Lecture 5 - Super resolution

What is super-resolution fluorescence microscopy?

A family of fluorescence imaging methods that distinguish structures closer together than conventional diffraction-limited microscopy can resolve • The lecture focuses on SIM, PALM/STORM and STED • Each method bypasses the conventional limit through a different physical or computational strategy

Who received the 2014 Nobel Prize in Chemistry for super-resolved fluorescence microscopy?

Eric Betzig — single-molecule localization/PALM • Stefan W. Hell — STED • William E. Moerner — single-molecule fluorescence • The prize recognized the development of super-resolved fluorescence microscopy

What is optical resolution and why is it limited by diffraction?

Resolution is the ability to distinguish two nearby objects as separate • A point emitter appears as a finite-sized point spread function (PSF), not a perfect point • When two PSFs overlap too strongly, the objects cannot be distinguished

What diffraction-limit formula is given in the lecture?

Δx = λ/(2 NA) • λ is the light wavelength and NA is the numerical aperture • The lecture quotes a typical lateral diffraction limit of approximately 200 nm • Shorter wavelength or higher NA improves conventional resolution

What is the point spread function (PSF)?

The intensity distribution formed by imaging a single point emitter • It includes a central peak and diffraction-related spread • Its width influences whether two neighboring emitters can be resolved • Super-resolution methods effectively narrow the observable response or localize emitters within it

What are the three major super-resolution approaches taught?

SIM — structured illumination microscopy • PALM/STORM — single-molecule localization microscopy (SMLM) • STED — stimulated emission depletion microscopy

Why are Fourier transforms important for SIM?

They relate image structure in real space to spatial frequencies • Fine, closely spaced features correspond to higher spatial frequencies • Conventional microscopy loses some high-frequency information • SIM shifts otherwise inaccessible sample frequencies into the observable range

What is the FFT?

Fast Fourier Transform • A computational method for transforming an image between spatial and spatial-frequency representations • Used to analyze and reconstruct structured-illumination images

What does a high-pass spatial-frequency filter do?

Retains higher-frequency image information • Emphasizes fine structures, edges and small details • Suppresses slower, broad intensity variations • Shown in the original/high-pass/low-pass comparison on slide 8

What does a low-pass spatial-frequency filter do?

Retains lower-frequency information • Preserves broad structures and slowly changing image intensity • Suppresses fine detail and sharp edges • Demonstrated in the Fourier-filter comparison on slide 8

What is the central idea of structured illumination microscopy (SIM)?

Illuminate a fluorescent specimen with a known periodic pattern instead of uniform illumination • The specimen pattern and illumination interact to produce lower-frequency information that can be captured • Acquire multiple pattern phases and orientations • Computationally reconstruct fine sample structure beyond the widefield limit

What are the key elements in the SIM Fourier-space illustration?

Object/sample structure • Known illumination pattern • Fluorescence emission resulting from their interaction • Emission blurred by convolution with the microscope PSF • Separation and recombination of shifted frequency components

How does patterned illumination alter the measured fluorescence?

The emitted fluorescence depends on the sample distribution multiplied by the illumination pattern • This mixing produces additional frequency components in the measured image • Components that would otherwise be beyond the detection passband can be shifted into it

What does convolution with the PSF mean in SIM?

The microscope records a blurred version of the fluorescence distribution • The PSF determines the optical blur and frequency cutoff • A single SIM frame is not by itself a complete super-resolved reconstruction • Multiple patterned images are needed to recover and combine shifted information

Why are SIM illumination patterns shifted in phase?

Different phases modulate the same structures differently • Multiple measurements allow the computational separation of frequency orders • A single pattern position does not provide enough independent information

Why are SIM illumination patterns rotated?

A stripe pattern preferentially reveals frequencies along particular directions • Rotating it captures missing high-frequency information in other orientations • Combining orientations improves resolution more uniformly in the image plane

How many input images are shown for a typical SIM acquisition in the lecture?

15 input images • 5 phase positions for each of 3 pattern angles • These images are processed together to reconstruct a super-resolved image • Illustrated on slide 17

What frequency orders are separated during SIM reconstruction?

The central or zero-order component • Negative first-order component (−1) • Positive first-order component (+1) • The components are shifted into their appropriate positions and combined

What are the principal SIM reconstruction steps?

Collect images at multiple pattern phases and angles • Fourier-transform the recorded images • Separate the frequency orders • Shift the components into their correct frequency-space positions • Combine components and reconstruct the final image

What are moiré fringes and how do they relate to SIM?

Moiré fringes are larger-scale patterns produced when two fine periodic patterns overlap • In SIM, the known illumination pattern interacts with fine sample structure • The observable moiré pattern carries information about otherwise unresolvable sample detail • Knowing the illumination pattern helps recover the unknown specimen structure

What does the widefield-versus-SIM comparison demonstrate?

Conventional widefield images merge nearby structures • SIM reconstruction reveals finer details and clearer separation • The improvement comes from structured acquisition plus computational processing

How do 2D and 3D SIM differ?

2D SIM primarily improves lateral resolution with patterned illumination • 3D SIM uses an illumination pattern varying in three dimensions • 3D illumination improves axial resolution as well • Both require specialized optics and reconstruction

What typical optical setup is shown for SIM?

An excitation beam • A diffraction grating or other pattern-forming optics • An objective that projects the structured pattern onto the sample • A camera/CCD that captures the sequence of fluorescence images

How much does conventional SIM improve resolution according to the lecture?

Approximately a twofold improvement • Typical lateral resolution around 120 nm • Improved axial resolution is possible with 3D illumination • Higher resolution may be possible with photoswitching-based SSIM

What is SSIM?

A photoswitching-based extension of structured illumination microscopy • Combines structured illumination with nonlinear/photoswitching behavior • The lecture notes that it can achieve higher resolution than conventional SIM

What are SIM’s advantages?

Compatible with essentially any fluorescent dye • Relatively low light dose and photobleaching • Can acquire images quickly with fast cameras • Recommended for live-cell imaging • Provides approximately twofold lateral resolution improvement

What are SIM’s disadvantages and practical requirements?

Specialized optics to generate accurate illumination patterns • Fast camera acquisition • Post-processing and demodulation algorithms • Pattern precision deteriorates with imaging depth • Depth performance is limited in scattering or thick specimens

Why is SIM described as mainly widefield?

The structured pattern illuminates a field rather than relying on point-by-point scanning • A camera captures the fluorescence over the field • The lecture notes that scanning implementations are also possible

What does SMLM stand for and which methods belong to it?

Single-Molecule Localization Microscopy • Includes PALM and STORM and related approaches • Reconstructs images from the coordinates of many individually localized fluorophores

How can the center of one fluorophore be localized more precisely than its PSF width?

A single emitter produces a diffraction-blurred spot • Fit the intensity profile, commonly with a Gaussian • Estimate the center/mean position (x₀,y₀) • With sufficient detected photons, the center can be determined much more precisely than the PSF width

What is Gaussian fitting in single-molecule localization?

Fit a Gaussian-like function to the recorded image of an isolated emitter • Estimate the spot center and width from pixel intensities • Record the fitted center as a molecular coordinate • Repeat across many molecules to reconstruct the structure

What localization-precision formula is shown in the lecture?

σ ≈ √[(s² + a²/12)/N] • σ is the standard error/localization precision • s is the standard deviation or width of the spot • a is the detector pixel size • N is the number of collected photons

How does photon number influence localization precision?

More collected photons improve the precision of the fitted molecular position • The lecture formula gives σ proportional to 1/√N when other factors are fixed • Four times as many photons approximately halves σ • Background and experimental instability can worsen practical performance

How do pixel size and spot width affect localization precision?

Larger spot width s increases the uncertainty in the fitted center • Detector pixel size a contributes the a²/12 term • Photon number N appears in the denominator • Good sampling and a strong signal improve localization

What is the difference between localization precision and optical resolution?

Localization precision describes uncertainty in the position of an isolated emitter • Optical resolution describes whether nearby structures can be distinguished • Very precise positions alone do not guarantee a well-resolved image • Labeling density, background and reconstruction quality also matter

Why must simultaneously emitting molecules be sparse in PALM/STORM?

Their diffraction-blurred PSFs must be separable • The lecture states that active emitters should be farther apart than the Abbe limit, roughly 250 nm • Sparse activation prevents overlapping spots from corrupting localization • Different subsets are activated across repeated frames

What does photoswitching mean?

A fluorophore changes between fluorescent ON and nonfluorescent OFF states • The state can be controlled by light or by the dye’s photochemistry • Switching permits only a small subset of labels to emit at one time • Essential for isolating emitters in many SMLM techniques

What is a photoswitchable fluorophore?

A label that can transition between dark and bright states • Its fluorescence can be turned on and off • The transitions may be reversible • Allows temporally separated localization of molecules

What is a photoactivatable fluorophore?

A label that initially has little or no fluorescence • Illumination activates it into a detectable fluorescent state • Allows controlled activation of sparse molecules

What is a photoconvertible fluorophore?

A label whose fluorescence properties or emission color change after light exposure • The lecture shows photoconversion as a distinct fluorophore behavior • Useful for controlling which molecules are detected during localization

Which fluorescent proteins are named for PALM?

EosFP — illustrated as a photoconvertible fluorescent protein • Dronpa — illustrated as a photoswitchable fluorescent protein • Dronpa switching is linked to reversible cis–trans isomerization in the lecture

Which organic fluorophores are named for dSTORM?

Cy5 • Rhodamine • The slides illustrate photochemical switching routes for organic dyes • The lecture contrasts organic fluorophores with fluorescent-protein labels

What properties make a fluorophore suitable for PALM or dSTORM?

High switching reliability • Low fatigue during repeated switching • Controllable switching kinetics • Appropriate balance of activation and inactivation rates • Only a small population should be active at a given moment

How are PALM and STORM distinguished in this lecture?

PALM is presented as using fluorescent proteins • STORM is presented as based on organic dyes • Both use sparse emission and localization over many cycles • Both belong to single-molecule localization microscopy

What is the PALM laser sequence shown in the lecture?

A blue laser activates fluorescent proteins • A green laser excites the activated proteins • Photons are collected until the labels return to a dark state or photobleach • The procedure is repeated to build the reconstructed image

What is the full PALM/STORM operating cycle?

Begin with most labels inactive • Activate a sparse subset — the lecture gives less than 1% • Excite and record isolated emitters • Fit their spots to determine nanometer-scale coordinates • Allow labels to darken or photobleach • Repeat many cycles and combine the coordinates

Why do SMLM fluorophores need to emit many photons before switching off?

The spot center is estimated from detected photons • More photons improve localization precision • The activated molecule must remain bright long enough to gather sufficient signal • It should then return to darkness so subsequent molecules can be measured

How many camera frames can be used to record a single SMLM emitter according to the slides?

The lecture describes low-power readout imaging over roughly 1–3 camera frames • Emission continues until the molecule photobleaches or returns to a dark state • The precise duration depends on the dye and experimental conditions

What does the STORM comparison with conventional microscopy illustrate?

Diffraction-limited images show thicker, merged fluorescent fibers • STORM reveals narrower, separated and more detailed filament patterns • Repeated localization makes it possible to reconstruct fine structural features

What is DNA-PAINT?

A single-molecule localization method based on transient binding of fluorescent DNA strands • Fluorescent imager strands bind briefly to complementary docking strands • Each binding event generates a localizable fluorescence signal • Many events reconstruct a super-resolved image

How does DNA-PAINT create blinking without conventional fluorophore photoswitching?

Unbound imager strands diffuse away from the target • Binding to a docking strand creates a temporary bright event • Unbinding ends the event • Repeated transient binding creates the blinking needed for localization

What roles do imager and docking strands play in DNA-PAINT?

The docking strand is attached to the target or targeting reagent • The fluorescent imager strand is complementary to the docking sequence • Transient hybridization marks the target position • Repeated events supply many localizations

What is Exchange-PAINT/multicolor PAINT?

An extension of DNA-PAINT for imaging multiple targets • Different docking sequences can identify different structures • Imager strands are exchanged to read out different targets • The lecture illustrates multicolor reconstruction

Which variables control SMLM resolution in the lecture?

Molecular labeling density • Number of photons collected • Background signal • Imaging speed and photon detection rate • Dye quantum yield and contrast • Stage drift and reconstruction algorithm

Why does molecular labeling density matter in SMLM?

A reconstruction requires enough localized labels to represent the true structure • Sparse overall labeling leaves gaps even if each localization is precise • Higher density improves structural sampling • Simultaneous emission must still be sparse enough to separate individual PSFs

How does background affect localization microscopy?

Background makes it harder to estimate the center of an emitter accurately • It reduces the contrast of single-molecule spots • It can worsen localization precision and reconstruction quality

Why is stage drift a problem in SMLM?

Localization coordinates are accumulated over many frames • Movement of the specimen or microscope shifts the apparent molecular positions • Uncorrected drift blurs the reconstructed image • Drift compensation or computational correction is therefore important

What photon-count examples are given for SMLM precision?

The lecture states that collecting 10,000 photons without background can give approximately 1–2 nm precision • With only 400 photons, the reported value is around 20 nm or worse • These are illustrative photon-count/precision examples, not guaranteed whole-image resolution

What practical resolution is given for PALM/STORM?

Approximately 20 nm in the lecture summary • Requires sparse switching, sufficient photons and computational reconstruction • Actual effective resolution depends on labeling density and experimental conditions

What equipment is shown for PALM/STORM?

Activation and excitation laser paths • A sensitive CCD camera • Fast electronics • Objective and fluorescence detection optics • Drift compensation and post-processing

What are the main drawbacks of PALM/STORM?

Relatively slow acquisition due to many required frames • Complex sample preparation and specialized switching labels • Limited depth resolution in the lecture summary • Dependence on drift control and computational reconstruction

What does STED stand for?

Stimulated Emission Depletion microscopy • A super-resolution fluorescence technique • Uses a second laser to suppress fluorescence around the edge of the excitation spot • Shrinks the effective fluorescent region

How is STED different from PALM/STORM and SIM?

STED physically confines fluorescence with a shaped depletion beam • PALM/STORM localizes sparse single molecules over many frames • SIM uses patterned illumination and computational frequency reconstruction • STED is commonly implemented as a scanning microscope

What is stimulated emission?

An incoming photon induces an excited molecule to return to a lower-energy state • A photon is emitted through a stimulated rather than spontaneous process • STED uses this process to deplete excited fluorophores before ordinary fluorescence occurs

How does the STED Jablonski diagram explain depletion?

The excitation beam raises fluorophores to an excited electronic state • The depletion beam stimulates their return toward the ground state • Molecules in the depleted region do not contribute ordinary detected fluorescence • Fluorescence remains concentrated in the region where depletion is minimal

Why is the STED depletion beam shaped like a doughnut?

It has high intensity around the outside of the excitation focus • It has a near-zero intensity at the center • Peripheral excited fluorophores are depleted • Central fluorophores remain able to fluoresce • The effective fluorescent PSF becomes smaller

What is PSF engineering in STED?

Designing excitation and depletion intensity distributions to control the effective fluorescent spot • A conventional excitation PSF is overlapped with a doughnut-shaped depletion PSF • The resulting ON region is much narrower than the original excitation spot

What does a vortex phase plate do in STED?

Modifies the phase of the depletion beam • Creates the doughnut-shaped intensity distribution • Produces a central intensity minimum needed to preserve fluorescence at the focus center

Why are two lasers used in the illustrated STED setup?

An excitation laser excites the fluorophore • A depletion laser drives stimulated emission around the excitation center • The beams have different spatial PSFs • Their overlap creates a small remaining fluorescence region

What fluorophore and wavelengths are used in the STED example?

Alexa Fluor 488 • Excitation at 488 nm • STED depletion at 592 nm • The slide states that the depletion beam should not be in the dye absorption range

Why must the depletion wavelength be chosen carefully?

It should efficiently induce stimulated emission from excited molecules • It should avoid substantial direct excitation through the dye absorption band • Fluorescence detection must be spectrally separated from the lasers • Available spectral windows restrict dye and color combinations

What is the STED resolution equation given in the lecture?

Δx = λ / [2 NA √(1 + I/Iₛ)] • λ is wavelength and NA is numerical aperture • I is STED depletion intensity • Iₛ is the dye saturation intensity • Increasing I/Iₛ reduces the effective resolution distance

What happens to STED resolution as depletion intensity increases?

The depletion ring more strongly suppresses peripheral fluorescence • The remaining central fluorescent region becomes smaller • The lecture illustrates approximate resolution changes from 200 nm to 120 nm to 70 nm • Higher intensity can increase photobleaching and other photophysical problems

What does the saturation intensity Iₛ mean in the STED equation?

A characteristic dye-dependent intensity scale for stimulated depletion • Determines how strongly a given depletion intensity affects the fluorescent region • Lower Iₛ makes the same I correspond to a larger I/Iₛ ratio

What is the difference between 2D STED and 3D STED?

2D STED primarily narrows the lateral (xy) fluorescence PSF • 3D STED additionally shapes depletion along the axial direction • 3D STED improves z resolution as well as lateral resolution • The lecture indicates that an additional depletion beam is required for its illustrated 3D configuration

What does the confocal-versus-2D-STED-versus-3D-STED PSF figure show?

Confocal has a comparatively broad three-dimensional fluorescent PSF • 2D STED narrows it laterally while the axial extent remains larger • 3D STED reduces the axial extent too • The figure appears on slide 45

What axial resolution is quoted for 3D STED?

The lecture states up to approximately 70 nm in xz • Achieved with additional axial depletion engineering • This is distinct from the approximately 50 nm lateral resolution quoted for STED

What is gated STED?

A STED acquisition variant using time-gated fluorescence detection • The lecture compares confocal and gated STED images • The illustrated gated STED image resolves finer filament structures

What biological targets are shown in the STED comparison on slide 47?

NUP135 is labeled green • Tubulin is labeled red • Confocal and STED regions are compared at a 1 µm scale • The example demonstrates better separation of subcellular structures

What does the Maastricht STED example show?

Confocal and STED images of Alexa 488-labeled structures • STED improves structural separation but can reduce signal-to-noise ratio and contrast • Detection range 500–550 nm • Photon-counting detection • Field of view 15.5 µm

What lateral resolution is quoted for STED?

Approximately 50 nm in xy • The precise resolution depends on depletion intensity, dye and imaging conditions • The lecture also shows an illustrative 70 nm resolution image

What are the main STED limitations?

Requires bright, suitable fluorophores • Significant photobleaching • More challenging sample preparation • Complex dye photophysics, including anti-Stokes effects • Restricted spectral windows and limited color combinations • Sensitive detectors required

What detectors are recommended for STED in the lecture?

Sensitive hybrid detectors or avalanche photodiodes (APDs) • Needed because depletion reduces the number of ordinary fluorescence photons • The setup diagrams also show a PMT detection path

Why can STED images have reduced SNR and contrast?

Fluorescence is intentionally suppressed outside the central spot • Fewer photons may remain available for detection • High depletion intensities can increase photobleaching • The lecture illustrates this trade-off in the Maastricht example

Why does probe size matter for effective super-resolution?

The fluorescent marker may not sit exactly at the molecular structure being labeled • Large tags introduce an offset between fluorophore and target • That offset can become comparable to the resolution of super-resolution microscopy • Small labels help preserve spatial accuracy

What probe sizes are listed in the lecture?

Small fluorophores: approximately 1 nm • Fluorescent proteins: approximately 3 nm • Antibodies: approximately 10–50 nm • These sizes matter when interpreting very high-resolution images

Compare the approximate lateral resolution of conventional, SIM, PALM/STORM and STED.

Conventional diffraction-limited fluorescence: approximately 200 nm • SIM: approximately 120 nm • PALM/STORM: approximately 20 nm • STED: approximately 50 nm • Values are representative lecture figures, not universal guarantees

Which super-resolution technique is recommended for live-cell imaging in the lecture?

SIM • Relatively low light dose and photobleaching • Fast camera-based acquisition • Compatible with many dyes • Its resolution gain is more modest than SMLM or STED

Which method requires many repeated sparse activation cycles?

PALM/STORM • Activate only a small fraction of labels • Localize their PSFs • Repeat and reconstruct from accumulated coordinates • The repeated cycles make acquisition comparatively slow

Which method uses a diffraction grating in the typical setup?

SIM • The grating creates the structured illumination pattern • A CCD records multiple phases and angles • Reconstruction combines the resulting images

Which method uses a vortex phase plate?

STED • The phase plate produces the doughnut-shaped depletion beam • The central depletion minimum allows a small fluorescent region to remain

Which method is most directly based on estimating molecular coordinates?

PALM/STORM and other SMLM techniques • Gaussian fitting localizes isolated PSFs • Coordinates from many frames are combined into a reconstructed image

Which methods depend strongly on computational reconstruction?

SIM requires demodulation and recombination of frequency components • PALM/STORM requires fitting and combining many single-molecule coordinates • STED chiefly uses engineered optical depletion to sharpen the effective fluorescence PSF

What are the typical setups compared on the final slides?

STED: excitation and depletion lasers, phase shaping, scanning optics, sensitive detector • SMLM: activation and excitation lasers, CCD camera, drift compensator • SIM: patterned illumination optics such as a diffraction grating and a CCD camera

What should be considered when choosing between SIM, SMLM and STED?

Required spatial resolution • Whether the sample is live or fixed • Acquisition speed and allowable light dose • Fluorophore compatibility and labeling density • Imaging depth and sample preparation • Equipment and reconstruction complexity

What is the central distinction between resolving two emitters and localizing one emitter?

Resolving two emitters means separating their overlapping images • Localizing one isolated emitter means estimating the center of its PSF • An isolated emitter can be localized to nanometer precision even though its spot is diffraction-blurred • SMLM uses temporal sparsity to make this distinction useful

Why is the practical resolution of a super-resolution image not just the nominal instrument resolution?

Label size offsets fluorophore positions from targets • Insufficient label density can leave structural gaps • Noise and background impair localization or contrast • Drift, bleaching and reconstruction artifacts affect results • Sample and acquisition conditions matter

What key examples in the lecture demonstrate SIM, STORM and STED improvements?

SIM: widefield versus reconstructed filament images • STORM: diffraction-limited versus reconstructed fluorescent fibers • STED: confocal versus STED filament and NUP135/tubulin images • The figures demonstrate different ways of revealing fine structures

What is the overarching lesson of super-resolution microscopy?

The conventional ~200 nm diffraction limit is not the ultimate limit of fluorescence imaging • SIM recovers additional spatial-frequency information • SMLM estimates sparse molecular positions • STED shrinks the fluorescent PSF with stimulated depletion • The most suitable technique depends on resolution, speed, sample and fluorophores