In vivo stem cell tracking in the myocardium of small laboratory animals: Feasibility and acquisition strategies on clinical magnetic resonance

Transcription

1 In vivo stem cell tracking in the myocardium of small laboratory animals: Feasibility and acquisition strategies on clinical magnetic resonance imaging (MRI) systems Piotr A. Wielopolski Erasmus MC Department of Radiology, Rotterdam, The Netherlands ENCITE Prague May 7 8, 2009

2 Introduction In an effort to better understand cardiovascular disease and possible therapeutic measures, small animal investigational models are increasingly used Although cardiac MRI excels at measuring myocardial mass, end systolic and end diastolic volumes, ejection fraction, stress and strain, blood flow patterns Yet, the evaluation in rodents presents so far enormous challenges

3 Introduction To understand the difficulties, differences in heart dimensions and functional parameters have to be considered between humans and rodents: Human Rat Mouse Body weight (g) 70, Heart weight (g) Heart diameter (mm) Heart rate (bpm) Blood volume (ml) Breathing rate (breaths/min) Total ventilation (L/min)

4 From human to rodents: required voxel sizes Taking the human heart as reference, a reduction in voxel volume greater than 300 or 3000 must be accounted for when imaging rat and mouse hearts, respectively A cine scans on humans at 1.5T using the CINE SSFP (steady state free precession) technique uses: voxel resolution ~ 1.8 x 1.8 x 8.0 mm 3 voxel volume of ~ 26 mm 3 temporal resolution of 40 ms comfortable breath hold To image rodent hearts with equivalent resolutions, voxel sizes of at least for rats ~ 0.09 mm 3 > x x 1.0 mm 3 for mice ~ mm 3 > x x 0.9 mm 3

5 Clinical MRI scanner availability Work with small animal models has been performed on dedicated small animal MRI systems at higher field strengths ( Tesla) With a widely available base of clinical scanners in research institutions, we need to explore in depth the imaging possibilities of clinical MRI scanners for small animal cardiac work

6 In a glimpse Understand the wide range of possibilities present in clinical MRI scanners for stem cell tracking in rodent hearts Appreciate the benefits of clinical whole body scanning platforms Know which artifacts and pitfalls to expect from the lower gradient performance of clinical imaging platforms and how to overcome the limitations

7 Comparison with animal scanners Clinical MRI systems with imaging gradient hardware up to 50 mt/m and high slew rates (200 mt/m/msec) are typically found in research institutions This gradient performance is poor in comparison to that found in animal scanners ( mt/m, slew rates mt/m/msec) Therefore, on clinical systems, spatial and temporal resolution must be compromised

8 Voxel resolution and gradient hardware considerations High resolution is virtually possible with any gradient imaging hardware However, the maximum resolution achievable by any MRI scanner, being clinical or dedicated animal, is not exclusively dependent on the strength of the gradient system alone

9 Voxel resolution and gradient hardware considerations the readout time (or TR), the time that it takes to acquire a specific number of points along the frequency direction, influences most dramatically the achievable spatial resolution There are limits to the choice of readout time for a particular imaging sequence dependent on the length of T2 or T2* of the tissues to image sensitivity to motion magnetic homogeneity

10 MR Histology 3.0T Rat heart A proton density weighted and T2 weighted FSE scans and a T1 weighted rf spoiled GRE scan voxel size of 40 x 40 x 600 µm cm loop PDW FSE T2W FSE T1W SPGR 1.5T < 35 minutes 3D T1 weighted scan compared to proton density and T2 weighted echo planar imaging (EPI) T1W SPGR PDW EPI T2W EPI voxel size of 700 x 500 x 600 µm cm loop

11 Voxel resolution and gradient hardware considerations Rat heart 3.0T 260 µm 170 µm 130 µm 1 cm Black blood FSE

12 Cardiac imaging potential on MRI clinical platforms 1 cm 1 cm 10 cm mouse rat human Functional and anatomical cardiac imaging can be performed effectively in clinical scanners with minor software modifications and with the inclusion of specialized surface coils for high SNR and acquisition throughput Scanning can be performed effectively down to 100 µm 2 in plane resolution with thin slices (0.5 mm) in mice with all relevant clinical sequences as applied to humans

13 SNR enhancement: dedicated receiver hardware Custom designed coils are necessary to enhance SNR Large field of view imaging accomplished with multi channel interfaces and multiple coils Coils Quadrature bird cage 2.0 cm loop 1.0 cm loop Interfaces 4 channel manifold 8 channel manifold

14 Comparison between coils used 3.0T 1 cm 1 cm Quadrature birdcage 4-channel interface with 2.0 cm loops Same acquisition time, voxel resolution 2 x SNR

15 Physiological monitoring Electrocardiographic (ECG) and pulse gating (PG) monitoring can be used with heart rates up to 400 beats per minute (bpm) For ECG gating, 4 electrode pads without the gel interface on front and back paws of the animals With PG, the optical sensor can be attached to the right/left front paw PG is preferred over ECG gating. Easier setup and no gradient interference Electrocardiographic gating (ECG) and setup Pulse gating (PG)

16 Clinical scanners: transparent work flow All cardiac pulse sequences available for humans can be applied for imaging rodents at high spatial resolution and reasonable temporal resolution 2D CINE rf spoiled GRE 2D CINE steady state free precession (SSFP) Line and grid tagging 2D CINE rf spoiled GRE 2D blood suppressed (black blood) fast spin echo (FSE) 2D and 3D T1 weighted inversion recovery prepared rf spoiled GRE 3D rf spoiled GRE cardiac gated or free running acquisition Clinical scanners offer the advantage of an improved user interface and pulse sequence design and programming and scanning possibilities

17 Rat heart myocardial infarction Rat heart 3.0T

18 Rat heart myocardial infarction

19 Rat functional imaging (strain stress)

20 Temporal resolution for scanning Most cardiac sequences use segmented k space acquisitions, the time resolution per phase is set accordingly depending on resolution A temporal resolution of 30 ms per cardiac phase for CINE scans and 50 ms for acquisitions limited to mid late diastole are reasonable This temporal resolution can be achieved using 1.0 mm and 0.9 mm sections in rats and mice, respectively, by adjusting number of lines/phase and readout bandwidth accordingly at the set target voxel volume Lower temporal resolution can be used for tracking labeled cells (augment averaging, improve scan consistency, higher SNR)

21 Labeled stem cell scanning strategies Bright blood gradient recalled echo imaging (GRE) sequences Gated Non gated ECG ECG + respiratory Self gated/navigated Multiple averaging 2D 3D Reconstructions in any instance in the respiratory and cardiac cycle radial scanning

22 Labeled stem cell scanning strategies 4.7T 2D short TE radial sequence 400 radials/image, TE=630μs, TR=80ms, flip angle=30deg, FOV=35mm, slice thickness = 1.2mm Concluded that overall image quality and sharpness were comparable. SNR per unit time are equivalent for SA and CRG. Due to discarded radial acquisitions, RSG method results in a reduced SNR/unit time ISMRM2009 Constant repetition time imaging protocols for high resolution lung proton MR Imaging in mice M. Zurek 1, A. Bessaad1, K. Cieslar1, and Y. Crémillieux1 Université de Lyon, CREATIS LRMN, Lyon, France

23 Rat heart Cryoinfarct + SPIO labeled cells 3.0T BB FSE TE=9 ms BB FSE TE=30 ms BB FSE TE=50 ms cine 3D cell track MDE 3D PD 3D ~T1w

24 Tracking SPIO loaded cells 5 x 10 5 SPIO labeled stem cells 3.0T MDE Multiple averaging freerunning 3D scan 20 minute scan 160 x 180 x 300 µm 3 Pre contrast Post contrast Extra T1 weighting

25 Visualization: sensitivity to magnetic susceptibility Dephasing / blooming effect increase with longer TE on GRE scans 3D 3.0T Day 0 TE=2.7 ms TE=3.7 ms TE=7.0 ms Longer TE, leads to increased signal loss close to the air interfaces, blood dephasing (no flow compensation used for imaging) Higher gradient strengths can eliminate blood dephasing (flow compensation) while higher magnetic fields could improve sensitivity with less artifacts at shorter TE

26 SPIO labeled stem cell tracking over time 5 x 10 5 SPIO labeled mesenchymal stem cells (MSCs) injected in healthy myocardium (arrows) can be tracked over the course of 4 weeks or longer 3D 3.0T Day 0 Day 4 Day 11 Day 15 Day 20 Day 22 Day 25 TR/TE/flip angle = 19/3.5 ms/4, FOV = 40 mm, 256 x 256, 65 Hz/pix readout bandwidth, 0.4 mm slices interpolated to 0.2 mm, number of excitations NEX = 6, 17 minute acquisition time 2D cine (TR/TE/flip angle = 18/4.5 ms/18, FOV = 40 mm, 256 x 256, 65 Hz/pix readout bandwidth, 0.7 mm slices, NEX = 10, 90 minute acquisition time)

27 SPIO tracking over time 2D cine 3.0T Day 0 Day 22 The cells could be visualized in a 3.5 mm axial volume with decreasing demarcation of the signal voids over time. Scans performed using a 4 channel receiver with 2.0 cm coils

28 Contrast improvements SPIO labeled cell tracking Gd loaded liposomes for vascular imaging: improved definition of cardiac chambers Multiple averaging free running T1 weighted 3D scan 3.0T 1 cm TR/TE = 41.7/2.8 ms, flip angle = 40, FOV = 60 mm, read bandwidth = 31.2 khz Matrix size = 512 x 320, NEX = 3, acquisition time = 35 minutes Voxel size = 117 x 187 x 100 µm 3

29 Take home points A balance between readout bandwidth and temporal resolution at lower magnetic field strengths using the slower imaging gradient set of the clinical scanner can provide effective cardiac imaging with appropriate in plane and through plane resolution with adequate SNR and CNR/time Stem cell tracking at high resolution and good SNR is readily possible on a clinical MRI for rodent cardiac applications given that specialized receiver hardware is available (coils)

30 Acknowledgements/ Collaborators Dept of Radiology Monique Bernsen Sandra van Tiel Jamal Guenoun Joost Haeck Gaby Doeswijk Gerben van Buul Leila Alic Jifke Veenland Erik Meijring Ihor Smal Esben Plenge Gyula Kotek Piotr Wielopolski Wiro Niessen Gabriel Krestin Dept of Surgical Oncology Gerben Koning Timo ten Hagen Lex Eggermont Dept of Orthopedics Eric Farrell Gerben van Buul Nicole Kops Koen Bos Gerjo van Osch Harrie Weinans FP7 ENCITE Dept of Exp. Cardiology Heleen van Beusekom Dirk Duncker Dept of Cardiology Robert Jan van Geuns Wim van der Giessen TU Delft Ulla Woronieck Dept of Cell Biology and Genetics. Paula van Heijningen Jeroen Essers TU Eindhoven Gustav Strijkers Klaas Nicolay GE Healthcare Gavin Houston