Septal Penetration and High-Energy Collimators

Introduction

A collimator is the lens of a gamma camera, and like any lens it is tuned to a specific energy — image a high-energy emitter like I-131 through a low-energy collimator and the photons bore straight through the lead septa, filling the picture with a haze that no reconstruction can undo. The collimator is the only part of the imaging chain that establishes direction, and it does so by absorbing the photons that are not traveling where they should. When that absorption fails, the image fails. ^{1, 2}

Septal penetration is the quiet failure mode behind many poor radioiodine and gallium images. It does not produce an error message; it produces a plausible-looking image with washed-out contrast, a faint star pattern, and counts that cannot be trusted for quantification. Because the cause is upstream — the wrong collimator for the photon energy — no amount of windowing or filtering at the workstation repairs it. ^{3, 4}

This article explains the physics of septal penetration, why high-energy isotopes demand medium- and high-energy collimators, and the resolution-and-sensitivity price those collimators charge. DRPS works through exactly these choices with facilities as part of its PET/CT and nuclear medicine physics services across Florida, Maryland, Virginia, Washington DC, California, and Nevada.

Topic Explanation

What the collimator does

A parallel-hole collimator is a slab of lead (or a lead-equivalent) perforated by thousands of parallel channels separated by thin walls called septa. Only photons traveling nearly parallel to the hole axis reach the detector; photons arriving at an angle strike a septum and are, ideally, absorbed. This is how a gamma camera forms a projection image — it rejects the mis-directed photons rather than focusing anything. ²

The word "ideally" is the whole problem. Septa are thin, lead is not perfectly opaque, and higher-energy photons are harder to stop. A photon that passes through one or more septa and still reaches the detector is counted — but it is assigned to the hole it came out of, not the hole it should have traveled down. It is a mispositioned count. ³

What septal penetration is

Septal penetration is the fraction of detected photons that reach the detector through the septa rather than through the holes. Penetrating photons form a characteristic pattern: for a hexagonal hole array, point sources develop a six-pointed star artifact, and extended sources lose contrast as a low-spatial-frequency "tail" is added to the collimator's response function. ^{3, 4}

Collimators are designed so that septal penetration is held below a conventional limit of about 5% for the maximum photon energy they are intended to image. Exceed that energy and penetration climbs steeply, because the stopping power of lead falls as photon energy rises. ^{1, 2}

Why photon energy is the deciding factor

The physics is captured by one quantity: the linear attenuation coefficient of lead, which measures how strongly lead absorbs photons of a given energy. Using published lead shielding data, the half-value layer (HVL) of lead is about 0.23 mm for Tc-99m's 140 keV photons but about 2.74 mm for I-131's 364 keV photons — roughly a twelve-fold difference. ⁵

Converting HVL to a linear attenuation coefficient, $μ = ln 2/ HVL$ , gives lead approximately $30 cm^{- 1}$ at 140 keV and only about $2.5 cm^{- 1}$ at 364 keV. ⁵ A septum that comfortably absorbs 140 keV is nearly transparent to 364 keV. That single fact drives the entire collimator-selection decision.

Key Technical Principles

The septal penetration design criterion

Consider the shortest possible path $w$ a photon can take through septal lead on its way to the detector. The transmitted fraction follows exponential attenuation:

penetration \approx e^{- μ w}

Holding penetration at or below 5% requires $μ w \geq 3$ (since $e^{- 3} \approx 0.05$ ), i.e. $w \geq 3/ μ$ . Geometry relates this minimum path to the septal thickness $t$ , hole diameter $d$ , and hole length $L$ , giving the classic minimum-septal-thickness condition: ^{1, 2}

t \geq \frac{6 d}{μL - 3}

The consequence is immediate. For I-131, $μ$ is about twelve times smaller than for Tc-99m, so $μL$ in the denominator shrinks, and the septal thickness $t$ required to satisfy the 5% criterion grows sharply. High-energy collimators carry thick septa not by preference but by necessity. ^{2, 5}

The price of thick septa: resolution and sensitivity

Lead spent on septa is space not spent on open holes, and that is where the penalty appears. The geometric (collimator) component of spatial resolution for a source at distance $b$ from the collimator face is:

R_{g} = \frac{d ( L _{eff} + b )}{L _{eff}}, L_{eff} = L - \frac{2}{μ}

and the geometric sensitivity scales as:

g \propto [\frac{d ^{2}}{L _{eff} ( d + t )}]^{2}

These two expressions pull against each other. Smaller holes ( $d$ ) and longer bores ( $L$ ) sharpen resolution but, as the sensitivity expression shows, cut sensitivity hard (it falls roughly as the square of $d^{2} / L$ ). Thicker septa ( $t$ ) increase the $(d + t)$ term in the denominator, lowering sensitivity further. A high-energy collimator, forced into thick septa, therefore ends up with inherently poorer resolution, lower sensitivity, or both, compared with a low-energy collimator. ²

A worked resolution example

Take a representative low-energy high-resolution (LEHR) collimator with hole diameter $d = 1.4 mm$ and hole length $L = 24 mm$ , imaging Tc-99m. With $μ \approx 30 cm^{- 1}$ , the effective length is $L_{eff} = 24 - (2/30.1) \times 10 \approx 23.3 mm$ . For a source 100 mm from the collimator face:

R_{g} = \frac{1.4 ( 23.3 + 100 )}{23.3} = \frac{1.4 \times 123.3}{23.3} \approx 7.4 mm

Resolution degrades linearly with distance $b$ — a core reason to keep the detector close to the patient. Now image I-131 through this same collimator and the result is dominated not by $R_{g}$ but by penetration: a large fraction of 364 keV photons never interact with a septum at all, and the clean 7.4 mm geometry is buried under the penetration tail and star artifact. The fix is not a better low-energy collimator; it is a high-energy collimator. ^{1, 3}

Matching collimator class to isotope

Collimator class	Max photon energy (approx.)	Example isotopes	Septa	Resolution	Sensitivity
Low-energy high-resolution (LEHR)	~150 keV	Tc-99m (140 keV)	Thinnest	Best	Lower (resolution-optimized)
Low-energy all-purpose (LEAP/LEGP)	~150 keV	Tc-99m	Thin	Good	Moderate
Medium energy (ME)	~300 keV	In-111 (171, 245 keV); Ga-67 (~185, 300 keV)	Thicker	Moderate	Moderate
High energy (HE/HEGP)	~364–400 keV	I-131 (364 keV)	Thickest	Poorest	Lowest

Photon energies here follow standard decay-data compilations. ⁸ The table's ordering is the trade-off in one view: as the isotope energy rises, septa thicken and both resolution and sensitivity give way.

Clinical Impact

Septal penetration is not an abstraction; it shows up in specific studies.

Radioiodine imaging (I-131). Diagnostic and post-therapy I-131 scans, and thyroid-cancer or neuroendocrine dosimetry studies, must use a high-energy collimator. A low- or medium-energy collimator produces star artifacts and overstated counts near hot foci, with the error worst exactly where it matters most — around intense uptake. ^{4, 6}
In-111 and Ga-67. These isotopes carry photons up to roughly 245–300 keV. Imaged on a low-energy collimator, the higher peaks penetrate, blurring octreotide, white-cell, or gallium images; the medium-energy collimator is the standard choice. ¹
Quantitative SPECT and dosimetry. Penetration adds mispositioned counts that bias activity estimates. For I-131 therapy dosimetry this bias flows straight into absorbed-dose calculations. Phantom work shows that modeling the collimator-detector response and scatter can bring I-131 SPECT quantification error to within a few percent, but that modeling assumes the correct collimator was used in the first place. ⁶
Dual-isotope and downscatter situations. When a high-energy emitter is present, its penetrating and downscattered photons can contaminate a lower-energy window, so collimator choice interacts with energy-window and scatter-correction strategy. ^{3, 6}

Practical Optimization Tips

Select the collimator by the highest significant photon energy, not the imaging peak. In-111's 245 keV photons and Ga-67's ~300 keV photons dictate a medium-energy collimator even though lower peaks are used for imaging. I-131's 364 keV dictates high energy. ¹
Never image I-131 on a low- or medium-energy collimator for anything you intend to interpret quantitatively or around intense uptake — the star artifact and count bias are not recoverable at the workstation. ⁴
Keep the detector as close to the patient as possible. Geometric resolution degrades linearly with distance; contouring the orbit tightly is the cheapest resolution improvement available. ²
Match the energy window to the collimator and isotope, and use scatter/penetration correction (for example, triple-energy-window or model-based methods) when quantifying, rather than relying on a single photopeak window. ^{3, 6}
Verify collimator condition and mounting as part of QC. Damaged or mis-seated collimators and foil defects create their own artifacts; routine quality-control procedures for SPECT systems include checks of collimator and detector-head mountings and collimator damage. ¹
For dosimetry studies, validate the full acquisition-and-reconstruction chain with a phantom of known activity so penetration and scatter corrections are demonstrated, not assumed. ⁶

Regulatory Considerations

Collimator selection and the performance characterization behind it sit within a facility's nuclear medicine quality-management and accreditation framework, even though no single regulation prescribes which collimator to mount. The governing documents are consensus performance standards and quality-assurance guidance, applied through the facility's QC program and overseen by its authorized users and medical physicist. ^{1, 7}

Key frameworks to reference:

NEMA NU 1-2023 defines how gamma camera performance parameters — including spatial resolution and sensitivity, which depend on the collimator — are measured and reported, giving a common basis for specification and acceptance testing. ⁷
IAEA Human Health Series No. 6 provides detailed quality-control test procedures for scintillation cameras and SPECT systems, including collimator and detector-head checks. ¹
ICRP Publication 107 is the standard compilation of nuclear decay data (photon energies and yields) that underlies every collimator-energy match. ⁸
SNMMI and EANM imaging practice standards specify collimator and energy-window choices for individual radiopharmaceutical procedures.

Accreditation programs expect that acquisition parameters, including collimator selection, are documented in protocols and that system performance is verified at acceptance and on a routine QC schedule. The medical physicist typically sets and validates these choices. For related performance testing, see our guides on gamma camera sensitivity QC and the bar-phantom resolution and linearity QC.

Frequently Asked Questions (FAQs)

What is collimator septal penetration?

Septal penetration is the fraction of detected photons that reach the detector by passing through the lead walls (septa) between collimator holes instead of traveling down a hole. Because photons that penetrate the septa are not aligned with the hole they appear to have come from, they are mispositioned, degrading contrast and resolution and biasing quantification. Collimators are designed to keep penetration below roughly five percent for the photon energy they are intended for.

Why can't I image I-131 with a low-energy collimator?

I-131's main gamma ray is 364 keV. Lead attenuates 364 keV photons roughly an order of magnitude less effectively than it attenuates Tc-99m's 140 keV photons, so a thin low-energy septum that stops 140 keV lets a large fraction of 364 keV photons through. The result is heavy septal penetration, a characteristic star artifact, loss of contrast, and unreliable counts. I-131 requires a high-energy collimator with thick septa.

What is the difference between low-, medium-, and high-energy collimators?

They differ mainly in septal thickness, matched to the maximum photon energy they are designed to image. Low-energy collimators (for Tc-99m, about 140 keV) have thin septa and the best resolution and sensitivity. Medium-energy collimators handle photons up to roughly 300 keV (In-111, Ga-67). High-energy collimators handle photons up to roughly 364 to 400 keV (I-131) with the thickest septa and the greatest resolution and sensitivity penalty.

How does septal thickness affect image resolution and sensitivity?

Thicker septa needed for higher photon energies take up space that could otherwise be open collimator holes. To preserve resolution, holes are made longer, which lowers sensitivity; to preserve sensitivity, holes are made larger or shorter, which worsens resolution. High-energy collimators therefore have inherently poorer spatial resolution and lower sensitivity than low-energy collimators.

Can image processing correct for septal penetration?

Partly. Modern quantitative SPECT reconstruction can model the full collimator-detector response, including septal penetration and scatter, and this improves accuracy substantially. But modeling cannot recover information that penetration has destroyed as well as simply using the correct collimator does. The first and most important control is choosing the right collimator for the isotope.

Which collimator should I use for In-111 or Ga-67?

In-111 (171 and 245 keV) and Ga-67 (with emissions near 185 and 300 keV) are typically imaged with a medium-energy collimator. A low-energy collimator would suffer significant septal penetration from the higher-energy photons of these isotopes, degrading image quality and quantitative accuracy, even though the lower-energy peaks alone might seem compatible.

Does septal penetration matter for dosimetry, or only for image appearance?

It matters for dosimetry. Penetration adds mispositioned counts that bias activity quantification, which propagates directly into absorbed-dose estimates for radionuclide therapy such as I-131. Quantitative imaging for dosimetry depends on controlling and correcting penetration, starting with the correct collimator.

Key Takeaways

Septal penetration is the collimator failing to stop mis-directed photons, and it produces star artifacts, lost contrast, and biased counts that cannot be fixed downstream.
Photon energy drives everything. Lead's attenuation coefficient falls roughly twelve-fold from 140 keV to 364 keV, so the collimator must be matched to the isotope's highest significant photon energy.
Collimators are classed by septal thickness: low energy for Tc-99m, medium energy for In-111 and Ga-67, high energy for I-131.
Thick septa cost resolution and sensitivity — the fundamental trade-off that makes high-energy imaging intrinsically harder.
Choose by the highest energy, keep the detector close, and correct penetration and scatter when quantifying.
Dosimetry depends on it. Penetration bias propagates into absorbed-dose estimates for radionuclide therapy.

Conclusion

The collimator decision looks like a menu choice — LEHR, medium, high — but it is really a physics calculation about one isotope and one number: the energy of its most penetrating significant photon. Lead that is opaque to Tc-99m is translucent to I-131, and the septa must thicken to compensate, paying for it in resolution and sensitivity. Understanding that trade-off is what separates a diagnostic radioiodine image from a star-filled haze, and a trustworthy dosimetry number from a biased one.

No workstation tool substitutes for the right collimator. Select it by the highest significant photon energy, keep the detector close, window and correct appropriately, and verify the whole chain with a phantom when the counts have to mean something.

How DRPS Can Help

Diagnostic Radiation Physics Services helps nuclear medicine programs match collimators to their radiopharmaceuticals, validate spatial resolution and sensitivity at acceptance and on a QC schedule, and build quantitative SPECT and dosimetry workflows that control septal penetration and scatter. This work is part of our PET/CT and nuclear medicine physics, accreditation support, and medical physicist consulting services.

DRPS supports facilities across our service locations, including Florida, Maryland, Virginia, Washington DC, California, Nevada, New York, Pennsylvania, New Jersey, and Delaware.

The lens has to match the light. For a gamma camera, that means matching the collimator to the photon.

Related Resources

References

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