Confocal Laser Scanning Microscopy and Related Techniques

Introduction to Confocal Laser Scanning Microscopy

• Conventional Fluorescence Limitations:

  • Problematic on thick sections (e.g., 20µm) at high numerical aperture.

  • Tissue thickness limits detail more than objective resolution (diffraction).

  • Superimposed photons from overlying structures blur detail. This scattering can significantly reduce image clarity, especially in deeper tissues, making it challenging to resolve fine structures.

• Alternative to Sectioning:

  • Cutting sections (physical sectioning). However, this method introduces its own set of artifacts and limitations.

Issues with Physical Sectioning

• Detail is lost at the interface between sections due to knife-edge damage. The physical cutting process can compress or distort the tissue, leading to inaccurate representations of the original structure.

• Distortion or loss of sections makes correlation difficult or impossible. Reconstructing a 3D image from serial sections requires precise alignment, which can be compromised by missing or damaged sections.

• Differential penetration of reagents in immunofluorescence studies. Antibodies or dyes may not uniformly penetrate each section, resulting in inconsistent staining and quantification.

• Exception: Serial block face SEM (aka volume EM or VEM). This technique avoids many of the sectioning artifacts by imaging the block face directly after each cut.

Deconvolution Limitations

• Fine detail above high-intensity fluorescence leads to detector saturation. Saturated pixels cannot be corrected by deconvolution algorithms.

• Detail cannot be recovered by deconvolution. Deconvolution can sharpen images by reassigning out-of-focus light, but it cannot restore information lost due to saturation or excessive scattering.

Confocal Microscopy: An Elegant Solution

• Optical Sectioning:

  • Confocal systems allow optical sectioning, aided by tissue clearing techniques. This allows for non-destructive 3D imaging of thick specimens.

  • Clearing tissues involves infiltration with media that equalize refractive index and reduce light scattering. Common clearing agents include glycerol, benzyl alcohol, and specialized commercial solutions.

  • Reduces contrast but allows light to penetrate deeper. By minimizing refractive index mismatches, scattering is minimized, and light can travel further into the sample.

  • Can clear whole organs or even entire animals. This enables large-scale 3D imaging projects to be carried out.

Confocal Imaging: Basic Principle

• Confocal Definition:

  • Pinhole at a conjugate focal plane to the subject. This spatial filtering is the key to rejecting out-of-focus light.

  • Out-of-focus light forms a large PSF and is excluded by the pinhole. Only light originating from the focal plane passes through the pinhole to reach the detector.

  • Modern designs use collimated laser light and infinity optical path. Collimated light ensures consistent focus and reduces aberrations.

• Light Path:

  • Laser light is deflected by a mirror into the light path (coaxial illumination). This ensures the illumination and detection paths are aligned.

  • Collimated beam forms a single 3D PSF in the sample volume. The shape and size of the PSF determine the resolution of the microscope.

  • No field aperture, unlike conventional fluorescence or bright field microscopy. The absence of a field aperture allows for efficient light collection from the entire field of view.

  • Single detector measures brightness of light passing through the pinhole. This provides a quantitative measurement of the fluorescence intensity at each point in the sample.

• Infinity Focus:

  • Exploited to mix light paths from multiple lasers with different dichroic mirrors. This allows for simultaneous excitation of multiple fluorophores.

  • Allows selective illumination of different fluorophores without colored emission filters. Dichroic mirrors selectively reflect certain wavelengths of light while allowing others to pass through.

• Dichroic Mirror:

  • Reflects short wavelengths (excitation) using interference reflection. The mirror is designed to efficiently reflect excitation light towards the sample.

  • Emitted light (longer wavelength fluorescence) passes back through the objective and becomes collimated. The collimated light is then directed towards the detector.

  • Light passes through the dichroic mirror and is refocused by a tube lens. The tube lens corrects for aberrations and forms a sharp image on the pinhole plane.

• Pinhole Aperture:

  • Located in the real image plane, confocal with the illumination spot. The pinhole size is critical for determining the amount of out-of-focus light that is rejected.

  • Typically 1 Airy unit in diameter. This size provides a good balance between resolution and signal intensity.

  • Signal from that location is measured by a sensitive detector (single point at a time). The detector converts the light signal into an electrical signal, which is then digitized and processed.

• Out-of-Focus Rejection:

  • Sharpness of the illumination spot (proportional to numerical aperture). A higher numerical aperture results in a smaller, more focused spot.

  • Size and location of the pinhole aperture (smaller=better, up to the diffraction limit). A smaller pinhole provides better out-of-focus rejection but also reduces signal intensity.

  • Pinhole diameter usually matches the Airy disk of the diffraction pattern. This ensures optimal rejection of out-of-focus light.

• Diffraction Image:

  • Wavelength dependent. Shorter wavelengths result in smaller diffraction patterns and higher resolution.

Scanning Laser Confocal Microscopy

• No Camera: Single detector reads a signal varying in time based on focus position in X, Y, and Z. The image is built up point-by-point by scanning the laser across the sample.

• Scanning Mechanism: Deflection of light path by mirrors (typically controlled by galvanometers).

  • Shifts X and Y location of the illumination spot. The galvanometers precisely control the movement of the mirrors, allowing for rapid and accurate scanning.

  • Pinhole stays fixed. The pinhole remains stationary while the laser spot is scanned across the sample.

Confocal Laser Scanning Microscope (CLSM)

• Pixel Acquisition:

  • Individual pixels at each X, Y location are acquired by integrating the signal from the detector over a ‘pixel dwell time’ (e.g., 20µs). Longer dwell times result in higher signal-to-noise ratios.

  • Builds a 2D image. Each pixel represents the fluorescence intensity at a specific location in the sample.

• Z Series:

  • 3D information obtained by shifting the specimen (stage) towards or away from the objective lens and repeating the scan. This process acquires a series of 2D images at different depths within the sample.

  • 1 voxel = 3-dimensional pixel. Each voxel represents the fluorescence intensity at a specific 3D location in the sample.

Strengths of Confocal Microscopy vs. Wide-Field Fluorescence

• Rejection of out-of-focus light in thick volumes. This results in sharper, more detailed images.

• Discrimination of multiple fluorophores. Confocal microscopy allows for the simultaneous imaging of multiple fluorescent labels with minimal crosstalk.

• Intrinsically higher resolution:

  • Lateral (x/y) resolution. The pinhole reduces the contribution of out-of-focus light, resulting in a sharper image.

  • Axial (z) resolution (depth). Confocal microscopy provides improved axial resolution compared to wide-field microscopy.

Resolution Enhancement

• Pinhole Effect: Reduces contribution of diffracted light from out-of-focus planes. This improves image clarity and resolution.

• Laser Light Coherence: Coherent laser light can be focused into a narrower Airy pattern than non-coherent light.

  • Smaller “focal volume” can be illuminated. This results in higher resolution and reduced photobleaching.

Resolution and Numerical Aperture

• $\lambda$: Light's wavelength

• $NA$: Objective's numerical aperture

• $NA = n \sin\theta$

• $n$: Refractive index of the immersion medium

Rayleigh's Formula Generalization

• Wide-Field (WF): $D = 0.61 \frac{\lambda}{NA}$

• Confocal (CLSM): $D = 0.46 \frac{\lambda}{NA}$

• 25% improvement in lateral resolution in confocal microscopy. This improvement is due to the pinhole's ability to reject out-of-focus light.

• Specialized imaging (1/2 AU pinhole, 2x oversampling) combined with deconvolution methods can double this (‘super-resolution’ techniques). These techniques push the resolution limits of confocal microscopy even further.

• Can resolve details to around 130nm with appropriate deconvolution. This allows for the visualization of subcellular structures and protein complexes.

Applications of Confocal Microscopy

• Resolve relationships and structure of major proteins and organelles (e.g., actin filaments, chromatin). Confocal microscopy is a powerful tool for studying cellular architecture.

• Protein localization (e.g., plasma membrane or cytoplasm). By labeling proteins with fluorescent tags, their location within the cell can be determined.

• Protein co-localization studies. Confocal microscopy can be used to determine whether two or more proteins are present in the same location within the cell.

Fluorescent Sources and Markers

• Sources: Autofluorescent, Fluorescent labels

• Ideal Markers:

  • “Bright” – high quantum yield. A high quantum yield ensures that the marker emits a strong signal.

  • “Stable” – slow bleach, insensitive to changes in pH and O2/CO2 balance. A stable marker will maintain its fluorescence properties over time.

  • Suitable for multifluorescence – i.e. separable from other fluorescent markers. This allows for the simultaneous imaging of multiple structures or proteins.

3D Analysis with Multiple Fluorophores

• Each location will generate maximum intensity for a structure that is in focus. This allows for the creation of 3D reconstructions of complex structures.

• “Projection” (maximum intensity projection or MIP) of all confocal planes shows all structures in focus simultaneously. This is a useful way to visualize the overall structure of a sample.

• Orthogonal views make it possible to investigate interrelationships of labeled structures (XZ, XY, YZ). This allows for the visualization of structures from different perspectives.

Aberrations

• Spherical aberration. This occurs when light rays from different parts of the lens do not converge at the same point.

• Chromatic aberration. This occurs when different wavelengths of light are focused at different points.

• Require the best objectives with full correction for spherical & chromatic ($\lambda$) aberration:

  • Confocal is very demanding: typically use plan-apochromats. These objectives are designed to minimize aberrations and provide the best possible image quality.

  • “super planapochromats” (corrected into near infrared and sometimes UV wavelengths). These objectives provide even better correction for aberrations across a wider range of wavelengths.

  • Often selected based on measured PSF from multiple objectives made in the same production line. This ensures that the objective meets the specific requirements of the experiment.

CLSM: Pros and Cons

• Advantages:

  • Better lateral (x/y) resolution. This is due to the pinhole's ability to reject out-of-focus light.

  • Better depth (z) resolution (ideal for 3D reconstruction of large volumes). Confocal microscopy provides improved axial resolution compared to wide-field microscopy.

  • Better visibility of fine details. The improved resolution and contrast of confocal microscopy allow for the visualization of fine details that would be missed by other techniques.

  • Better discrimination for multiple fluorophores. Confocal microscopy allows for the simultaneous imaging of multiple fluorescent labels with minimal crosstalk.

• Disadvantages:

  • Sequential scanning of each point can be very slow. This can be a limitation when imaging large volumes or dynamic processes.

  • High illumination intensity can bleach fluorescence. Photobleaching can be minimized by using lower laser power and shorter exposure times.

  • Very high cost. Confocal microscopes are expensive to purchase and maintain.

Alternatives to CLSM

Light-Sheet Microscopy (SPIM)

• Similar in some respects to CLSM, but using a second light path and a lens that forms a flat sheet of light orthogonal to the imaging path. This allows for rapid, low-phototoxicity imaging of large samples.

• May use a simple cylinder lens to form a continuous sheet of light, or a low-numerical aperture illumination objective (requires scanning). The choice of illumination method depends on the desired resolution and field of view.

• Whole image captured by camera. This allows for faster acquisition times compared to point-scanning methods.

• Allows long integration time for low-intensity illumination. This improves the signal-to-noise ratio and reduces photobleaching.

• The light sheet can be very thin to match the XY resolution of the imaging objective. This ensures optimal resolution in all three dimensions.

Advantages of Light-Sheet Microscopy

• Much lower intensity/better signal:noise. This reduces photobleaching and allows for longer imaging times.

• 2D image (sCMOS camera versus single pixel at a time). This allows for faster acquisition times compared to point-scanning methods.

• Very high speed acquisition. Light-sheet microscopy is well-suited for imaging dynamic processes.

• Suitable for large volume imaging (transparent specimens). The low phototoxicity and high speed of light-sheet microscopy make it ideal for imaging large, complex samples.

• Live imaging. Light-sheet microscopy is a powerful tool for studying living organisms.

• Resolution in Z ultimately lower than CLSM. The axial resolution of light-sheet microscopy is typically lower than that of confocal microscopy.

Spinning Disc Confocal Microscopy

• Uses multiple pinholes spun in the confocal plane and a 2D imaging system (typically sCMOS camera). This allows for faster acquisition times compared to traditional confocal microscopy.

• Individual pixels in the camera sensor act as the confocal pinhole. This eliminates the need for a separate pinhole and detector.

• Very high sampling rate

Principle similar to scanning confocal

• Collimated beam and series of illumination pinholes scans area in parallel. This allows for faster acquisition times compared to point-scanning methods.

• Microlens array forms intermediate image confocal with objective back focal plane. This improves the efficiency of light collection.

• By having many locations simultaneously illuminated, each pinhole can spend more time at each location than in CLSM, reducing excitation energy needed to illuminate a sample. This reduces phototoxicity and photobleaching.

• Reduces phototoxicity / bleaching – excellent for imaging live cells or whole animals. Spinning disc confocal microscopy is a powerful tool for studying living organisms.

• Dual disc system uses microlens arrays to increase light focused through each pinhole. This further improves the efficiency of light collection.

2-Photon (or Multi-Photon) Microscopy

• Principle: Two simultaneous lower energy photons can combine and have the same effect as one higher energy photon. This allows for deeper penetration into thick samples.

• Similar but simpler light path to single-photon (conventional CLSM), but differs in laser used:

• Laser light pulsed in extremely short bursts so that many photons arrive at the focal point in a very brief period. This increases the probability of two-photon absorption.

• 2-photon absorption by fluorophore can stimulate the same fluorescence as a single photon of double the energy (i.e., half the wavelength). This allows for the use of near-infrared light to excite visible light fluorophores.

• Infra-red laser can excite fluorescence of visible light fluorophores. Near-infrared light is scattered less by tissue, allowing for deeper penetration.

Advantages

• 2-photon excitation limits excitation outside the focal plane. This reduces photobleaching and improves image contrast.

• Allows detection without a confocal pinhole (more efficient light collection). This increases the signal-to-noise ratio.

• Long wavelength excitation highly compatible with living tissue. Near-infrared light is less damaging to living cells.

• Better penetration into thick samples. This is due to the reduced scattering of near-infrared light.

• Scanning mechanism can be placed immediately at the laser.

• No pinhole means fewer alignment issues.

Functional Probes

• e.g., Calcium (or voltage) sensitive indicators allow imaging electrical activity in vivo. These probes change their fluorescence properties in response to changes in calcium concentration or membrane potential.

• Can be injected into cells (specific) or added to medium. The method of delivery depends on the specific probe and application.

• Can be expressed genetically (e.g., Cre/Lox system in mouse brain, Gal4/UAS in Drosophila). This allows for targeted expression of the probe in specific cell types.

Older Confocal Light Path (Zeiss LSM 510 meta)

• Light Sources (Lasers):

  • Blue laser diode (405 nm)

  • Ar ion laser (458, 477, 488, 514 nm)

  • Green DPSS laser (561 nm)

  • HeNe gas laser (633 nm)

Pinholes (confocal apertures)

• Eliminate out-of-focus light. The pinhole size is critical for determining the amount of out-of-focus light that is rejected.

• One per detector.

• Individually adjustable. This allows for optimization of the pinhole size for each wavelength.

• Typically adjusted to match the diffraction image for each wavelength (longer wavelength=larger aperture)

Detectors

• PMTs (photomultiplier tubes). These are highly sensitive detectors that convert light into an electrical signal.

• Spectral detector (e.g., “metadetector”) – spectrometer that allows multiple wavelengths to be analysed. This allows for the simultaneous detection of multiple fluorophores.

• Photon-avalanche detectors.

• Lambda ($\lambda$) scan

Lambda ($\lambda$) Scan

• Meta-detector array can detect up to 32 different channels (=wavelength intervals) simultaneously. This allows for the acquisition of spectral information for each pixel.

• Allows spectral properties of emitted fluorescence to be quantified. This can be used to identify and separate different fluorophores.

Lambda ($\lambda$) Scan allows for spectral ‘unmixing’

• spectral emission curves can be obtained for each pixel or region of interest

• 2 or more fluorophores can be ’unmixed’ by comparison with reference spectra

Dichroic mirrors at the detector are inefficient

• Pass desired wavelength (to match fluorophore emission spectrum). The dichroic mirrors selectively transmit the desired wavelengths of light to the detector.

• Reflect other wavelengths. The reflected light is then directed to other detectors.

• Detectors typically arranged in sequence so that reflected light can be scavenged (e.g. during simultaneous excitation with 2 or more lasers). This improves the efficiency of light collection.

• Reflected light lost due to limit on the number / specificity of the filters that can be included in the system

New Generation CLSM

• Solid-state lasers.

• Replace dichroic mirrors with a spectrophotometer mechanism for more efficient light collection.

• Typically use detectors (e.g., GaAsP) with higher quantal efficiency. These detectors are more sensitive to light and provide a better signal-to-noise ratio.

• Replaces dichroic mirrors in the detector path with a spectrophotometer mechanism

• Light (all colors) passes a single pinhole before a prism splits the wavelengths spatially.

• Broadband mirrors are then combined with moveable knife- like spectral ‘sliders’ to split the spectrum up between detectors.

• Detection wavelength band can be adjusted in very small increments (~1 nm).