Conventional radiography is also unable to evaluate the activity and extension of the inflammatory process through and beyond intestinal mucosa.