Interventional Radiology Notes

Interventional Radiology

Objectives

  • Radiation Protection:
    • Describe measures for radiation protection for patients and personnel during Interventional Radiology (IR).
  • Minimally Invasive Procedures:
    • Explain why minimally invasive procedures are often more beneficial than traditional surgeries.
  • Contrast Media:
    • Discuss advantages of non-ionic contrast over ionic contrast.
  • Equipment:
    • Describe the equipment found in an interventional suite.

Digital Fluoroscopy

  • Similar to traditional fluoroscopy but includes:
    • Additional computers and monitors.
    • Ability to control the fluoroscopy unit from outside the room.
  • Key functionalities:
    • Data acquisition.
    • Post-processing.
    • Image display.
  • Control Devices:
    • Trackball.
    • Joystick or mouse.
  • X-ray Tube:
    • Typically located under the couch.
    • Operates in general radiographic mode as pulses.
  • Pulse Rates:
    • Common rates are 1/sec or 10/sec.
    • Each image is projected until a new image is prepared.
  • Interrogation Time:
    • Time required for the x-ray tube to switch on and reach adequate kVp and mA levels for the next exposure.
  • Extinction Time:
    • Time required for the exposure to switch off.
  • High-Frequency Generators:
    • DF systems use high-frequency generators with interrogation/extinction times of <1ms.
  • Duty Factor (DF):
    • The fraction of time the imaging system is exposing.
    • Example: A fluoro machine exposing 10ms every second has a DF of 10%.
    • DF=Exposure TimeTotal TimeDF = \frac{\text{Exposure Time}}{\text{Total Time}}
  • Dose Reduction:
    • Low DF decreases patient dose.
    • Pulsing at the lowest rate possible is recommended.

Image Receptor Types in Digital Fluoroscopy

  • Charge-Coupled Device (CCD)
  • Flat Panel Detector
CCD
  • Mechanism:
    • A layer of crystalline silicon, when illuminated by output phosphor, creates an electrical signal.
    • A pixel at the base of the Detector Element (DEL) captures this signal.
  • Automatic Brightness Control (ABC):
    • When ABC is used, the entire CCD signal is sampled and drives the ABC system.
  • Benefits:
    • Small size.
    • Does not demonstrate distortion.
    • Higher Detective Quantum Efficiency (DQE) and Signal-to-Noise Ratio (SNR).
    • Good spatial resolution if pixel size is small.
    • Unlimited life.
    • No maintenance.
    • Linear response.
Flat Panel Detector
  • Capture Elements:
    • Cesium Iodide (CsI) or amorphous Silicon (a-Si) with a Thin-Film Transistor (TFT).
  • Function:
    • Works like a flat panel detector in radiography but with faster interrogation/extinction times.
  • Advantages:
    • Smaller/lighter and more easily manipulated than Image Intensifiers (II).
    • Distortion-free images.
    • High DQE.
    • Improved consistent image quality.
    • Rectangular image area.
    • Insensitive to external magnetic fields, allowing for image-guided catheter navigation.
      • A magnetic tip catheter is introduced into a vessel and manipulated using external steering magnets.

What is Interventional Radiology?

  • Interventional radiology is a specialized field using imaging guidance to perform minimally invasive procedures.

Types of Interventional Radiology (IR) Procedures

  • Angiography (1930s):
    • Opacification of vessels through injection of contrast media.
      • Transbrachial (1960s).
      • Transfemoral (1960s).
  • Angioplasty
  • Thrombolysis
  • Embolization
  • Vascular Stents
  • Biopsies
    • Completed through vascular access.

Digital Subtraction Angiography (DSA)

  • Minicomputers and microprocessors in Digital Radiography (DR) are important for controlling:

    • Matrix size
    • Dynamic range
    • Image acquisition rate

  • Analog-to-Digital Converter (ADC):

    • For data to be compatible, the ADC must have the same dynamic range as the digital system.
    • Electrical signal → ADC → Digital Signal → memory → matrix.
    • If the image storage is in the primary memory, acquisition and transfer can be as fast as 30 images/sec.
    • As matrix size doubles, the time for image acquisition/transfer takes 4x as long, which can limit temporal resolution or frame rate.

Digital Subtraction Methods

  • Temporal, energy, or hybrid subtraction.
Temporal Subtraction
  • Most common type of DSA.
  • Uses computer-assisted techniques.
  • Mask Mode:
    • A mask image is obtained before contrast injection.
    • Contrast is injected, and subsequent images are acquired.
    • The computer recognizes the initial mask image and removes that data from the subsequent images, leaving only the contrast-enhanced anatomy.
    • Subsequent images can be taken manually or preprogrammed to enhance the exam.
    • Remasking is necessary if movement occurs.
    • Image Integration:
      • Several video frames can be summed in memory to create a single image to compensate for noise due to rapid imaging (33 microseconds).
      • This process increases dose by increasing pulses.
Time Interval Mode
  • Each subtracted image requires its own new mask.
  • Commonly used in cardiac evaluation.

Digital Subtraction Artifacts

  • Misregistration Artifact:
    • Occurs when motion happens between the mask image and the subsequent image.
    • The anatomy is not present in the same pixel in both images.
      • Correction:
        • Can be corrected through reregistration of the mask by shifting the mask one or more pixels to achieve superimposition again.
  • Energy Subtraction:
    • Uses two different x-ray beams of two different emission spectrums alternately to provide a subtraction image resulting from differences in photoelectric (PE) interactions.
    • Requires alternating pulsing of different kVp beams.
  • Hybrid Subtraction:
    • Combination of temporal and energy subtraction.
    • Mask and subsequent images are created using the energy subtraction technique.
    • Produces the highest contrast images.

Roadmapping

  • Used in Digital Subtraction Angiography.
    • Mask image is acquired and stored.
    • Contrast is injected.
    • Subsequent subtracted images are obtained.
    • The catheter is entered into the vessel.
    • An image is formed by the subtraction of the second mask (the vessel with contrast) so the catheter can be visualized advancing.
    • It appears as a black guidewire in a white vessel.

Interventional Radiology Suite

Personnel and Equipment
  • Expensive, advanced radiographic and fluoroscopic equipment.
  • Ceiling-mounted x-ray tube and C-arm are common.
  • X-ray tube:
    • Designed for magnification and repetitive imaging.
    • Large anode and small target angle.
    • Small focal spot allows good resolution for small arteries.
  • High-frequency generators are popular, but some cases require 3-phase 12-pulse generators.
  • The Room:
    • Large rooms to accommodate equipment needs.
    • Door to allow easy transfer of hospital bed.
  • The Personnel:
    • Specialized radiographer (2-3 in the room).
      • ARRT offers Cardiovascular and Interventional Exams.
    • Interventional Radiologist.
    • Radiology Nurse.
    • Anesthesiologist (when anesthesia is necessary).

Arterial Access Procedure

  • Insertion of needle with a stylet.
  • Stylet removed.
  • Insertion of guidewire.
  • Removal of needle.
  • Catheter threaded onto guidewire.
  • Common Femoral Artery (CFA) is the most often accessed artery.

Consumable Equipment Details

Guidewires
  • Made of stainless steel with a hydrophilic polymer coating.
  • Classified by length, size, stiffness, and coating.
  • May be J-tipped for areas of arteriosclerosis.
Catheters
  • Diameter described in French units (3 Fr = 1mm).
    • Diameter (mm)=French Size3\text{Diameter (mm)} = \frac{\text{French Size}}{3}
  • Come in different shapes.
  • Have side holes for contrast injection without whiplash.
  • Once inserted, the guidewire can be removed.
  • Must be flushed immediately to prevent clotting (heparinized saline).

Patient Preparation and Monitoring

Pre-Procedure
  • Patient is referred.
  • Meets with interventional radiologist.
  • History and physical are acquired.
  • Patient begins preparation, including IV hydration and a clear liquid diet.
Monitoring Throughout the Procedure
  • Blood Pressure (BP).
  • Pulse oximetry.
  • Electrocardiogram (ECG).
Post-Procedure
  • Compression to access site.
  • Immobilization and monitoring for several hours.

Risks of Arteriography

  • Continuation of bleeding.
  • Contrast media reactions.
  • Kidney failure.
Risk Minimization
  • Medical examination.
  • Allergy history evaluation.
  • Kidney function labs.
  • Medication evaluation.

Contrast Media: Ionic vs. Non-ionic

  • Risks associated with different types of contrast media.

Summary of Interventional Imaging Benefits

  • Less invasive
  • Less expensive
  • Faster
  • Often no hospital stay
  • Has replaced many surgical procedures
  • Highly reliant on skilled interventional radiologists and technologists
  • Highly team-dependent