What is High-Intensity Focused Ultrasound (HIFU)?

High-Intensity Focused Ultrasound (HIFU): Mechanism and Applications

High-Intensity Focused Ultrasound (HIFU) is a non-invasive therapeutic technique that uses focused ultrasound waves emitted from a transducer to cause tissue damage through mechanical and thermal effects as well as cavitation, allowing targeted treatment without affecting surrounding tissues. 1

Mechanism of Action

• HIFU delivers high-energy ultrasound waves that are precisely focused on a target area, causing:

  • Thermal effects: Heating tissue to temperatures of 70°C or higher 1
  • Mechanical effects: Physical disruption of tissue structure
  • Cavitation: Formation and collapse of microbubbles that enhance tissue damage 1

• The focused nature of HIFU allows for:

  • Concentrated energy delivery at the focal point
  • Minimal damage to surrounding tissues in the beam path
  • Acoustic intensity at the focal point up to 100 times higher than outside the focal zone 1

Technical Aspects

• HIFU systems typically consist of:

  • Piezoelectric transducer that generates ultrasound waves 2
  • Focusing mechanism (either geometric design or phased array)
  • Imaging guidance system (MRI or ultrasound)

• Key parameters that affect HIFU treatment:

  • Frequency: Typically ranges from 0.8-10 MHz depending on application 1
  • Power: Determines the intensity of the ultrasound beam
  • Exposure time: Affects the extent of tissue damage
  • Focal length: Distance from transducer to target tissue

Clinical Applications

Approved Applications

1. Prostate Cancer Treatment:

   • Used for focal therapy in small low- or intermediate-risk unifocal tumors
   • Aims to ablate tumors while sparing neurovascular bundles, sphincter, and urethra
   • Shows 88% failure-free survival at 5 years in case series 1
   • Currently recommended only within clinical trials or well-designed prospective registries 1

2. Benign Prostatic Hyperplasia (BPH):

   • Considered investigational according to AUA guidelines 1
   • Creates coagulation necrosis in targeted areas of the prostate
   • Described as "moderately promising" but requires additional long-term studies 1

3. Dermatological Applications:

   • Used for skin rejuvenation and body fat reduction 3
   • High-frequency HIFU (20 MHz) specifically developed for dermal interventions 4
   • Produces controlled lesions at 1-2 mm depth, corresponding to human dermis 4

4. MRI-guided HIFU (MRgFUS):

   • Non-invasive treatment for essential tremor
   • Considered a low-risk procedure with major adverse event rate of only 0.2% 5
   • Patients can generally proceed without additional testing due to low risk profile 5

Limitations and Safety Considerations

• Delivery challenges:

  • HIFU delivered from the rectal cavity faces challenges in reaching the anterior part of large prostates 1
  • Requires specialized training and equipment 1

• Outcomes and complications:

  • Poor long-term oncological outcomes for high-risk localized prostate disease 1
  • Functional outcomes in prostate treatment show 12% incontinence and 61% erectile dysfunction rates 1
  • Dermatological applications may have complications despite being minimally invasive 3

Current Status in Medical Practice

• HIFU is typically performed as a single-session, day-case procedure with patients either fully conscious, lightly sedated, or under light general anesthesia 6

• Advantages over other ablation techniques:

  • No need for transcutaneous insertion of probes into target tissue 6
  • Precise targeting with minimal damage to surrounding tissues 2
  • Non-invasive nature makes it suitable for multiple applications 7

• Current recommendation status:

  • For prostate cancer: Should only be performed within clinical trials or well-designed prospective registries 1
  • For BPH: Considered investigational and not recommended as standard treatment 1
  • For essential tremor: Approved with favorable safety profile 5

HIFU represents an evolving technology with promising applications across multiple medical fields, but its use should be guided by the specific evidence for each indication and the latest clinical guidelines.