In an ideal radiograph every photon passes through the patient in a straight line to the detector, carrying the information of the tissue it crossed. In reality, a large fraction of photons are scattered in the tissue — they change direction and strike the detector at the wrong location. These scattered photons add no real information; they only add a "fog" spread everywhere, lowering contrast and SNR. Scatter is image quality's biggest hidden enemy in radiography.
What is scatter?
At diagnostic energies, one of the main interactions of photons with tissue is Compton scattering: a photon collides with an electron, changes direction and continues. Because these scattered photons reach the detector at random locations, in digital radiography they act essentially as a source of noise and lower the signal-to-noise ratio.1
SPR and fraction
The amount of scatter is measured by the scatter-to-primary ratio (SPR): the energy deposited at a given detector location by scattered photons divided by that deposited by primary (non-scattered) photons.1
If SPR = 1, half the energy at that point comes from scatter — i.e. half the information is useless.1 SPR is not fixed: it increases as the irradiated tissue volume grows. Both field size and patient thickness raise SPR; so scatter is a much bigger problem in the abdomen than in extremity radiography (e.g. SPR can reach ~4.5 at 25 cm thickness).1
How to reduce it
The classic ways to reduce scatter:
- Collimation: limiting the beam field to the needed region directly lowers the irradiated volume — and hence SPR. One of the simplest and most effective steps.1
- Anti-scatter grid: placed between patient and detector, the grid absorbs angled scattered photons and largely passes the primary beam; it is the most widely used scatter-reduction technology in radiography. Detail: What Is an Anti-scatter Grid?.
- Air gap: opening the gap between patient and detector lets some angled scattered photons miss the detector.
References
- Bushberg JT, Seibert JA, Leidholdt EM, Boone JM. The Essential Physics of Medical Imaging, 3rd ed. Lippincott Williams & Wilkins, 2011. §7 (Radiography): saçılma bir gürültü kaynağı olarak SNR'yi düşürür; saçılan/primer oranı SPR = S/P (Denklem 7-8); saçılma fraksiyonu F = SPR/(SPR+1) (Denklem 7-9, 7-10); SPR alan boyutu ve hasta kalınlığıyla artar — abdomen, ekstremiteye göre çok daha sorunlu (Şekil 7-21); kolimasyon SPR'yi azaltır; örn. 25 cm hastada SPR ~4,5 (s.231). Sayfa numaraları bu baskıya aittir.
- İlişkili: Grid (Izgara) Nedir? · Işınlama Parametreleri · Görüntü Kalitesi