Hospitals hand scans over as DICOM files, usually on a CD or USB stick with a Windows viewer that will not run on a Mac, a Chromebook, a phone or a computer without a disc drive. This online DICOM viewer opens the files themselves, free and with no sign-up: point it at the whole disc and it reads the disc's index (DICOMDIR), sorts every image into patient, study and series with thumbnails, and lets you scroll through the slices, adjust the contrast, zoom, turn and measure. A CT or MRI stack can be rebuilt in three planes (axial, coronal and sagittal, known as MPR), and two series can be compared side by side. With no scan to hand, Try a sample scan opens a synthetic head CT made for this page.
It reads the compressed forms discs actually use - JPEG, JPEG lossless, JPEG-LS, JPEG 2000 and RLE - as well as uncompressed files, in greyscale and colour. Scans are about as private as files get, so they are read in the page and never sent anywhere.
A CT or MRI scan is a stack of slices: each series is one stack, and an MRI usually has several (T1, T2, FLAIR and so on) taken in different directions - choose them from the thumbnails. CT values are Hounsfield units, which is what the CT windows and the area measurement use. An X-ray or mammogram is usually one image per series. Ultrasound often comes as a cine loop, which plays at the speed recorded in the file; change the frames per second to slow it down. Reports from the radiologist (structured reports) and radiotherapy plans have no image, so their details are listed under All tags.
- How do I view an MRI or CT scan CD on a Mac or a computer without the disc's viewer?
- The program on the disc usually only runs on Windows, but the scans themselves are ordinary DICOM files. Insert the disc (or copy it to the computer first), press Choose a folder and pick the whole disc. The page reads the disc's index, DICOMDIR, and every scan, sorts them into series and opens the largest one. If it says the index lists more images than were found, choose the whole disc rather than one folder in it.
- What is the DICOMDIR file on my scan disc?
- It is the disc's table of contents: it lists the patient, the studies, the series and where each image file is, but holds no images itself. Choosing the folder that contains DICOMDIR (usually the whole disc) opens everything it lists; opening DICOMDIR on its own shows only a message saying so.
- Does the saved picture include my name?
- Not from this page. Saved images and videos contain only the picture, with your measurements if you leave them ticked: the name, date of birth and study details in the corners are drawn over the picture on screen, not into it - but a screenshot would catch them, which is what Hide patient details is for. Some scans, often ultrasound and scanned documents, have the name burned into the image itself by the machine that made them; the page says so when a file declares this, and that text stays in the saved picture.
- How do I share a DICOM scan without my personal details?
- Press Anonymise and save .zip. The ZIP holds copies of the DICOM files with the patient's name, ID, birth date, address, the hospital and doctors' names, the dates and all private tags removed or emptied, and new study, series and image IDs, following the DICOM standard's basic de-identification profile (PS3.15), down to the details nested inside other details. The files are renamed too, and the disc's index (DICOMDIR), which lists the patient's name, is left out. The images are untouched, so anyone with a DICOM viewer sees the same pictures. A file that is cut short or whose structure is damaged is left out and named, rather than passed on half cleaned, and details stored with no type that the profile cannot recognise are removed. Tick Keep scan dates and descriptions if the person needs the dates, the study and series descriptions and the protocol name. Those descriptions are kept exactly as staff typed them and may contain names — check them before sharing. Text burned into the picture itself cannot be removed: the page warns when a file says it has some, and when it is an ultrasound, a scanned document or a screen capture, where that is common.
- Can I measure on the scan, and how accurate is it?
- Yes: Length measures in millimetres, Angle in degrees, and Area draws an ellipse and gives its area with the mean and spread of the values inside (Hounsfield units on CT). The size comes from the pixel spacing the scanner wrote into the file. Measuring works in the single view; in the three-plane view it is off. On X-rays that spacing is often at the detector, which makes things look a few percent larger than in the body, and the page says when that is so; when a file gives no spacing, lengths are in pixels.
- Why does a scan look too dark or washed out?
- A CT or MRI holds far more shades than a screen can show, so a viewer has to pick which range to spread across black to white. The file usually suggests one, sometimes several by name; Auto uses the first. Dragging on the image with Contrast changes it, Full range shows everything at once, and the CT windows are the settings radiologists use for lung, bone, brain, abdomen, liver and soft tissue.
- Can it read every DICOM file?
- Most of what is on a patient disc, yes. The exceptions are 12-bit extended JPEG, an older compression now rarely used (8-bit extended JPEG does open), and files with no DICOM header that are also big-endian. For those the details and tags still show, and the page says why the picture does not. Structured reports and radiotherapy plans have no image at all, so only their tags are shown.
- Will a big CT with thousands of slices work on a phone?
- Yes. Each file is read only as far as its details to sort it, and an image is decoded when it is shown; only a limited number of decoded images are kept for scrolling back, fewer on phones, so a long study does not fill the phone's memory. Reading a disc with thousands of files still takes a little while, and the page counts them as it goes.
- Can it show coronal and sagittal views of a CT or MRI (MPR)?
- Yes, for a series that is one stack of slices: the same size, pixel spacing and orientation, each further along the same direction. Press Three planes and the slices are sorted by their position in the body (not by their numbers, which can be wrong) and rebuilt into the other two planes at their true proportions, using the pixel spacing and the real distance between slices; unevenly spaced slices are placed where they really are and blended in between, and the page says so. It is not offered, with the reason shown, for a single image, a cine loop or other multi-frame file, slices in mixed orientations, the same slices repeated (several time points), a tilted scanner gantry, or a stack with four or more slices missing in one place. When one to three slices seem to be missing, the page says about how many and the views blend across the gap. A very large scan is shown at half or a quarter of its resolution on a phone or small computer to fit its memory, or refused if even that is too much. Measuring is off in the three planes, and works in the single view.
- What do MIP and Average slab do?
- They look through a thicker slice than one plane: MIP (maximum intensity projection) shows the brightest value through the slab, which makes contrast-filled vessels and bone stand out, and Average shows the mean, which smooths noise like a thicker slice would. Set the thickness in millimetres; it applies to all three views.
- Can I compare two scans side by side?
- Yes. Open both together (drop both folders on the page at once, or choose a folder that holds both, such as the whole disc when both studies are on it), press Side by side and pick a series on each side, such as a scan before and after treatment. When both are stacks facing the same way from one scan session (the same frame of reference), scrolling one moves the other to the same slice position even if the slices are spaced differently, and a side shows Outside this series when the other is beyond its ends. Positions from different visits do not line up, so there the two keep the distance they had when linking started: line them up by hand first. Other series scroll by image number. The page says which it is using. Zoom and pan are linked, and contrast can be; each link can be switched off.
- Can I save every image of a series as JPG or PNG?
- Yes. All as .png or All as .jpg saves the whole series as numbered pictures in one ZIP, at the contrast, turn and flip you set (or each image's own contrast while it is on Auto), up to 1,000 images (300 on a phone). Contact sheet .pdf makes a printable PDF of small pictures with their image numbers; a series of more than 300 images shows one in every few, with the real numbers kept. Both show their progress and can be cancelled. The patient's name and details in the corners are never in the pictures, and with Hide patient details ticked the file names and the sheet's heading use only the type of scan.
- Is there a sample scan to try it with?
- Yes. Try a sample scan (under the drop box) opens a synthetic head CT of 64 slices: the Shepp-Logan phantom, a standard test object made of ellipsoids, computed for this page with Hounsfield-like values for air, skull and brain. It is not anyone's scan; it is fetched (about 100 KB) only when you press the button, and it opens like any other series, three planes included.
- How does it compare with RadiAnt, MicroDicom and Weasis?
- Those are desktop programs you install, and they do much more: all three have MPR with MIP and 3D volume rendering. RadiAnt and MicroDicom are Windows programs (each says a Mac can run it only through Wine or CrossOver, which they do not support); MicroDicom is free for non-commercial use, and RadiAnt is paid by subscription after a free trial. Weasis is free and open source (EPL 2.0 or Apache 2.0), built on Java with its own runtime included, and runs on Windows, macOS and Linux. This page needs no install or account and runs in a browser on a Mac, Chromebook, phone or a work computer where you cannot install software, with the scans staying on your device. This page is not certified as a medical device for diagnosis, and RadiAnt and Weasis say on their sites that they are not either.
- Can I use this DICOM viewer for a diagnosis?
- No. It is for looking at your own images, not a medical device, and is not cleared or certified for clinical use; its measurements are for interest only. Any question about what a scan shows is one for the radiologist’s report or your doctor. It was built by John McMillan, a software engineer, for johnmcm.com.