Recommended MRI Imaging Protocols
Static and dynamic sequences should be acquired; T2-weighted images (T2WI) are recommended for both,
due to its greater anatomic detail.
( Fig. 1 ).
[3]
T1WI should complement the examination,
whenever a pelvic lesion is detected.
- Static images → Detection and classification of structural abnormalities
→ Pelvic floor anatomy
→ Defects of the supporting structures
→ Anal sphincter complex
- Dynamic images (during squeezing,
straining and defecation) → functional abnormalities that are assessed by metric measurements of the three compartments
→ Pelvic organ mobility
→ Pelvic floor relaxation
→ Pelvic organ prolapse (POP)
→ Associated compartment defects
No oral or intravenous contrast is necessary.
[3]
Anatomy brief review
◊ Pelvic compartments
Pelvic floor is one functional unity commonly divided into three different compartments ( Fig. 2 ):
- Anterior Compartment (AC) - Bladder and Urethra
- Cystocele
- Urethral Hypermobility
- Middle Compartment (MC) - Uterus,
Cervix and Vagina
- Uterine/Vaginal prolapse
- Posterior Compartment (PC) - Anus,
Anal Canal,
Rectum,
Sigmoid colon
- Pelvic Floor Relaxation
- Rectocele
- Intussusception
The cul-de-sac is often considered a "virtual" compartment,
where pathological processes also occur:
- Enterocele
- Peritoneocele
- Sigmoidocele
◊ Supporting Structures
Endopelvic Fascia - located immediately beneath the peritoneum; it has various thickenings or condensations in specific areas.
[5]
⇒ Most of Endopelvic Fascia ( Fig. 3 ) is not directly visualized in MRI → Defects are inferred based on secondary signs! [6]
° Level I (suspension) - Cephalic 2–3 cm of the vagina and above
- Provides the upper vaginal support (Parametrium and Paracolpium) [6,7]
° Level II (attachment) - Mid-portion of the vagina (between level I and III)
- Transverse support of the vagina (Paracolpium)
- Urinary bladder support (through the anterior vaginal wall)
- Prevents the anterior protrusion of the rectal wall (through the posterior vaginal wall and rectovaginal fascia)
- Prevents the bowel from herniating inferiorly (rectovaginal fascia) [4]
° Level III - At the level of the hymen ring and the 2–3 cm above it → directly attached to the surrounding structures (urethra,
perineal body, levator ani muscles)
- Urethral support [4,5,7]
⇓
Static images in normal patients show ( Fig. 4 ):
- Symmetrical urethral ligaments on an axial scan (no distortion or defect)
- Bladder neck positioned close to the pubis symphysis
- Vaginal lumen with a widened H-appearance in axial sequences and the lateral vaginal walls are close to the puborectalis muscle [3,4]
Pelvic Diaphragm - The levator ani (pubococcygeus,
puborectalis and iliococcygeus muscles) and ischiococcygeus muscles.
[4,5,7] ( Fig. 5 )
- Iliococcygeus forms,
posteriorly,
a firm midline raphe ⇒ Levator plate
- Puborectalis ⇒ U-shaped sling - elevates the bladder neck and compress it against the pubic symphysis [4,5,7]
Urogenital Diaphragm - Connective tissue and the deep transverse muscle of the perineum (anterior to the anorectum) [4,8]
Pelvic organ prolapse vs pelvic floor relaxation
Different entities,
but frequently coexistent in pelvic floor weakness
Pelvic organ prolapse - abnormal descent of a pelvic organ through the hiatus beneath it [3]:
- bladder (cystocele)
- vagina (vaginal prolapse)
- uterus (uterine prolapse)
- mesenteric fat (peritoneocele)
- small intestine (enterocele)
- sigmoid colon (sigmoidocele)
Pelvic floor relaxation - excessive descent and widening of the entire pelvic
floor during rest and/or evacuation,
regardless of whether prolapse is present.
[3]
How to look at the MRI?
◊ Dynamic Images - Identifying and grading the abnormality(ies)
1 - Maximum straining - Is there:
- Loss of urine through the urethra?
- Urethral hypermobility? ( Fig. 6 )
- Kinking of the urethrovesical junction?
- An enterocele? Which is the content of the peritoneal sac?
- Loss of rectal gel during straining? ( Fig. 7 )
2 - Rest,
squeezing and defectation - Trace the Anorectal Angle (ARA) ( Fig. 8 ) - expresses the functioning of the Puborectal muscle
- At rest – 93º
- During squeezing - sharpening of 10-15°
- During straining or defecation - 15-25° more obtuse [3]
ARA should close with squeezing and open with defecation ⇒ The change of ARA with dynamic sequences is more important than the absolute values.
[3]
3 - Rest,
maximum straining and defecation - Trace the pubococcygeal line (PCL) ( Fig. 9 )
3.1 - Measure the perpendicular distance between the PCL and each reference point (values above the reference line have a minus sign,
values below a plus sign): [3,4,6]
- Anterior compartment → the most inferior aspect of the bladder base
- Middle compartment → the most distal edge of the cervix or the vaginal vault in case of previous hysterectomy
- Posterior compartment → the anorectal junction
⇒ Cystocele and POP are graded accordingly to the “rule-of-three", starting at 1 cm below the PCL [3]:
- Grade I - 1–3 cm
- Grade II - 3–6 cm
- Grade III - > 6 cm
Refer cystocele and POP as pathological just when grade II or III.
[3]
⇒ Anorectal Junction descent (ARJ) is graded starting at 3 cm below the PCL [3]:
- Grade I - 3 - 5 cm below the PCL
- Grade II - > 5 cm below the PCL
〉 The difference between rest and maximum straining should be noted - Pelvic Organ Mobility.
〉 The full extent of POP is only visible during evacuation! [3]
4 - End of evacuation phase
4.1 -Is there a rectocele? (Measure the distance between the anterior rectal wall and a vertical line that goes through the anterior wall of the anal canal) ( Fig. 10 )
⇒ Grade rectoceles using the “rule-of-two” - pathological starting from II [3]
- Grade I - < 2 cm
- Grade II - 2–4 cm
- Grade III - > 4 cm
4.2 - Is there any mucosal invagination/rectal prolapse? ( Fig. 11 )
4.3 -Was rectal emptying adequate? Was it delayed?
→ In the presence of incomplete evacuation or delayed evacuation time (more than 30 seconds to evacuate 2/3 of the rectal content),
anismus should be considered.
[3,6]
___________________________________________________________
Δ Routinely measuring pelvic floor relaxation reached no consensus! [2]
- H-line (hiatus length)
- M-line (descent of the levator plate)
- Levator plate angle
⇓
Measured in the sagittal plane ( Fig. 12 )
Normal values [6]:
- H-line during straining 5.8 ± 0.5 cm
- M-line during straining 1.3 ± 0.5 cm
- Levator plate angle during straining 11.7 ± 4.8° [9]
- Transverse width of the levator hiatus (axial plane)
- Iliococcygeus angle (coronal plane)
⇓
Measured in the axial and coronal planes - Optional sequences [3]
___________________________________________________________
◊ Static Images - Finding the underlying structural defect responsible for the previously identified abnormalities.
- Urethral support system - maintains urinary continence
- Level III endopelvic fascial defects
→ Widening of the retropubic space - Moustache sign ( Fig. 4, Fig. 18, Fig. 19 ) [3,4,7]
- Puborectalis muscle detachment,
disruption,
atrophy or avulsion ( Fig. 13 ) [3,4,7]
- Vaginal support system - prevents prolapse
- Level I and II paravaginal fascial defects
→ Loss of the H-shape of the vagina
→ Sagging of the posterior bladder wall → "Saddlebag" sign ( Fig. 14 ) [3,4,7]
- Iliococcygeus diffuse or focal muscle abnormality
- Anal sphincter complex - maintains anal continence [3,4,6]
- Mucosa/Submucosa
- Internal anal sphincter
- External anal sphincter