Put the gaze in motion—sweep it across a field: take it all in, without pause. Binoculars along the horizon. Or if not the gaze, sweep with an energy that can lend itself to vision. Ultrasound, across the ocean bottom. X-rays, around a body. Electron beam, across a sample. Or sweep with the Archive’s electronic eye: inventory gun, across the barcode. Starfleet tricorder, across the alien landscape. Or, sweep with an energy that subtends some sense other than vision. Spin the FM dial for a clear channel. Sweep an algorithm through recorded speech-acts, for words that plot terror.

A scan is a sense, or some sensory prosthesis, in transit through space-time, gathering without discrimination. The scan is not the glance—nor the rapt stationary gaze. It is observation in regulated motion. The motion can serve various motives: anxiety for potential prey, or mastery via surveillance.

After the sweep, the transit, the gathering of impressions…then comes the culling. The scan accumulates a swath of data for review, analysis, selection, examination, comparison. Focus the binoculars back on that smudge at bearing 0-5-6: what is that? This moment of closer inspection, interpretation, classification culminates the scan. It is hinged to the scan, is one of the teloi of the scan.

In medicine, we scan through the body, or some body part. We scan to observe corporeal structures, flows, or functions. Structure came first, befitting a clinic of lesions, of diseases arising from “seats and causes”: ultrasound scans, CT scans (x-ray computed tomography), MRI scans (magnetic resonance imaging). Along with structures came flows and functions: Doppler ultrasound scans, nuclear medicine tracer scans (thyroid scans; bone scans; lung ventilation and perfusion scans; heart muscle perfusion scans; PET scans (positron emission tomography); fMR scans (functional magnetic resonance).

Medical scanning modalities come to bodies, arriving from extra-medical theatres. Ultrasound came from undersea warfare. Computed tomography came from computer pattern-recognition of letters, math of line integrals, and a machine-shop lathe. Radioisotopes for nuclear scans emerged from weapon sciences’ postwar retooling. And even in the clinic, we scan at scales beyond those of bodies. We scan genomes, microbiotic proteomes: from the molecular to the populational.

Machines are crucial to these and other modes of scanning. Yet machine-operators—persons with know-how and tact—remain integral to scans’ logistics in the clinic. In ultrasound, a technologist handles a wand, sliding it across a region of interest. In CT scanning, a circumferential sweeping gesture is “black-boxed” in the machine, but a technologist still sets the schedule for this movement’s itinerary around each bodily volume.

Machine and operator are just the beginning of the assemblage necessary to perform a scan. There are special shielded rooms and utilities (cooling & air exchanges for x-ray tubes and computers; radioisotope production facilities for nuclear scans). There are other and various personnel: schedulers, transporters, IT support, custodial staff; nurses, radiologists, and client clinicians. And there are other materials flowing in and out of the scanning spaces and the bodies that occupy them, some special—contrast materials—and some quite ordinary—sheets and towels, urine and shit.

Inside the box? In a CT scanner, the donut houses a ring of x-ray detectors and a spinning x-ray tube aimed across the ring. As the tube spins, detectors across the ring measure energy. Each detector’s momentary measurement correlates with x-ray absorption along the column of tissue between it and the tube—part of a fleeting x-ray “cut” through the tissue. A computer accumulates data from hundreds of such column-cuts from one spin, and then performs complex mathematical operations to derive a grid of x-ray absorption coefficients distributed across the plane of the spin. This planar distribution of absorption numbers (which correlate with local tissue density), once converted to grey-scale “pixels,” constitute one slice-image. This is repeated dozens or hundreds of times, at regular intervals along the body. A bit like the spin-cutting of a ham-hock—it’s even done in a tight spiral now. Other kinds of scanning use other black-boxed energy sources, detectors, and computers to translate data into legible images. Many of them likewise accumulate their data as slices—and eventually, often, display their findings as slices. Scans have taught the clinic to see in slices.

The slice cuts but it also sutures. “Scan” references both a technical procedure (a patient undergoes a scan) and the image/representation that is produced in the process (here are scans from the other hospital, let’s pull that scan out of the archives). This bit of reification collapses the work of scanning into a durable, mobile image. Yet perhaps it also anticipates the further interpretive work required. With CT, MR, ultrasound, etc., an embodied and trained gaze must review the images to extract actionable intelligence. Is this thing real or an artifact? Normal or abnormal? Friend or foe? Is this a sign of once or future illness? Some of this process—“reading” or interpreting—may be supplemental to the mechanical part of a scan, but it also sometimes constitutes another “scanning”—a sweeping look across, or scrolling through, a set of images.

Today’s clinical scanning assemblages articulate hardware with informatics with human “wetware.” Humans bring the diagnostic curiosity and angst. Are there fully automated scanning procedures out there, assemblages that do not require machine-operator tact, or submit results to a play of human intelligence after their conclusion? Perhaps. Roles of machines and algorithms are at least expanding, enough to make some radiologists concerned about their eventual replaceability. There is no escaping the affinity of scanning with mechanism. Mechanism is ingredient to the scan: to move a sensor through a measured, even sweep, one not susceptible to distraction or premature closure, one must at least imitate a machine. The merely human, all-too-human gaze is otherwise easily waylaid by worry, desire, reflection.

There are perhaps few ways in which a scan, or a scanner, requires bodies. The roles of its operators and patient-subjects are minimized. The Scan is a regularized gesture, a machinic logistics of sweep-sensing. It is deployed across platforms. And the drama of detection we associate with it, the economy of affect that distributes responsibilities of humans for each others’ fates, may be a contingent supplement, if not a historical residue. The Scan’s findings can be plugged directly into the Archive.

Barry Saunders is a cultural anthropologist of biomedicine at UNC-Chapel Hill’s Department of Social Medicine. His book CT Suite: The Work of Diagnosis in the Age of Noninvasive Cutting is an ethnography and philosophical history of CT scanning. He is currently working on a history of “chiasms”—optic, rhetorical, and philosophical—at particular junctures of modernity. And extending the teaching of STS perspectives in medical school.

Image: “Lego toy MRI.” voxel123, flickr.

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