What Is DICOM? Difference from JPEG, Window/Level, Header and PACS

When you save a CT image to your phone and send it to a friend, you have a picture. But radiology stores that image not as a picture but as measurable data: each pixel has a numeric meaning (e.g. a Hounsfield unit), which patient it belongs to, what kVp it was acquired at... The standard that keeps all of this together and carries it from device to device is DICOM (Digital Imaging and Communications in Medicine). Without DICOM, neither PACS nor the modern radiology workflow would be possible.

What is DICOM?

Connecting devices to a network is not enough on its own to transfer images: vendors may use different, proprietary formats.¹ To solve this, the ACR (American College of Radiology) and NEMA (National Electrical Manufacturers' Association) jointly developed the DICOM standard.¹ DICOM is not just a file format; it is also a network language. It defines three things at once: the standard format of the images and information to be transferred, the services one device can request from another, and the messages between devices.¹ DICOM defines information objects like "patient", "study" and "image", which combine into CT, CR, digital x-ray (DX), mammography (MG), ultrasound (US), MR and nuclear medicine (NM) image objects.¹

Difference from JPEG

This is the most commonly confused point. A JPEG (or PNG) carries only pixels: typically 8 bits per channel (256 tones), compressed, with no medical context. DICOM is a container: it holds both full-dynamic-range pixel data (in CT usually 12–16 bit, i.e. 4096 or more gray levels) and an extensive header.² The difference: if you export a CT as a JPEG you lose the bit depth, the calibration (the HU values) and the entire header — you can no longer measure density or re-window. DICOM preserves not the picture but the measurement. (DICOM can also compress; but options like lossless JPEG/JPEG2000 preserve diagnostic information.)

A JPEG carries only pixels; DICOM keeps full-dynamic-range pixel data plus the medical header together. That is why density can be measured and the window re-adjusted in DICOM.²

Why does window/level change?

This is DICOM's most visible consequence. CT data carries thousands of gray levels (12 bit = 4096), but a monitor shows ~256 tones and the eye discriminates fewer. The solution: select not all values but a range, and spread it from black to white. This is window/level, done with a look-up table (LUT).¹ The level (window center) sets the midpoint of the values shown, and the window width sets the range about that point.¹ Values below the chosen window saturate to black, those above to white:¹

lower = Level − Window/2 · upper = Level + Window/2

The key point: window/level does not change the data, only which range is shown on screen at that moment. The same DICOM file looks completely different in a "soft-tissue window", a "bone window" and a "lung window" — because the pixel values stay fixed while the LUT changes.

The chosen window slides and widens along the HU axis; values inside it spread from black to white, those outside saturate. The data never changes — only the mapping (LUT).¹

In the clinic

The wrong window can hide a real finding: a pneumothorax missed in a soft-tissue window appears in a lung window; a faint bleed seen in a brain window vanishes completely in a bone window. This is why CT is routinely reviewed in several windows.

The DICOM header carries everything that makes the image "medical". Information is stored as data elements, each addressed by a tag — a (group, element) pair, e.g. (0010,0010) = Patient Name.² The header typically holds:

Patient and study identity: name, ID, date of birth; unique identifiers (UIDs) for study/series/image.
Modality and acquisition parameters: modality (CT/MR/US…), kVp, mAs, slice thickness, pixel spacing, acquisition date/time, equipment info.
Image-data description: rows/columns, bit depth, and the rescale slope/intercept — coefficients that convert the stored raw pixel value into a meaningful value such as HU.²
Presentation information: suggested window/level presets and the Presentation State — storing, together with the image on PACS, the radiologist's zoom, crop, flip, windowing and annotations such as arrows and notes.¹

Why it matters

Without the rescale slope/intercept, a "pixel value" is just a number; with these coefficients it becomes a Hounsfield unit and density measurement gains meaning. In an image flattened to JPEG this link is broken, so HU can no longer be read.

Relationship to PACS

PACS (Picture Archiving and Communication System) is the system that stores and supplies digital medical images for display.¹ The relationship is simple: DICOM is the language, PACS is the system that speaks it. The modality produces the image in DICOM format and sends it to PACS with a storage service; the radiologist workstation retrieves it from PACS with query-retrieve services. DICOM also manages workflow: the modality worklist (MWL) tells which patient is to be imaged on which device, so patient information arrives automatically from the RIS rather than by hand.¹ In short, DICOM is the common protocol that gets an image from the device to the radiologist's screen.

The modality sends the image to PACS with DICOM (C-STORE); the radiologist retrieves it from PACS (C-FIND/C-MOVE). The modality worklist pulls patient data automatically from the RIS.¹

How the image forms: How Is an X-ray Image Formed? · Pixel value and HU: Dose in CT · Image quality: Image Quality

References

Bushberg JT, Seibert JA, Leidholdt EM, Boone JM. The Essential Physics of Medical Imaging, 3rd ed. Lippincott Williams & Wilkins, 2011. Bölüm 5 (Medical Imaging Informatics): DICOM — ACR ve NEMA ortak standardı; bilgi nesneleri (CT, CR, DX, MG, US, MR, NM), servisler (depolama, sorgula-getir), modalite çalışma listesi (MWL) ve Presentation State (s.147–149); PACS tanımı (s.143); DICOM GSDF ile görüntü tutarlılığı (s.135–137); window/level — en yaygın LUT, L−W/2 altında siyaha, L+W/2 üstünde beyaza doygunluk (Şekil 4-31, s.90); pencereleme matematiği — seviye = orta nokta, pencere = aralık, alt sınır = Seviye − Pencere/2, üst sınır = Seviye + Pencere/2 (Bölüm 5). Sayfa numaraları bu baskıya aittir.
NEMA. DICOM Standard (PS3) — Digital Imaging and Communications in Medicine; veri elemanları/etiketler (group,element), bilgi nesneleri, ağ servisleri ve yeniden ölçekleme (rescale slope/intercept). dicomstandard.org
İlişkili konular: piksel değerinin HU'ya dönüşümü için bkz. BT'de Doz ve görüntü kalitesi kavramları için Görüntü Kalitesi.

Note: This content is for education; for clinical decisions or regulatory compliance, consult a qualified medical physicist and current regulations.

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What Is DICOM? JPEG, Window/Level, Header, PACS