Scan Converter, Display & Image Storage

The Digital Scan Converter: The Bridge to the Display

After the receiver has amplified, compensated (TGC), compressed (dynamic range), and processed (Section 7.4) the echo data, the signal must be translated into a format a video display can show. This is the job of the digital scan converter (DSC). The scan converter takes the beam former's line-by-line acoustic data — acquired along radial or sequential scan lines that follow the transducer's actual scanning geometry — and converts (or "maps") it into a rectangular grid of picture elements, or pixels, arranged in rows and columns that match standard video display geometry. This conversion step is essential because raw ultrasound scan-line data is not naturally arranged in the row-and-column grid that a monitor displays; the scan converter performs the geometric transformation (and interpolates data to fill in gaps between scan lines) so that a curved or sector-shaped acquisition pattern still displays correctly as a properly proportioned image on a rectangular screen.

The scan converter also holds the image memory referenced throughout Section 7.4 — the storage into which pre-processing writes final values and from which post-processing reads and modifies them for display.

Pixels, Bits per Pixel, and Gray Shades

Each pixel in the displayed image stores a numeric value representing brightness. The number of distinct brightness levels (gray shades) any given pixel can display is determined by how many bits are allocated to store that pixel's value — the pixel's bit depth.

⚠ Gray Shades = 2^(bits per pixel)

Because each bit is a binary digit (0 or 1), the number of unique values (gray shades) representable by n bits is:

Number of gray shades = 2ⁿ, where n = bits per pixel.

Most clinical grayscale ultrasound systems use 8 bits per pixel, yielding 256 (2⁸) shades of gray — the conventional benchmark cited across SPI review material. Increasing bit depth increases the number of distinguishable gray shades, which increases the image's potential contrast resolution, but only up to the point where the human eye and ambient viewing conditions can still discriminate adjacent shades; beyond a certain bit depth, additional shades add no perceptible benefit. Color and power Doppler information, layered on top of the grayscale B-mode image, is stored and displayed using additional bits assigned separately from the grayscale bits.

Image Memory and Frame Rate Storage

Because the scan converter stores a complete frame in digital memory before displaying it, it can also hold and quickly recall a sequence of recent frames — the basis for cine review (scrolling back through recently acquired frames) and for persistence/frame-averaging processing described in Section 7.4. The scan converter's digital memory is what makes both freeze-frame review and all forms of post-processing possible, since post-processing by definition operates on data already resident in this memory.

The Monitor: Final Display

The monitor (display) presents the scan-converted, pixel-mapped image to the sonographer. Clinical ultrasound monitors are calibrated to render the full range of stored gray shades as accurately and consistently as possible; ambient lighting, monitor brightness/contrast settings, and monitor calibration all affect how many of the theoretically available gray shades (e.g., 256 at 8 bits/pixel) a viewer can actually distinguish in practice. This is why SPI candidates should understand that "number of gray shades stored" (a bit-depth property) and "number of gray shades perceptible" (a viewing-conditions property) are related but distinct concepts.

Image Storage: DICOM and PACS

Once an image or clip is finalized, it must be archived in a standardized, retrievable format for clinical use, comparison with prior exams, and interpretation by the physician.

• DICOM (Digital Imaging and Communications in Medicine) is the universal standard file format and communication protocol for storing and transmitting medical images (including ultrasound), ensuring that images captured on one vendor's equipment can be opened, measured, and interpreted correctly on another vendor's viewing software.
• PACS (Picture Archiving and Communication System) is the networked hospital/clinic system that stores, indexes, retrieves, and distributes DICOM images across a healthcare enterprise, allowing images from an ultrasound suite to be reviewed at any authorized workstation, archived long-term, and compared against prior studies.

Together, DICOM (the format standard) and PACS (the storage/distribution infrastructure) form the final link in the pulse-echo imaging chain covered across this chapter: from the pulser generating the transmit pulse (7.1), through receiver amplification and TGC (7.2), dynamic range compression (7.3), and signal processing (7.4), to scan conversion, pixel-based display, and long-term digital archiving (7.5).

Chapter 7 Chain Summary