Contrast Enhanced MRA How I do it

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1 Contrast Enhanced MRA How I do it Scott B. Reeder, MD, PhD International Society for Magnetic Resonance in Medicine Sociedad Mexicana de Radiologia e Imagen (SMRI) Mexico City June 4, 2014 Department of Radiology University of Wisconsin Madison, WI

2 Disclosure UW receives support from GE and Bracco Off-label uses of Gadolinium contrast Investigational Pulse Sequences

3 Learning Objectives Understand the basic physics of contrast enhanced MRA Be familiar with common applications and diagnoses Advantages and considerations of CE-MRA at 3.0T

4 MRA Methods Renal Artery MRA Time-of-Flight Phase Contrast Contrast-Enhanced

5 Contrast Enhanced MRA*: T1 Shortening of Gadolinium Gadolinium agents shorten T1 dramatically 1 r1 Gd T1 During IV bolus, Gadolinium concentrates in arteries for about 1 minute Gadolinium is a potent T1 relaxation agent T1 of blood: 1200 ms <100 ms *Contrast-enhanced MRA is an off label use

6 Contrast Enhanced MRA: T1 Shortening of Gadolinium 30o Increasing Gd concentration (shortened T1) Poor contrast at low flip angles 5mM Background suppression in non-enhancing tissues

7 k-space ky Image Detail kx Contrast

8 Contrast Curve signal Elliptic Centric Acquisition time k-space Data

9 Contrast-enhanced MRA Pre During Post

10 Contrast-enhanced MRA During Post

11 Principles of MRA: Contrast Timing Critical to understand k-space behavior Center of k-space: contrast/brightness Periphery of k-space: detail Goal: time peak of bolus while acquiring central lines of k-space

12 Case: 81yo F with HTN resistant to medication Differential Perfusion Phase Contrast

13 Visualization and Grading of Stenoses Grading based on diameter stenoses Mild <50% Moderate 50-75% Severe >75% Do NOT rely on MIP images to grade stenosis Must use thin source images or thin MPR reformats

14 MRA: Cross-sectional Multi-Planar Reformats Case Courtesy Stefan Schoenberg, MD

15 Courtesy of G. Schneider Case: 36yo F with h/o HTN

16 Case: 29yo F, 3 days s/p MVA with new HTN

17 Case: 29yo F, 3 days s/p MVA with new HTN Page Kidney from Intracapsular Hematoma

18 Why Time Resolved MRA? Physiological information Collaterals (eg. coarctation, occlusion) Shunting (eg. AV fistula, congenital disease) Reversed flow (eg. Pelvic congestion syndrome, endoleaks) Filling patterns in aortic dissection Enhancement of hypervascular tumors Many more applications Timing Timing may be difficult or impossible in some applications Avoids the need for timing bolus: point and shoot

19 Approaches to Time Resolved MRA Brute force Commercial Repeat acquisition many times State of the Art Parallel imaging can help Inefficient, usually inadequate Exploit k-space behavior of 3D-MRA TRICKS/TWIST, keyhole imaging Exploit sparsity of MRA imaging Undersampled projection reconstruction On the near Exploit constraints of fixed anatomy Horizon HYPR, compressed sensing methods Combinations of the above

20 k-space ky Image Detail kx Contrast

21 5% 10 % 20 % 50 %

22 5% 10 % 20 % 50 %

23 3D Time Resolved Imaging of Contrast Kinetics (TRICKS) (TWIST, TRACKS) kz kz ky D CB A B C D ky Korosec et al., Magn. Reson. Med. 1996

24 3D TRICKS: Technique Artery Contrast curve Vein Time frame D A C A B A D A C A B A D... A B(I) C(I) D(I) FFT Image at time frame 15...

25 3D TRICKS TR = 10.8 (1996) 512 x 128 x16 Frame Time 5.6 s construction time 1996: 6 hours, one graduate studen

26 Pelvic Congestion Syndrome

27 Bolus-chase MRA 0.1mmol/kg Gd shared btw 2 & 3 2. Pelvis: Reverse EC 2 3. Thighs: EC 3 1. Distal Station Time-resolved MRA 10 cc Gd at 2cc/sec 1

28 Indications: Pulmonary Embolus Difficult to diagnose clinically Potentially fatal CTA commonly used to diagnose PE PE uncommon (5% of CTA positive) Young patients Large radiation dose Historically, MRA limited by scan time and spatial coverage Parallel imaging for improved coverage

29 1.5T Thoracic MRA 2D Parallel Imaging (ARC) Coronal (Acquisition Plane) Axial Reformats 1.2 x 1.4 x 2.0mm3 in 13-19s Sagittal Reformats

30 Normal Pulmonary Angiogram

31 Pulmonary MRA: r/o Pulmonary Embolus LLL PE

32 Pulmonary MRA: r/o Pulmonary Embolus RUL PE

33 Pulmonary MRA: Value of Perfusion 33 yr old woman with right pleuritic chest pain Coronal Axial MPR Sagittal MPR Cutoff vessel leading into perfusion defect Double Oblique Thin Slab MIP

35 Case: 61yo M with Pneumonia, r/o PE Poor opacification of LLL PA s Could not exclude PE. Pulmonary MRA no PE on MRA

36 Case: 61yo M with Pneumonia, r/o PE Pulmonary abscess, empyema

37 Advantages of High Field Strength Why use 3T for MRA??

38 Physics of 3.0T MRI: Increased SNR Higher field strength SNR proportional to field strength: 2x SNR Increased spatial resolution Shortened Scan Times Opportunity to reduce scan time with parallel imaging with minimal to no SNR penalty All methods benefit from increased SNR

39 Physics of 3.0T MRI: Improved CNR Longer T1 at 3T Improved Background Suppression Results in improved CNR of enhancing tissue Overall: improved CNR increased SNR background suppression Most important benefit of TOF and CE-MRA at 3T

40 Relaxivity of Gadolinium Drops at Higher Field 1 = r1 [Gd ] Enhancement Gad T1 Concentration Relaxivity: Bang for your $$ Bo (T) Gd-DPTA Gd-BT-DO3A Gd-BOPTA (Magnevist) (Gadavist) (Multihance) 0.2 T T T Pintaske et al, Invest. Radiol. 2006

41 Physics of 3.0T MRI: Decreased Relaxivity Decrease in the relaxivity of Gadolinium at 3T Reduces the bang for your buck of contrast Relatively small effect, outweighed by improved SNR and increased background T1

42 Physics of 3.0T MRI: Improved Background Suppression Enhancing Arterial Blood Unenhancing Tissue at 1.5T Unenhancing Tissue at 3.0T Longer T1 Suppresses background signal and improves CNR

43 Improved CNR at 3.0T: Summary Improved SNR alone improves CNR by 2x Also have improved contrast from T1 effect Small reduction in the relaxivity of Gd for CEMRA Overall, Contrast to Noise Ratio improves by much larger factor, perhaps as high at 3x (prediction) Precise improvement difficult to measure

44 Physics of 3.0T MRI: Parallel Imaging Parallel imaging works better at higher field strength! Smaller wavelengths at 3T Better SNR performance (lower g-factor) More SNR to burn 2x field strength translates to 4x acceleration with same SNR!! Particularly true at higher accelerations, 2D acceleration Full capabilities of parallel imaging at 3T still being explored Parallel Imaging Gives 3T imaging the Flexibility to trade the 2x SNR for 4x faster scanning

45 Disadvantages: Challenges of MRA at 3T Field (Bo) Inhomogeneities RF (B1) Inhomogeneities SAR

46 Physics of 3.0T MRI: Bo Field Homogeneity Increasing field strength worsens magnetic field inhomogeneity due to increased susceptibility Susceptibility from stainless steel implants or ferromagnetic foreign bodies does NOT worsen at 3T: induced magnetization is already saturated Relatively small impact on most MRA sequences Short TR high bandwidth Not a major issue Inject contrast more slowly to prevent T2* effect

47 Physics of 3.0T MRI: RF (B1) Inhomogeneities Created by dielectric effects and shorter wavelength of the higher field imaging Results in non-uniform B1 amplitude Manifests as non-uniform flip angle

48 Physics of 3.0T MRI: RF (B1) Inhomogeneities Enhancing Arterial Blood Broad signal response Unenhancing Tissue MRA methods relatively insensitive to flip angle and RF inhomogeneitites

49 RF Heating: Specific Absorption Rate (SAR) Major challenge for high flip angle sequences Fast Spin-Echo (FSE) Steady State Free Precession * (SSFP, FIESTA, truefisp, BFFE) These sequences not typically used for MRA RF Pulse Duration RF Amplitude - doubles at 3T

50 RF Heating: Specific Absorption Rate (SAR) CE-MRA uses body coil for transmit Very bad for SAR, but High flip angle (30o), short TR (3ms) Scan duration limited (20-30s), which limits SAR Parallel imaging reduces SAR if shorter scan times used

51 Pulmonary MRA: 3T 19s breath-hold 1.2 x 1.3 x 1.6 mm3 (0.6 x 0.6 x 0.8 mm3)

52 Pre-surgical Localization : Artery of Adamkiewicz 1. The Artery of Adamkiewicz (AOA) is a tiny artery ( mm) Requires very high spatial resolution 2. The AOA has a variable origin A large field of view (FOV) is needed to ensure visualization Vein Artery 3. Great anterior radiculomedullary vein can mistaken for the AOA 4. Bolus tracking techniques are difficult to time precisely in TAA patients Nijenhuis et al, AJNR 2006

53 Anterior Spinal Artery MRA: Excellent anatomical detail Time-resolved MRA at 3T Yoshioka, K et al. Radiographics 2006

54 Results: Examples Bley et al Radiology 2010, 255(3):873-81

55 Results: Arterial-Venous Separation with TR-MRA at 3.0T No se puede mostrar la imagen. Puede que su equipo no tenga suficiente memoria para abrir la imagen o que ésta esté dañada. Reinicie el equipo y, a continuación, abra el archiv o de nuev o. Si sigue apareciendo la x roja, puede que tenga que borrar la imagen e insertarla de nuev o. Phase 3 No se puede mostrar la imagen. Puede que su equipo no tenga suficiente memoria para abrir la imagen o que ésta esté dañada. Reinicie el equipo y, a continuación, abra el archiv o de nuev o. Si sigue apareciendo la x roja, puede que tenga que borrar la imagen e insertarla de nuev o. Phase 5 No se puede mostrar la imagen. Puede que su equipo no tenga suficiente memoria para abrir la imagen o que ésta esté dañada. Reinicie el equipo y, a continuación, abra el archiv o de nuev o. Si sigue apareciendo la x roja, puede que tenga que borrar la imagen e insertarla de nuev o. Phase 7

56 Learning Objectives Understand the basic physics of contrast enhanced MRA Be familiar with common applications and diagnoses Advantages and considerations of CE-MRA at 3.0T

57 Thank you! Tom Grist, MD Chris Francois, MD Scott Nagle, MD, PhD Mark Schiebler, MD

GE 3.0T NPW,TRF,FAST,F R NPW,TRF,FAST,F R

GE 3.0T NPW,TRF,FAST,F R NPW,TRF,FAST,F R GE 3.0T 3.0T WRIST Invivo 8CH Wrist Coil Sequence Ax T2 Cor PD Cor PDFS Cor T1 Cor PD (Small FOV) FOV (mm) 80 80 80 80 40 Matrix 384x224 384x256 320x256 384x320 320x192 Phase Direction RL RL RL RL RL #