Calcium ions represent one of the key second messengers accompanying neural activity and synaptic signaling. Accordingly, dynamic imaging of calcium fluctuations in living organisms represents a cornerstone technology for discovering neural mechanisms that underlie memory, determine behavior, and modulate emotional states as well as how these mechanisms are perturbed by neurological disease and brain injury. While optical technologies are well established for high resolution imaging of calcium dynamics, physical limits on light penetration hinder their application for whole-brain imaging in intact vertebrates. Unlike optics, magnetic resonance imaging (MRI) enables noninvasive large-scale imaging across vertebrates of all sizes. This has motivated the development of several sensors that leverage innovative physicochemical mechanisms to sensitize MRI contrast to intracellular and extracellular changes in calcium. Here, we review the current state-of-the-art in MRI-based calcium sensors, focusing on fundamental aspects of sensor performance, in vivo applications, and challenges related to sensitivity. We also highlight how innovations at the intersection of reporter gene technology and gene delivery open potential opportunities for mapping calcium activity in genetically targeted cells, complementing the benefits of small molecule probes and nanoparticle sensors.

1.
H.
Dana
,
Y.
Sun
,
B.
Mohar
,
B. K.
Hulse
,
A. M.
Kerlin
,
J. P.
Hasseman
,
G.
Tsegaye
,
A.
Tsang
,
A.
Wong
, and
R.
Patel
, “
High-performance calcium sensors for imaging activity in neuronal populations and microcompartments
,”
Nat. Methods
16
(
7
),
649
657
(
2019
).
2.
Y.
Zhao
,
S.
Araki
,
J.
Wu
,
T.
Teramoto
,
Y.-F.
Chang
,
M.
Nakano
,
A. S.
Abdelfattah
,
M.
Fujiwara
,
T.
Ishihara
, and
T.
Nagai
, “
An expanded palette of genetically encoded Ca2+ indicators
,”
Science
333
(
6051
),
1888
1891
(
2011
).
3.
T.-W.
Chen
,
T. J.
Wardill
,
Y.
Sun
,
S. R.
Pulver
,
S. L.
Renninger
,
A.
Baohan
,
E. R.
Schreiter
,
R. A.
Kerr
,
M. B.
Orger
, and
V.
Jayaraman
, “
Ultrasensitive fluorescent proteins for imaging neuronal activity
,”
Nature
499
(
7458
),
295
300
(
2013
).
4.
C.
Grienberger
and
A.
Konnerth
, “
Imaging calcium in neurons
,”
Neuron
73
(
5
),
862
885
(
2012
).
5.
M. S.
Rad
,
Y.
Choi
,
L. B.
Cohen
,
B. J.
Baker
,
S.
Zhong
,
D. A.
Storace
, and
O. R.
Braubach
, “
Voltage and calcium imaging of brain activity
,”
Biophys. J.
113
(
10
),
2160
2167
(
2017
).
6.
A.
Birkner
,
C. H.
Tischbirek
, and
A.
Konnerth
, “
Improved deep two-photon calcium imaging in vivo
,”
Cell Calcium
64
,
29
35
(
2017
).
7.
A.
Cheng
,
J. T.
Gonçalves
,
P.
Golshani
,
K.
Arisaka
, and
C.
Portera-Cailliau
, “
Simultaneous two-photon calcium imaging at different depths with spatiotemporal multiplexing
,”
Nat. Methods
8
(
2
),
139
142
(
2011
).
8.
D. A.
Dombeck
,
A. N.
Khabbaz
,
F.
Collman
,
T. L.
Adelman
, and
D. W.
Tank
, “
Imaging large-scale neural activity with cellular resolution in awake, mobile mice
,”
Neuron
56
(
1
),
43
57
(
2007
).
9.
F.
Helmchen
and
W.
Denk
, “
Deep tissue two-photon microscopy
,”
Nat. Methods
2
(
12
),
932
940
(
2005
).
10.
G.
Hong
,
S.
Diao
,
J.
Chang
,
A. L.
Antaris
,
C.
Chen
,
B.
Zhang
,
S.
Zhao
,
D. N.
Atochin
,
P. L.
Huang
, and
K. I.
Andreasson
, “
Through-skull fluorescence imaging of the brain in a new near-infrared window
,”
Nat. Photonics
8
(
9
),
723
730
(
2014
).
11.
N. G.
Horton
,
K.
Wang
,
D.
Kobat
,
C. G.
Clark
,
F. W.
Wise
,
C. B.
Schaffer
, and
C.
Xu
, “
In vivo three-photon microscopy of subcortical structures within an intact mouse brain
,”
Nat. Photonics
7
(
3
),
205
209
(
2013
).
12.
J.-H.
Park
,
W.
Sun
, and
M.
Cui
, “
High-resolution in vivo imaging of mouse brain through the intact skull
,”
Proc. Nat. Acad. Sci.
112
(
30
),
9236
9241
(
2015
).
13.
K. T.
Takasaki
,
D.
Tsyboulski
, and
J.
Waters
, “
Dual-plane 3-photon microscopy with remote focusing
,”
Biomed. Opt. Express
10
(
11
),
5585
5599
(
2019
).
14.
K.
Takasaki
,
R.
Abbasi-Asl
, and
J.
Waters
, “
Superficial bound of the depth limit of two-photon imaging in mouse brain
,”
eNeuro
7
(
1
),
ENEURO.0255–19.2019
(
2020
).
15.
G.
Cui
,
S. B.
Jun
,
X.
Jin
,
M. D.
Pham
,
S. S.
Vogel
,
D. M.
Lovinger
, and
R. M.
Costa
, “
Concurrent activation of striatal direct and indirect pathways during action initiation
,”
Nature
494
(
7436
),
238
242
(
2013
).
16.
K. K.
Ghosh
,
L. D.
Burns
,
E. D.
Cocker
,
A.
Nimmerjahn
,
Y.
Ziv
,
A. E.
Gamal
, and
M. J.
Schnitzer
, “
Miniaturized integration of a fluorescence microscope
,”
Nat. Methods
8
(
10
),
871
(
2011
).
17.
L. A.
Gunaydin
,
L.
Grosenick
,
J. C.
Finkelstein
,
I. V.
Kauvar
,
L. E.
Fenno
,
A.
Adhikari
,
S.
Lammel
,
J. J.
Mirzabekov
,
R. D.
Airan
, and
K. A.
Zalocusky
, “
Natural neural projection dynamics underlying social behavior
,”
Cell
157
(
7
),
1535
1551
(
2014
).
18.
S.
Ghosh
,
P.
Harvey
,
J. C.
Simon
, and
A.
Jasanoff
, “
Probing the brain with molecular fMRI
,”
Curr. Opin. Neurobiol.
50
,
201
210
(
2018
).
19.
T.
Lee
,
L. X.
Cai
,
V. S.
Lelyveld
,
A.
Hai
, and
A.
Jasanoff
, “
Molecular-level functional magnetic resonance imaging of dopaminergic signaling
,”
Science
344
(
6183
),
533
535
(
2014
).
20.
S.
Ogawa
,
T.-M.
Lee
,
A. R.
Kay
, and
D. W.
Tank
, “
Brain magnetic resonance imaging with contrast dependent on blood oxygenation
,”
Proc. Nat. Acad. Sci.
87
(
24
),
9868
9872
(
1990
).
21.
X.
Yu
,
Y.
He
,
M.
Wang
,
H.
Merkle
,
S. J.
Dodd
,
A. C.
Silva
, and
A. P.
Koretsky
, “
Sensory and optogenetically driven single-vessel fMRI
,”
Nat. Methods
13
(
4
),
337
340
(
2016
).
22.
W.-h.
Li
,
S. E.
Fraser
, and
T. J.
Meade
, “
A calcium-sensitive magnetic resonance imaging contrast agent
,”
J. Am. Chem. Soc.
121
(
6
),
1413
1414
(
1999
).
23.
W.-h.
Li
,
G.
Parigi
,
M.
Fragai
,
C.
Luchinat
, and
T. J.
Meade
, “
Mechanistic studies of a calcium-dependent MRI contrast agent
,”
Inorg. Chem.
41
(
15
),
4018
4024
(
2002
).
24.
T.
Atanasijevic
,
M.
Shusteff
,
P.
Fam
, and
A.
Jasanoff
, “
Calcium-sensitive MRI contrast agents based on superparamagnetic iron oxide nanoparticles and calmodulin
,”
Proc. Nat. Acad. Sci.
103
(
40
),
14707
14712
(
2006
).
25.
G.
Angelovski
,
P.
Fouskova
,
I.
Mamedov
,
S.
Canals
,
E.
Toth
, and
N. K.
Logothetis
, “
Smart magnetic resonance imaging agents that sense extracellular calcium fluctuations
,”
ChemBioChem
9
(
11
),
1729
1734
(
2008
).
26.
K.
Dhingra
,
P.
Fousková
,
G.
Angelovski
,
M. E.
Maier
,
N. K.
Logothetis
, and
E.
Tóth
, “
Towards extracellular Ca2+ sensing by MRI: Synthesis and calcium-dependent 1H and 17O relaxation studies of two novel bismacrocyclic Gd3+ complexes
,”
JBIC J. Biol. Inorg. Chem.
13
(
1
),
35
46
(
2007
).
27.
K.
Dhingra
,
M. E.
Maier
,
M.
Beyerlein
,
G.
Angelovski
, and
N. K.
Logothetis
, “
Synthesis and characterization of a smart contrast agent sensitive to calcium
,”
Chem. Commun.
29
,
3444
3446
(
2008
).
28.
V.
Kubíček
,
T.
Vitha
,
J.
Kotek
,
P.
Hermann
,
L.
Vander Elst
,
R. N.
Muller
,
I.
Lukeš
, and
J. A.
Peters
, “
Towards MRI contrast agents responsive to Ca(II) and Mg(II) ions: Metal‐induced oligomerization of dota–bisphosphonate conjugates
,”
Contrast Media Mol. Imag.
5
(
5
),
294
296
(
2010
).
29.
I.
Mamedov
,
S.
Canals
,
J.
Henig
,
M.
Beyerlein
,
Y.
Murayama
,
H. A.
Mayer
,
N. K.
Logothetis
, and
G.
Angelovski
, “
In vivo characterization of a smart MRI agent that displays an inverse response to calcium concentration
,”
ACS Chem. Neurosci.
1
(
12
),
819
828
(
2010
).
30.
G.
Angelovski
,
T.
Chauvin
,
R.
Pohmann
,
N. K.
Logothetis
, and
É.
Tóth
, “
Calcium-responsive paramagnetic CEST agents
,”
Bioorganic Medic. Chem.
19
(
3
),
1097
1105
(
2011
).
31.
A.
Bar-Shir
,
A. A.
Gilad
,
K. W. Y.
Chan
,
G.
Liu
,
P. C. M.
van Zijl
,
J. W. M.
Bulte
, and
M. T.
McMahon
, “
Metal ion sensing using ion chemical exchange saturation transfer 19F magnetic resonance imaging
,”
J. Am. Chem. Soc.
135
(
33
),
12164
12167
(
2013
).
32.
G.
Angelovski
,
S.
Gottschalk
,
M.
Milošević
,
J.
Engelmann
,
G. E.
Hagberg
,
P.
Kadjane
,
P.
Andjus
, and
N. K.
Logothetis
, “
Investigation of a calcium-responsive contrast agent in cellular model systems: Feasibility for use as a smart molecular probe in functional MRI
,”
ACS Chem. Neurosci.
5
(
5
),
360
369
(
2014
).
33.
P.
Kadjane
,
C.
Platas-Iglesias
,
P.
Boehm-Sturm
,
V.
Truffault
,
G. E.
Hagberg
,
M.
Hoehn
,
N. K.
Logothetis
, and
G.
Angelovski
, “
Dual-frequency calcium-responsive MRI agents
,”
Chem. – A Eur. J.
20
(
24
),
7351
7362
(
2014
).
34.
S.
Gündüz
,
N.
Nitta
,
S.
Vibhute
,
S.
Shibata
,
M. E.
Mayer
,
N. K.
Logothetis
,
I.
Aoki
, and
G.
Angelovski
, “
Dendrimeric calcium-responsive MRI contrast agents with slow in vivo diffusion
,”
Chem. Commun.
51
(
14
),
2782
2785
(
2015
).
35.
A.
Moussaron
,
S.
Vibhute
,
A.
Bianchi
,
S.
Gündüz
,
S.
Kotb
,
L.
Sancey
,
V.
Motto‐Ros
,
S.
Rizzitelli
,
Y.
Crémillieux
, and
F.
Lux
, “
Ultrasmall nanoplatforms as calcium‐responsive contrast agents for magnetic resonance imaging
,”
Small
11
(
37
),
4900
4909
(
2015
).
36.
K. W.
MacRenaris
,
Z.
Ma
,
R. L.
Krueger
,
C. E.
Carney
, and
T. J.
Meade
, “
Cell-permeable esterase-activated Ca(II)-sensitive MRI contrast agent
,”
Bioconjugate Chem.
27
(
2
),
465
473
(
2016
).
37.
A. T.
Preslar
,
L. M.
Lilley
,
K.
Sato
,
S.
Zhang
,
Z. K.
Chia
,
S. I.
Stupp
, and
T. J.
Meade
, “
Calcium-induced morphological transitions in peptide amphiphiles detected by 19F-magnetic resonance imaging
,”
ACS Appl. Mater. Interfaces
9
(
46
),
39890
39894
(
2017
).
38.
S.
Okada
,
B. B.
Bartelle
,
N.
Li
,
V.
Breton-Provencher
,
J. J.
Lee
,
E.
Rodriguez
,
J.
Melican
,
M.
Sur
, and
A.
Jasanoff
, “
Calcium-dependent molecular fMRI using a magnetic nanosensor
,”
Nat. Nanotechnol.
13
(
6
),
473
477
(
2018
).
39.
A.
Barandov
,
B. B.
Bartelle
,
C. G.
Williamson
,
E. S.
Loucks
,
S. J.
Lippard
, and
A.
Jasanoff
, “
Sensing intracellular calcium ions using a manganese-based MRI contrast agent
,”
Nat. Commun.
10
(
1
),
897
(
2019
).
40.
K.
Du
,
A. E.
Thorarinsdottir
, and
T. D.
Harris
, “
Selective binding and quantitation of calcium with a cobalt-based magnetic resonance probe
,”
J. Am. Chem. Soc.
141
(
17
),
7163
7172
(
2019
).
41.
T.
Savić
,
G.
Gambino
,
V. S.
Bokharaie
,
H. R.
Noori
,
N. K.
Logothetis
, and
G.
Angelovski
, “
Early detection and monitoring of cerebral ischemia using calcium-responsive MRI probes
,”
Proc. Nat. Acad. Sci.
116
(
41
),
20666
20671
(
2019
).
42.
C. J.
Adams
,
R.
Krueger
, and
T. J.
Meade
, “
A multimodal Ca(II) responsive near IR-MR contrast agent exhibiting high cellular uptake
,”
ACS Chem. Biol.
15
(
2
),
334
341
(
2020
).
43.
G.
Gambino
,
T.
Gambino
,
R.
Pohmann
, and
G.
Angelovski
, “
A ratiometric 19F MR-based method for the quantification of Ca2+ using responsive paramagnetic probes
,”
Chem. Commun.
56
(
24
),
3492
3495
(
2020
).
44.
F.
Garello
,
S.
Gündüz
,
S.
Vibhute
,
G.
Angelovski
, and
E.
Terreno
, “
Dendrimeric calcium-sensitive MRI probes: The first low-field relaxometric study
,”
J. Mater. Chem. B
8
(
5
),
969
979
(
2020
).
45.
A.
Bar‐Shir
,
L.
Avram
,
S.
Yariv-Shoushan
,
D.
Anaby
,
S.
Cohen
,
N.
Segev‐Amzaleg
,
D.
Frenkel
,
O.
Sadan
,
D.
Offen
, and
Y.
Cohen
, “
Alginate‐coated magnetic nanoparticles for noninvasive MRI of extracellular calcium
,”
NMR Biomedicine
27
(
7
),
774
783
(
2014
).
46.
U.
Heinemann
,
H.
Lux
, and
M.
Gutnick
, “
Extracellular free calcium and potassium during paroxysmal activity in the cerebral cortex of the cat
,”
Exp. Brain Res.
27
(
3–4
),
237
243
(
1977
).
47.
C.
Nicholson
,
G. T.
Bruggencate
,
R.
Steinberg
, and
H.
Stöckle
, “
Calcium modulation in brain extracellular microenvironment demonstrated with ion-selective micropipette
,”
Proc. Nat. Acad. Sci.
74
(
3
),
1287
1290
(
1977
).
48.
G. E.
Hagberg
,
I.
Mamedov
,
A.
Power
,
M.
Beyerlein
,
H.
Merkle
,
V. G.
Kiselev
,
K.
Dhingra
,
V.
Kubìček
,
G.
Angelovski
, and
N. K.
Logothetis
, “
Diffusion properties of conventional and calcium‐sensitive MRI contrast agents in the rat cerebral cortex
,”
Contrast Media Mol. Imag.
9
(
1
),
71
82
(
2014
).
49.
F.
Garello
,
S.
Vibhute
,
S.
Gündüz
,
N. K.
Logothetis
,
E.
Terreno
, and
G.
Angelovski
, “
Innovative design of Ca-sensitive paramagnetic liposomes results in an unprecedented increase in longitudinal relaxivity
,”
Biomacromolecules
17
(
4
),
1303
1311
(
2016
).
50.
J. M.
Perez
,
L.
Josephson
,
T.
O'Loughlin
,
D.
Högemann
, and
R.
Weissleder
, “
Magnetic relaxation switches capable of sensing molecular interactions
,”
Nat. Biotechnol.
20
(
8
),
816
820
(
2002
).
51.
P. C.
Van Zijl
and
N. N.
Yadav
, “
Chemical exchange saturation transfer (CEST): What is in a name and what isn't?
,”
Magn. Resonance Medicine
65
(
4
),
927
948
(
2011
).
52.
K. K.
Watson
and
M. L.
Platt
, “
Of mice and monkeys: Using non-human primate models to bridge mouse-and human-based investigations of autism spectrum disorders
,”
J. Neurodevelopmental Disorders
4
(
1
),
21
(
2012
).
53.
A. S.
Mitchell
,
A.
Thiele
,
C. I.
Petkov
,
A.
Roberts
,
T. W.
Robbins
,
W.
Schultz
, and
R.
Lemon
, “
Continued need for non-human primate neuroscience research
,”
Curr. Biol.
28
(
20
),
PR1186
(
2018
).
54.
M. G.
Shapiro
,
R. M.
Ramirez
,
L. J.
Sperling
,
G.
Sun
,
J.
Sun
,
A.
Pines
,
D. V.
Schaffer
, and
V. S.
Bajaj
, “
Genetically encoded reporters for hyperpolarized xenon magnetic resonance imaging
,”
Nat. Chem.
6
(
7
),
629
634
(
2014
).
55.
A.
Mukherjee
,
D.
Wu
,
H. C.
Davis
, and
M. G.
Shapiro
, “
Non-invasive imaging using reporter genes altering cellular water permeability
,”
Nat. Commun.
7
,
13891
(
2016
).
56.
A.
Mukherjee
,
H. C.
Davis
,
P.
Ramesh
,
G. J.
Lu
, and
M. G.
Shapiro
, “
Biomolecular MRI reporters: Evolution of new mechanisms
,”
Prog. Nuclear Magnetic Resonance Spectroscopy
102–103
,
32
42
(
2017
).
57.
K. Y.
Chan
,
M. J.
Jang
,
B. B.
Yoo
,
A.
Greenbaum
,
N.
Ravi
,
W.-L.
Wu
,
L.
Sánchez-Guardado
,
C.
Lois
,
S. K.
Mazmanian
, and
B. E.
Deverman
, “
Engineered AAVs for efficient noninvasive gene delivery to the central and peripheral nervous systems
,”
Nat. Neurosci.
20
(
8
),
1172
1179
(
2017
).
58.
J. O.
Szablowski
,
A.
Lee-Gosselin
,
B.
Lue
,
D.
Malounda
, and
M. G.
Shapiro
, “
Acoustically targeted chemogenetics for the non-invasive control of neural circuits
,”
Nat. Biomed. Eng.
2
(
7
),
475
(
2018
).
59.
M.
Barth
,
F.
Breuer
,
P. J.
Koopmans
,
D. G.
Norris
, and
B. A.
Poser
, “
Simultaneous multislice (SMS) imaging techniques
,”
Magn. Resonance Medicine
75
(
1
),
63
81
(
2016
).
60.
J. C.
Ye
, “
Compressed sensing MRI: A review from signal processing perspective
,”
BMC Biomed. Eng.
1
(
1
),
8
(
2019
).
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