Intravascular Magnetic Resonance Imaging Albert C. Lardo, Ph.D.

Assistant Professor of Radiology, Biomedical Engineering and Surgery - Johns Hopkins University School of Medicine

andChief Scientific Officer - Surgi-Vision, Inc.

Intravascular Coils – Two Modes

• Active visualization of catheters, guidewires and devices

• Enables MR-guided intervention

Tracking

• Very high local signal for imaging structures directly adjacent to the coil

• Enables diagnostic vascular imaging

Imaging

External surface coils• Cardiac mechanical function• Perfusion• Metabolic Imaging• Angiography

• Plaque characterization and therapy assessment??

Problem

Spatial resolution limitation (~1 mm)

Cardiovascular MRI - From the Outside In

Why do we need more spatial resolution?

External

Aorta ~10

Renal ~7

Iliac ~5

Coronary ~7

Artery

Distance from Coil (cm)

Chest depth (180lb male) ~300mm

Quantitative assessment of deep thoracic arteries commonly prone to atherosclerotic disease (e.g. aorta, renal, iliac, coronary)

Goal: Detection of small but potentially unstable plaques

Fibrous cap detection (70-400m)

Intravascular MRI - From the Inside Out

MR Coil Miniaturization

Endovascular MRI – Placement of small radiofrequency receiver antennas (coils) directly into blood vessels for active tracking and high resolution imaging (Atalar et al 1996).

cm100-185 cm

Coaxial transmission line

DTC

Decoupling and tuning circuit

Scanner

Inner conductorOuter conductor

MR Signal Strength Depends on Coil Distance from the Energy Source

Sign

al

Distance

Energy Source

CoilTissue

Distance from coil (cm)

SNR

Internal vs. External Coil

External

Aorta ~10 0.4, 2

Renal ~7 ~0.4

Iliac ~4.5 ~0.6

Coronary ~7.5 ~0.2

InternalArteryDistance from Coil (cm)

Chest depth (180lb male) ~300mm

Distance from Coil and Structure of Interest

Internal Coil

2 106

External Coil

10 26

Endovascular Coils - Two Designs

• High local signal and moderate resolution

• Signal falls off with distance (1/R)

• Can be made very small and flexible (>1F)

• Very high local signal and thus resolution

• Signal falls off very rapidly with distance (1/R2)

• Larger in size and less flexible

Loopless Loop

Resolution ~ 200 m Resolution ~ 60 m

MRI Intravascular Guidewire

• Loopless antenna design• Gold plated Nitinol core• Nitinol tubing• Intravascular placement• OD = 0.032” and 0.014”• Fully tuned and decoupled

Antenna whip

Nitinol tube

Interactive Interface

Gutman, McVeigh, NHLBI

MR-guided Applications

• Intravascular MR imaging• MR-guided Catheterization• Real-Time Angiography• MR-guided Balloon Angioplasty• MR-guided Stent Placement• MR-guided Invasive Electrophysiology•MR-Guided Gene Therapy

Require Imaging and Tracking

IVUS versus IVMRI

Intravascular UltrasoundIntravascular Ultrasound Intravascular MRIIntravascular MRI

Explanted Human Aortic Specimen

Atalar et al.

Intravascular Rabbit Aortic Imaging

DIR-FSETE=20ms, ETL= 20, FOV=6cm

DIR-FSETE=26.4ms, ETL= 20, FOV=4cm

Resolution – 150mResolution – 234m

Yucel et al. Serfaty et al.

Intravascular MRI of Watanabe Rabbits

mmB

FSE, 1200/13-msec TR/TE, Double IR blood suppression, 16 ETL, 4-cm FOV, 32 NEX,256x256 matrix

ACM

• Resolution: 150 µm

Watanabe rabbit with a 0.032” MRI-Guidewire

Serfaty et al.

Intravenous Guidewire - Swine

0.16x0.16x4 mm, TR 531, TE 11, BSP, 6 NEX, 2:25

stent

iliac a.

iliac v.

Bluemke et al.

Cardiac Coil Ch2 Cardiac Coil Ch3Cardiac Coil Ch1

All Channels

External vs. Endovascular CoilIliac Imaging - Pig

Lederman et al.DIR-FSE TR=1558ms, TE=26ms, ETL= 24, FOV=4cm

Endovascular coil

156 m resolution

MR ScoutX-ray Fluoroscopy High resolution MR

Iliac wallvein

R Iliac

DIR-FSETR=2000ms, TE=6.1ms,

ETL= 24, FOV=10cm, st=4 mm

Human IVMRI Iliac Imaging

Yucel et al.

Design

Plaq

ue

Plaq

ue

Vessel

0.014” MRIGBalloon - 3% Gd

Gd/blue-dye medium or Gd/GFP-lentivirus medium

Xiaoming Yang, et al.Johns Hopkins Hospital

Control

Gd/GFP-lentivirus

Immunohistochemistry

G

H

Xiaoming Yang, et al.Johns Hopkins Hospital

MR imaging of Gd/GFP-lentivirus transfer

a

fed

cb

Summary

• Intravascular MR coils and guidewires provide active device localization and high resolution imaging

• MRI guided cardiovascular intervention is feasible on currently available scanners

• Further improvements in spatial and temporal resolution, device localization and MR scanner design are required for general clinical use

Challenges

• Further loop coil miniaturization for coronary imaging

• Effects of cardiac motion during coronary imaging

• Real-time quantitative device localization• Reduction of image acquisition time