Molecular Imaging

Molecular imaging is a rapidly emerging biomedical research in diagnostics and therapeutic fields due to current indispensable tools in modern diagnostics.[1] Various imaging modalities (Table 1) are currently employed in the field of molecular imaging either individually or in hybrid in order to optimize their resolution and sensitivity [2]. As portrayed in figure 1, each modality utilizes different energy to generate the image and thus offers different spatial, temporal resolution and sensitivity.

Table 1: Characteristics of various imaging modalities

Imaging Technique Spatial resolution

Depth Temporal resolution

Sensitivity Molecular Probe used

Amount of Molecular Probe used

Positron emission tomography (PET)

1-2mm no limit

10 sec to minutes

10-11-10-12

mole/L radiolabeled, direct or indirect

nanograms

Single photon emission computed tomography (SPECT)

1-2 mm no limit

minutes 10-10-10-11

mole/L radiolabeled, direct or indirect

nanograms

Optical bioluminescence imaging

3-5 mm 1-2 cm seconds to minutes

10-15-10-17

mole/La activatable, indirect

micrograms to milligrams

Optical fluorescence imaging

2-3 mm < 1 cmb

seconds to minutes

10-9-10-12

mole/La activatable, direct or indirect

micrograms to milligrams

Magnetic resonance imaging (MRI)

25-100 µm

no limit

minutes to hours

10-3-10-5

mole/L activatable, direct or indirect

micrograms to milligrams

Computed tomography (CT)

50-200 µm

no limit

minutes not well characterized

not applicable

Ultrasound 50-500 µm

mm to cm

seconds to minutes

not well characterized

limited activatable, direct

micrograms to milligrams

aNot well characterized; bThis depth applies to reflectance fluorescence. Fluorescence tomography can likely image objects at greater depths (2–6 cm); c

Definitions: Spatial resolution is a measure of the accuracy or detail of graphic display in the images expressed in millimeters. It is the minimum distance between two independently measured objects that can be distinguished separately. It is a measure of how fine the image is.

Temporal resolution is the frequency at which the final interpretable version of images can be recorded/ captured from the subject once the imaging process is initiated. This relates to the time required to collect enough events to form an image, and to the responsiveness of the imaging system to rates of any change induced by the operator or in the biological system at hand.

Sensitivity, the ability to detect a molecular probe when it is present, relative to the background, measured in moles per liter.

Figure 1: The electromagnetic spectrum showing energy expended for image generation

Molecular imaging is currently being sought in current medicine practice in order to:

1. Develop non-invasive in vivo imaging methods that reflect specific cellular and molecular processes such as gene therapy and protein-protein interactions

2. Monitor multiple molecular events concurrently 3. Follow trafficking and targeting cells 4. Optimize drug and gene therapy 5. Image drug effects at molecular and cellular level 6. Assess disease progression at a molecular pathological level[1]

References:

[1] J. M. Hoffman and S. S. Gambhir, Radiology 2007, 244, 39-47. [2] P. J. Ell, Br J Radiol 2006, 79, 32-36.