**5. Reference lines**

Image interpretation from conventional and upright open MRI evaluates the three compartments of the pelvic floor [43]. The three compartments are evaluated for morphologic changes such as POP at various pelvic floor positions. To define the existence and descent of POP, the use of a point of reference is beneficial. Several points and lines of reference for measuring POP have been reported [44]. The more commonly used lines are the pubococcygeal line (PCL) and the mid-pubic line (MPL), both applied on midsagittal images. The PCL is the line drawn from the inferior part of the symphysis pubis to the last coccygeal joint. Extending the posterior portion of the PCL to the sacrococcygeal joint also has been proposed because there is movability of the coccyx with straining [21]. The MPL is a line extending along the long axis of the symphysis pubis. The PCL represents the levator plate, while the MPL correlates with the level of the hymen, which is the landmark applied for clinical staging [45]. To measure pelvic organ prolapse a perpendicular line is drawn from the reference line (PCL or MPL) to the bladder base (anterior compartment), the cervix or vaginal vault (middle compartment), and the anal rectal junction (ARJ) (posterior compartment) [23].

Another classification system, H line, M line, organ prolapse (HMO), has been proposed for measuring prolapse [35]. The H line is drawn from the pubis to the posterior anorectal junction and measures the levator hiatus width. Organ prolapse is measured relative to that line. The M line measures the descent of the levator plate from the pubococcygeal line. The angle of the levator plate relative to the pubococcygeal line and the width and part of the pelvic hiatus on axial images can be measured as well [22].

The choice of which reference line is used is mostly made by the radiologist and/or the referring clinician, as neither of the two lines has shown distinct superiority [44]. The PCL, however, is the most-used reference line, particularly by surgeons and gastroenterologists. The MPL is better known among urogynecologists, as it is compatible with their clinical staging system. Both reference lines display only moderate to poor agreement with clinical staging of pelvic organ prolapse [38]. This might be partly because anatomical landmarks used for MR measurements and for clinical examination differed in most of the studies [23]. Standards exist for diagnosing prolapse on physical examination [1]. Congruity between this clinical standard and MRI imaging analysis should be used to document the utility of MRI and the success of treatment [22].

However, different criteria are currently used for diagnosing prolapse on MRI. Most research that reported using the bony reference lines uses one of the following criteria: (a) descent of the bladder base more than 1 cm inferior to the pubococcygeal line, (b) position of the cervix or vaginal vault less than 1 cm over the PCL or below it, and (c) descent of the posterior compartment more than 2.5 cm below the PCL (International Urogynecological Association and International Continence Society) [1]. There are also other minor differences in the diagnostic criteria applied for prolapse; cystocele has been defined as when the bladder descends to any area below the PCL, and uterocervical prolapse and enterocele are when the cervix or small bowel are below the PCL .
