High Intensity Focused Ultrasound

HIFU is a non-invasive, thermal ablation procedure in the treatment of uterine fibroids. It uses focused ultrasound beams directed towards the tumor to cause coagulative necrosis. HIFU has a shorter recovery period and can be done as an out-patient procedure compared to surgery.

Non-perfused volume ratio (NPVR) is used to determine HIFU efficacy. It is defined as the ratio of the volume of the non-perfused area in post-ablation to the total volume of the fibroid. 

• Calculated by: volume of non-enhancing area of fibroid in post-treatment / total fibroid volume in post-treatment
• NPVR of >80% post-treatment is considered sufficiently ablated.

(Figures 1 to 4 are images of our HIFU machine and its parts.)

Fig 1: Haifu Focused Ultrasound Tumor Therapeutic System control console

Fig 2: Haifu Focused Ultrasound Tumor Therapeutic System treatment table

Fig 3: Components of the thermal energy delivery system: energy emitter for ablation (yellow arrow) and ultrasound probe for real time monitoring (red circle with arrow). The patient is in prone position during the procedure. The target fibroid is positioned at the center of the dish (heat energy emitter).

Fig 4: Control console interface

Important Factors Affecting HIFU Efficacy:

1. Signal Intensity of fibroid on T2WI 

1. 1. Higher signal intensity relative to myometrium = less efficient ablation.
   2. T2-hypointense fibroids are denser with more collagen fibers and less mucin/fluid than T2-hyperintense fibroids, hence, more susceptible to ablation.

2. Enhancement of fibroid relative to myometrium 

1. 1. Efficiency of ablation relative to blood supply: more enhancing/more vascular = greater heat dispersion (less efficient).
   2. Mildly enhancing (most susceptible to ablation), moderate or iso enhancing relative to myometrium (less susceptible to ablation), markedly enhancing (least susceptible to ablation).

3. Fibroid size 

1. 1. Larger fibroids are less vascular and have greater surface area preventing quick thermal dispersion; therefore, more susceptible to thermal ablation.
   2. Fibroid beyond the umbilical level
      1. Not conducive to sufficient ablation; fractional ablation is recommended.
      2. More prone to complications as more intraabdominal structures can lie along the sonication path.

4. Platelet count 

1. 1. Correlates with HIFU efficiency but specific mechanism is still for further investigation.

Magnetic Resonance Imaging

MRI has superior soft-tissue contrast relative to ultrasound and computed tomography (CT) scan; hence the preferred imaging modality for detection, localization and characterization of fibroids.

MRI evaluation of the uterine fibroids is required before and within a week following HIFU treatment. At our institution, imaging of the lower abdomen is done using 3 Tesla (Philips Achieva) or 1.5 Tesla (Philips Ingenia). Surface coil is used to increase signal-to-noise ratio. Image acquisition protocols for pre-HIFU and post-HIFU in our institution are composed of sequences (Figure 5) necessary to plan treatment, describe fibroids, assess response of fibroids to treatment and evaluate for complications. The field of view includes the uterus, cervix, vagina, myomas, lower abdominal wall, pelvic side walls and back. Slice thickness is 5-6 mm without gap.

Fig 5: Standard Protocol for Pre-HIFU and Post-HIFU MRI

Reporting of fibroids before and after treatment

• Knowledge of zonal anatomy of uterus is the key to localizing fibroids. T2WI/T2FSE is the best sequence for demonstrating the uterine anatomy (Figure 6). The location determines the type of fibroids based on FIGO classification (Figure 7). (Figures 8-15 are series of images of some of the different types of fibroids.)

Fig 6: Sagittal (a) and axial (b) T2WI of a normal uterus showing the 4 distinct zones

Fig 7: FIGO Classification of Uterine Fibroids

Fig 8: FIGO 1: Submucosal myoma, with <50% intramural component

Fig 9: FIGO 2 : Submucosal myoma, with >50% intramural component

Fig 10: FIGO 3: Intramural myoma, with endometrial contact

Fig 11: FIGO 4: Completely intramural myoma

Fig 12: FIGO 5: Intramural myoma (subserosal with >50% intramural component)

Fig 13: FIGO 6: Subserosal myoma with <50% intramural component

Fig 14: FIGO 7: Subserosal pedunculated myoma

Fig 15: FIGO 3-5: Hybrid myoma

• Size is reported in three dimensions: anteroposterior (AP), transverse (T) and craniocaudal (CC).
• Pre-treatment contrast enhancement patterns. These are reported as mild, moderate or marked enhancement with reference to the adjacent normal myometrium (Figure 16).

Fig 16: Guide in assessing enhancement of fibroid on T1FS post-gadolinium (GD).

• In post-HIFU, immediate follow up MR imaging is done ideally within 1 week after treatment. The two most important sequences in assessing the effectiveness of ablation are T2WI and T1FS post-gadolinium sequences. These sequences are used to compute for the NPVR, where the volume of non-enhancing area of the fibroid on T1FS post-gadolinium is divided by the volume of the fibroid on T2WI and multiplied by 100%. The fibroid volume on T2WI is also used to determine the decrease in fibroid volume, where pre-HIFU volume – post-HIFU volume / pre-HIFU volume x 100%. In addition, the T2 sequences will demonstrate relative increase in signal of ablated fibroid due to coagulative necrosis. There may also be relative increase in T2 signal of the myometrium due to post-treatment edema. Other finding on T2 sequences, although rarely seen, is subcutaneous edema in the abdominal wall. Occasionally there may be development of T1 hyperintense areas in the ablated fibroid which may suggest, but not specific for subacute blood. (Figures 17 and 18 demonstrate post-treatment changes.)

Fig 17: Comparison of pre-HIFU and post-HIFU myoma characteristics on T2 images. Post-HIFU MRI was done 1 week after treatment.

Fig 18: Comparison of pre-HIFU and post-HIFU myoma characteristics on post-gadolinium images. Post-HIFU MRI was done 1 week after treatment.

Complications and Adverse Effects of HIFU treatment is graded according to the Society of Interventional Radiology (SIR) Classification Standard

Complication classification range from A to F, with increasing severity

• A & B: Minor adverse effects
• C to F: Major adverse effects         

Majority of adverse effects fall under classes A and B, rarely above C

• Most common mild adverse effects: mild lower abdominal pain, sacrococcygeal pain, and abnormal vaginal discharge.
• Most common major adverse effect: skin burns (more severe in patients with surgical scars).
• Class D to F adverse effects are rare and less frequent in HIFU compared to other treatment methods.

Fig 19: Complications and Adverse Effects of HIFU treatment graded according to the Society of Interventional Radiology (SIR) Classification Standard

Our templates for reporting before and after HIFU treatment are shown in figures 20 and 21.

Fig 20: Reporting template before HIFU treatment.

Fig 21: Reporting template after HIFU treatment.