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M i c r o s c o p y f r o m C a r l Z e i s s
3 Steps to Get a Confocal Image
How to Enhance Image Quality
Carl Zeiss
· 07740 Jena · Germany · E-mail: [email protected] ·
www.zeiss.de/lsm
The Confocal Laser Scanning Microscope
45-0024 e
/08.03
•
Light source
– projected into specimen
• Laser power: adjustable via attenuation device
(AOTF, AOM, MOTF) and tube current setting (Ar)
• Lifetime Ar: prolonged by using lower tube current;
but laser noise will be increased (8 A = minimum noise)
• Stand-by mode: prolongs laser lifetime; not suitable for
image acquisition
• Laser line: can be chosen via selection device (AOTF, MOTF)
dependent on fluorescent dye. Generally: the shorter
the wavelength, the higher the resolution
• Application goals: (1) Protect specimen (reduction of dye
bleaching and phototoxicity) by reduction of laser power.
(2) Maximize fluorescence signal (higher SNR) by longer
pixel dwell times or averaging
Laser
•
Scanning unit
– moves focused laser beam across
specimen line by line
• Scanning speed: defines frame rate (frames/sec) and
pixel time, i.e. time the specimen is illuminated
• Pixel time: influences SNR of image; the longer the pixel time,
the more photons per pixel, the less noise in the picture;
but bleaching of fluorochromes may increase
• Pixel resolution: maximum resolution can be achieved if
pixel size is set correctly (at least 4 x 4 pixels (x, y) per
smallest detail)
➝
directly adjustable via scan zoom
• x/y frame size: variable from 4 x 2 up to 2048 x 2048 pixels;
maximum frame rate with 512 x 512 pixels 5 frames/sec
(bidirectional scan ); unidirectional scan : slower by
factor 2
Scanning Mirrors
•
Focusing the specimen
– acquisition of image stacks
or x-z sections
• z-interval: distance between two optical slices (step size
of z-motor: min. 25 nm)
• Optimum z-motor step size: 0.5 x optical slice thickness
(compare: min. slice thickness about 340 nm for NA =1.4,
n =1.52,
λ
= 488 nm)
• Optional: fast z-scanning stage (HRZ)
fast piezo objectiv focus
Z-Motor
Collimator
Laser source
Focal plane
Specimen
Objective
Scanning
mirrors
Main dichroic
beamsplitter
Confocal
pinhole
Detector
Emission filter
Z-motor
View specimen in VIS mode
Focus the specimen in epi-fluorescence mode using the binocular
and center the part of interest; select fluorescence filter cube according
to application (e.g. FITC or Cy3) via SW (window "Microscope Control");
match the field of view: change to appropriate objective magnification
(consider use of correct immersion medium).
Scan an image
Click on "Find" button (right row in window "Scan Control")
=> System automatically opens image window, optimizes detector
settings (matches PMT gain and offset to dynamic range of 8 or 12 bit),
and scans an image.
See operating manual for scanning a stack of slices, time series etc.
More reliability !
• Use Multitracking: very fast switching of excitation wavelengths; prevents
crosstalk of signals between channels; predefined configurations available.
• Use ROI (Region Of Interest) function: significantly reduces excited area
of specimen and increases acquisition rate at constant SNR; several ROIs
of any shape can be defined and used simultaneously.
More details !
• Use objective with higher numerical aperture (NA); x/y-resolution ~ 1/NA,
z-resolution ~ 1/NA2.
• Increase "FrameSize"= number of pixels per line + lines per frame,
e.g. 1024 x 1024 or 2048 x 2048 (min. 4 x 2).
• Optimize scan zoom (Z), i.e. pixel size
≤
0.25 x diameter of Airy disk
(e.g.: Objective 40x, NA 1.3, l = 488 nm => Z = 4.56).
• Increase dynamic range (change from 8 to 12 bit per pixel).
More signal !
• Change to longer pixel dwell times by reducing scanning speed
• Use "Average" method: Calculation of "Sum"or "Mean" value
of pixels of consecutive "Line" or "Frame" scans.
• Increase bandwidth of emission filter (e.g. LP instead of BP).
• Enlarge pinhole diameter; Note: optical slice thickness increases accordingly.
• Increase excitation energy (laser power); but pay attention
to bleaching, saturation and phototoxic effects.
•
Detector
– pixelwise detection of photons emitted /
reflected by the respective specimen detail
• Parameters: "Detector Gain"= PMT high voltage,
"Amplifier Offset"= black level setting, "Amplifier
Gain"= electronic post-amplification
• Calibration: "Amplifier Offset" on image background
(object-free area), "Detector Gain" according to scanned
image (object); setting aid = "Range Indicator"
(
➝
"Palette"). Goal: least number of overmodulated
(red, Gain) and undermodulated (blue, Offset) pixels
• Signal amplification: First exploit "Detector Gain" slider
before "Amplifier Gain" >1
Photomultiplier (PMT)
•
Depth discrimination
– confocal aperture to prevent
detection of out-of-focus light (optical sectioning)
• Diameter: determines thickness of optical slice; optimum
diameter: 1 Airy unit = best trade-off between depth
discrimination capability and efficiency
• x/y position: factory-adjusted for all beam path configu-
rations; can be modified manually (
➝
"Maintain-Pinhole")
Confocal Pinhole
•
Fluorescence beam path
– definable by combination
of main (HFT) and secondary (NFT) dichroic mirrors and
emission filters (BP = bandpass, LP = longpass,
KP = shortpass) (
➝
"Acquire"–"Config")
• HFT: separates excitation and emission light
• NFT: effects spectral division of (different) fluorescence
emissions (e.g. NFT 545: reflects light of
λ
< 545nm and
transmits light of
λ
> 545nm)
• BP, LP, KP: determines bandwidth of fluorescence
emission for the respective channel (e.g. LP 505:
λ ≥
505 nm
➝
detection)
Beam Splitter
•
Optical image formation
– determines properties
of image quality such as resolution (x, y, z)
• Numerical Aperture (N.A.): determines imaged spot size
(jointly with wavelength), and substantially influences
the minimum optical slice thickness achievable
• Refractive index (n): match n
immersion liquid
with
n
specimen mounting medium
for better image quality.
• Best confocal multifluorescence images (VIS, UV):
use water immersion objectives with apochromatic
correction (C- Apochromat)
Objective Lens
Load an LSM configuration
Activate LSM mode (operate manual tube slider or
button "LSM"). Open window "Configuration control", and
select a predefined configuration from list (Single Track).
A click on "Apply" automatically sets up the system: laser lines, attenuation,
emission filters, beam splitters (HFT, NFT), pinhole diameter, detector settings
(channels, gain, offset). Or: Click on "Reuse" button (stored image/image
database window) to restore settings of a previous experiment.
We make it visible.