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If you are planning on using the democratiz3D service to automatically convert a medical scan to a 3D printable STL model, or you just happen to be working with medical scans for another reason, it is important to know if you are working with a CT (Computed Tomography or CAT) or MRI (Magnetic Resonance Imaging) scan. In this tutorial I'll show you how to quickly and easily tell the difference between a CT and MRI. I am a board-certified radiologist, and spent years mastering the subtleties of radiology physics for my board examinations and clinical practice. My goal here is not to bore you with unnecessary detail, although I am capable of that, but rather to give you a quick, easy, and practical way to understand the difference between CT and MRI if you are a non-medical person. Interested in Medical 3D Printing? Here are some resources: Free downloads of hundreds of 3D printable medical models. Automatically generate your own 3D printable medical models from CT scans. Have a question? Post a question or comment in the medical imaging forum. A Brief Overview of How CT and MRI Works For both CT (left) and MRI (right) scans you will lie on a moving table and be put into a circular machine that looks like a big doughnut. The table will move your body into the doughnut hole. The scan will then be performed. You may or may not get IV contrast through an IV. The machines look very similar but the scan pictures are totally different! CT and CAT Scans are the Same A CT scan, from Computed Tomography, and a CAT scan from Computed Axial Tomography are the same thing. CT scans are based on x-rays. A CT scanner is basically a rotating x-ray machine that takes sequential x-ray pictures of your body as it spins around. A computer then takes the data from the individual images, combines that with the known angle and position of the image at the time of exposure, and re-creates a three-dimensional representation of the body. Because CT scans are based on x-rays, bones are white and air is black on a CT scan just as it is on an x-ray as shown in Figure 1 below. Modern CT scanners are very fast, and usually the scan is performed in less than five minutes. Figure 1: A standard chest x-ray. Note that bones are white and air is black. Miscle and fat are shades of gray. CT scans are based on x-ray so body structures have the same color as they don on an x-ray. How does MRI Work? MRI uses a totally different mechanism to generate an image. MRI images are made using hydrogen atoms in your body and magnets. Yes, super strong magnets. Hydrogen is present in water, fat, protein, and most of the "soft tissue" structures of the body. The doughnut of an MRI does not house a rotating x-ray machine as it does in a CT scanner. Rather, it houses a superconducting electromagnet, basically a super strong magnet. The hydrogen atoms in your body line up with the magnetic field. Don't worry, this is perfectly safe and you won't feel anything. A radio transmitter, yes just like an FM radio station transmitter, will send some radio waves into your body, which will knock some of the hydrogen atoms out of alignment. As the hydrogen nuclei return back to their baseline position they emit a signal that can be measured and used to generate an image. MRI Pulse Sequences Differ Among Manufacturers The frequency, intensity, and timing of the radio waves used to excite the hydrogen atoms, called a "pulse sequence," can be modified so that only certain hydrogen atoms are excited and emit a signal. For example, when using a Short Tau Inversion Recovery (STIR) pulse sequence hydrogen atoms attached to fat molecules are turned off. When using a Fluid Attenuation Inversion Recovery (FLAIR) pulse sequence, hydrogen atoms attached to water molecules are turned off. Because there are so many variables that can be tweaked there are literally hundreds if not thousands of ways that pulse sequences can be constructed, each generating a slightly different type of image. To further complicate the matter, medical scanner manufacturers develop their own custom flavors of pulse sequences and give them specific brand names. So a balanced gradient echo pulse sequence is called True FISP on a Siemens scanner, FIESTA on a GE scanner, Balanced FFE on Philips, BASG on Hitachi, and True SSFP on Toshiba machines. Here is a list of pulse sequence names from various MRI manufacturers. This Radiographics article gives more detail about MRI physics if you want to get into the nitty-gritty. Figure 2: Examples of MRI images from the same patient. From left to right, T1, T2, FLAIR, and T1 post-contrast images of the brain in a patient with a right frontal lobe brain tumor. Note that tissue types (fat, water, blood vessels) can appear differently depending on the pulse sequence and presence of IV contrast. How to Tell the Difference Between a CT Scan and an MRI Scan? A Step by Step Guide Step 1: Read the Radiologist's Report The easiest way to tell what kind of a scan you had is to read the radiologist's report. All reports began with a formal title that will say what kind of scan you had, what body part was imaged, and whether IV contrast was used, for example "MRI brain with and without IV contrast," or "CT abdomen and pelvis without contrast." Step 2: Remember Your Experience in the MRI or CT (CAT) Scanner Were you on the scanner table for less than 10 minutes? If so you probably had a CT scan as MRIs take much longer. Did you have to wear earmuffs to protect your hearing from loud banging during the scan? If so, that was an MRI as the shifting magnetic fields cause the internal components of the machine to make noise. Did you have to drink lots of nasty flavored liquid a few hours before the scan? If so, this is oral contrast and is almost always for a CT. How to tell the difference between CT and MRI by looking at the pictures If you don't have access to the radiology report and don't remember the experience in the scanner because the scan was A) not done on you, or you were to drunk/high/sedated to remember, then you may have to figure out what kind of scan you had by looking at the pictures. This can be complicated, but don't fear I'll show you how to figure it out in this section. First, you need to get a copy of your scan. You can usually get this from the radiology or imaging department at the hospital or clinic where you had the scan performed. Typically these come on a CD or DVD. The disc may already have a program that will allow you to view the scan. If it doesn't, you'll have to download a program capable of reading DICOM files, such as 3D Slicer. Open your scan according to the instructions of your specific program. You may notice that your scan is composed of several sets of images, called series. Each series contains a stack of images. For CT scans these are usually images in different planes (axial, coronal, and sagittal) or before and after administration of IV contrast. For MRI each series is usually a different pulse sequence, which may also be before or after IV contrast. Step 3: Does the medical imaging software program tell you what kind of scan you have? Most imaging software programs will tell you what kind of scan you have under a field called "modality." The picture below shows a screen capture from 3D Slicer. Looking at the Modality column makes it pretty obvious that this is a CT scan. Figure 3: A screen capture from the 3D Slicer program shows the kind of scan under the modality column. Step 4: Can you see the CAT scan or MRI table the patient is laying on? If you can see the table that the patient is laying on or a brace that their head or other body part is secured in, you probably have a CT scan. MRI tables and braces are designed of materials that don't give off a signal in the MRI machine, so they are invisible. CT scan tables absorb some of the x-ray photons used to make the picture, so they are visible on the scan. Figure 4: A CT scan (left) and MRI (right) that show the patient table visible on the CT but not the MRI. Step 5: Is fat or water white? MRI usually shows fat and water as white. In MRI scans the fat underneath the skin or reservoirs of water in the body can be either white or dark in appearance, depending on the pulse sequence. For CT however, fat and water are almost never white. Look for fat just underneath the skin in almost any part of the body. Structures that contained mostly water include the cerebrospinal fluid around the spinal cord in the spinal canal and around the brain, the vitreous humor inside the eyeballs, bile within the gallbladder and biliary tree of the liver, urine within the bladder and collecting systems of the kidneys, and in some abnormal states such as pleural fluid in the thorax and ascites in the abdomen. It should be noted that water-containing structures can be made to look white on CT scans by intentional mixing of contrast in the structures in highly specialized scans, such as in a CT urogram or CT myelogram. But in general if either fat or fluid in the body looks white, you are dealing with an MRI. Step 6: Is the bone black? CT never shows bones as black. If you can see bony structures on your scan and they are black or dark gray in coloration, you are dealing with an MRI. On CT scans the bone is always white because the calcium blocks (attenuates) the x-ray photons. The calcium does not emit a signal in MRI scans, and thus appears dark. Bone marrow can be made to also appear dark on certain MRI pulse sequences, such as STIR sequences. If your scan shows dark bones and bone marrow, you are dealing with an MRI. A question I am often asked is "If bones are white on CT scans, if I see white bones can I assume it is a CT?" Unfortunately not. The calcium in bones does not emit signal on MRI and thus appears black. However, many bones also contain bone marrow which has a great deal of fat. Certain MRI sequences like T1 and T2 depict fat as bright white, and thus bone marrow-containing bone will look white on the scans. An expert can look carefully at the bone and discriminate between the calcium containing cortical bone and fat containing medullary bone, but this is beyond what a layperson will notice without specialized training. Self Test: Examples of CT and MRI Scans Here are some examples for you to test your newfound knowledge. Example 1 Figure 5A: A mystery scan of the brain Look at the scan above. Can you see the table that the patient is laying on? No, so this is probably an MRI. Let's not be hasty in our judgment and find further evidence to confirm our suspicion. Is the cerebrospinal fluid surrounding the brain and in the ventricles of the brain white? No, on this scan the CSF appears black. Both CT scans and MRIs can have dark appearing CSF, so this doesn't help us. Is the skin and thin layer of subcutaneous fat on the scalp white? Yes it is. That means this is an MRI. Well, if this is an MRI than the bones of the skull, the calvarium, should be dark, right? Yes, and indeed the calvarium is as shown in Figure 5B. You can see the black egg shaped oval around the brain, which is the calcium containing skull. The only portion of the skull that is white is in the frontal area where fat containing bone marrow is present between two thin layers of calcium containing bony cortex. This is an MRI. Figure 5B: The mystery scan is a T1 spoiled gradient echo MRI image of the brain. Incidentally this person has a brain tumor involving the left frontal lobe. Example 2 Figure 6A: Another mystery scan of the brain Look at the scan above. Let's go through our process to determine if this is a CT or MRI. First of all, can you see the table the patient is lying on or brace? Yes you can, there is a U-shaped brace keeping the head in position for the scan. We can conclude that this is a CT scan. Let's investigate further to confirm our conclusion. Is fat or water white? If either is white, then this is an MRI. In this scan we can see both fat underneath the skin of the cheeks which appears dark gray to black. Additionally, the material in the eyeball is a dark gray, immediately behind the relatively white appearing lenses of the eye. Finally, the cerebrospinal fluid surrounding the brainstem appears gray. This is not clearly an MRI, which further confirms our suspicion that it is a CT. If indeed this is a CT, then the bones of the skull should be white, and indeed they are. You can see the bright white shaped skull surrounding the brain. You can even see part of the cheekbones, the zygomatic arch, extending forward just outside the eyes. This is a CT scan. Figure 6B: The mystery scan is a CT brain without IV contrast. Example 3 Figure 7A: A mystery scan of the abdomen In this example we see an image through the upper abdomen depicting multiple intra-abdominal organs. Let's use our methodology to try and figure out what kind of scan this is. First of all, can you see the table that the patient is laying on? Yes you can. That means we are dealing with the CT. Let's go ahead and look for some additional evidence to confirm our suspicion. Do the bones appear white? Yes they do. You can see the white colored thoracic vertebrae in the center of the image, and multiple ribs are present, also white. If this is indeed a CT scan than any water-containing structures should not be white, and indeed they are not. In this image there are three water-containing structures. The spinal canal contains cerebrospinal fluid (CSF). The pickle shaped gallbladder can be seen just underneath the liver. Also, this patient has a large (and benign) left kidney cyst. All of these structures appear a dark gray. Also, the fat underneath the skin is a dark gray color. This is not in MRI. It is a CT. Figure 7B: The mystery scan is a CT of the abdomen with IV contrast Example 4 Figure 8A: A mystery scan of the left thigh Identifying this scan is challenging. Let's first look for the presence of the table. We don't see one but the image may have been trimmed to exclude it, or the image area may just not be big enough to see the table. We can't be sure a table is in present but just outside the image. Is the fat under the skin or any fluid-filled structures white? If so, this would indicate it is an MRI. The large white colored structure in the middle of the picture is a tumor. The fat underneath the skin is not white, it is dark gray in color. Also, the picture is through the mid thigh and there are no normal water containing structures in this area, so we can't use this to help us. Well, if this is a CT scan than the bone should be white. Is it? The answer is no. We can see a dark donut-shaped structure just to the right of the large white tumor. This is the femur bone, the major bone of the thigh and it is black. This cannot be a CT. It must be an MRI. This example is tricky because a fat suppression pulse sequence was used to turn the normally white colored fat a dark gray. Additionally no normal water containing structures are present on this image. The large tumor in the mid thigh is lighting up like a lightbulb and can be confusing and distracting. But, the presence of black colored bone is a dead giveaway. Figure 8B: The mystery scan is a contrast-enhanced T2 fat-suppressed MRI Conclusion: Now You Can Determine is a Scan is CT or MRI This tutorial outlines a simple process that anybody can use to identify whether a scan is a CT or MRI. The democratiz3D service on this website can be used to convert any CT scan into a 3D printable bone model. Soon, a feature will be added that will allow you to convert a brain MRI into a 3D printable model. Additional features will be forthcoming. The service is free and easy to use, but you do need to tell it what kind of scan your uploading. Hopefully this tutorial will help you identify your scan. If you'd like to learn more about the democratiz3D service click here. Thank you very much and I hope you found this tutorial to be helpful. Nothing in this article should be considered medical advice. If you have a medical question, ask your doctor.

Hi, I am looking for specific individually CT scan reconstructed and exported cervical vertebrae C1-C7. I need them to be separated as most of the STL exported models contain them together and as I don't have the expertise I cannot separate the mesh myself Thank you

Hi all, I have a patient who needs to take a very fine CT (1mm slices) prior to surgery for the manufacture of a cranial implant. The equivalent dose for this scan is approximately 10mSv. If the patient were to have this scan done at low dose - 30 to 50% of the regular dose, would the CT scan be acceptably accurate for the implant maufacture? I have found materials detailing how detection rate of tumors and other malignancies in soft tissues using low-dose is similar to that of normal does. In this case, since we are interested in just the shape, the contour of the bone, is low-dose a good solution? Thank you very much, Ted

This is the second in a series of articles about skull models created from CT scan data and designed to provide a low-cost means of anatomy teaching. To see my past article about the skull base model, click here. Learning detailed anatomy is a grueling process that doctors, nurses, and other health science students must go through. Traditionally, learning anatomy involved detailed study of textbooks, but learning 3D structures from 2D pages just doesn't work well. Dissecting cadavers is the traditional means of teaching doctors, but this process is tedious, messy, very expensive, and only available in select educational institutions (i.e. med schools). Most students of anatomy do not have access to these resources. 3D printing is putting the power of real 3D anatomy within reach of ordinary students at very low cost. These models are created from highly detailed CT scan data from real human bodies, not an artist's conceptualization. This half skull and cervical spine has been cut along median sagittal plane. This clearly shows the external bony anatomy (zygomatic arch, orbit, etc.) as well as intracranial anatomy (skull base formina, paranasal sinuses, etc.). Bony details of the cervical spine are also clearly shown. You can 3D print your own model by downloading the free files. These files are available on this website in STL or COLLADA format, in full size and half-size versions. You can get them here: full size (STL, COLLADA), half-size (STL, COLLADA). Check out more downloadable files in the File Vault. If you would rather have a high quality model made for you, you can buy one from Shapeways here (full-size, half-size). Feel free to modify the files as you would like, just please don't use them for commercial purposes. If you create something cool, please give back to the community by sharing it on the Embodi3d website in the File Vault. For updates on news and new blog entries, follow us on Twitter at @Embodi3D

I've been working on ways to artistically expand on 3D printed anatomic models beyond an exact replica of the anatomy. My first project is this Lace Skull. The skull is based on an anatomically accurate skull generated from a CT scan. I have made several of the earlier skull models available for download on the Embodi3D website here and here. Using a variety of methods, I have transformed the skull and given it a unique lace-like appearance. The overall surface contours are still anatomically accurate. The lace-like texture gives the model its unique aesthetic but also cuts down on printing material while maintaining mechanical strength. I have made the STL files available for FREE download in the File Vault section of the Embodi3D website. You can find the STL files here (HALF-SIZE | FULL-SIZE). If you 3D print this file, please report back regarding your outcome. I printed a half-size model using the "White, Strong & Flexible" nylon material on Shapeways. If you would rather have Shapeways print the model and ship it to you for a fee, you can go Shapeways to directly order the models here (HALF-SIZE | FULL-SIZE). I hope you enjoy this 3D printable model. Please report back on your experiences with printing the model. Also, please share your own 3D printable creations with the community in the File Vault section of the website.

UPDATED TUTORIAL: A Ridiculously Easily Way to Convert CT Scans to 3D Printable Bone STL Models for Free in Minutes Hello and welcome back. I hope you enjoyed my last tutorial on creating 3D printable medical models using free software on Macintosh computers. In this brief video tutorial I'll show you how to create a 3D printable skull STL file from a CT scan in FIVE minutes using only free and open source software. In the video I use a program called 3D Slicer, which is available from slicer.org. 3D Slicer works on Windows, Macintosh, and Linux operating systems. Also, I use Blender, which is available from blender.org, to perform some mesh cleanup. Finally, I check my model prior to 3D printing using Meshmixer from Autodesk. This is available at meshmixer.com. All software programs are free. If you like this, view my complete tutorial where I go through each step shown here in detail. I hope you enjoy the video.

Please note that any references to “Imag3D” in this tutorial should be replaced with “democratiz3D” In this tutorial we will discuss how to share, sell, organize, and reprocess 3D printable medical models you make using the free online democratiz3D service from embodi3D. democratiz3D is a powerful tool that automatically converts a medical CT scan into a 3D printable file in minutes with minimal user input. It is no longer necessary to master complicated desktop software and spend hours manually segmenting to create a 3D printable model. Learn how to make high quality medical 3D models with democratiz3D by following my introductory guide to creating medical 3D printing files and my more advanced 3D printing file processing tutorial. Once you create your medical masterpiece, you can share, sell, organize, or tweak your model to make it perfect. This tutorial will show you how. Resubmit your CT Scan for Reprocessing into Bone STL If you are trying to learn the basics of how to convert CT scans into 3D printable STL models, please see my earlier tutorials on basic creation of 3D printable models and more advanced multiprocessing. If you are not 100% satisfied with the quality of your STL model, you can resubmit the input scan file for repeat processing. To do this, go to the page for your input NRRD file. IMPORTANT: this is the NRRD file you originally uploaded to the website, NOT the STL file that was generated for you by the online service. Since both the original NRRD file and the processed STL file have similar titles, you can tell the difference by noting that the NRRD file you uploaded won't have any thumbnails, Figure 1. In most cases, the processed file will have the word "processed" appended to the file name. Figure 1: Choose the original NRRD file, not the generated STL file. You can find your files underneath your profile, as shown in Figure 2. That will show you your most recent activity, including recently uploaded files. Figure 2: Finding your files under your profile. If you uploaded the file long ago or contribute a lot of content to the site, your uploaded NRRD file may not be among the first content item shown. You can search specifically for your files by clicking on See My Activity under your Profile, and selecting Files from the left hand now bar, as shown in Figures 3 and 4. Figure 3: Showing all your activity. Figure 4: Showing the files you own. Once you have found your original NRRD file, open the file page and select File Actions on the lower left-hand corner, as shown in Figure 5. Choose Edit Details as shown in Figure 6. Figure 5: File Actions – start making changes to your file Figure 6: Edit Details Scroll down until you reach the democratiz3D Processing section. Make sure that the democratiz3D Processing slider is turned ON. Then, make whatever adjustments you want to the processing parameters Threshold and Quality, as shown in Figure 7. Threshold is the value in Hounsfield units to use when performing the initial segmentation. Quality is a measure of the number of polygons in the output mesh. Low quality is quick to process and generates a small output file. Low quality is suitable for small and geometrically simple structures, such as a patella or single bone. High quality takes longer to process and produces a very large output file, sometimes with millions of polygons. This is useful for very large structures or complex anatomy, such as a model of an entire spine where you wish to capture every crack and crevice of the spine. Medium quality is a good balance and suitable in most cases. Figure 7: Changing the processing parameters. When you're happy with your parameter choices, click Save. The file will now be submitted for reprocessing. In 5 to 15 minutes you should receive an email saying that your file is ready. From this NRRD file, an entirely new STL file will be created using your updated parameters and saved under your account. Sharing your 3D Printing File on embodi3D.com Sharing your file with the embodi3D community is easy. You can quickly share the file by toggling the privacy setting on the file page underneath the File Information box on the lower right, as shown in Figure 8. If this setting says "Shared," then your file is visible and available for download by registered members of the community. If you wish to have more detailed control over how your file is shared, you can edit your file details by clicking on the File Actions button on the lower left-hand side of the file page, also shown in Figure 8. Click on the Edit Details menu item. This will bring you to the file editing page which will allow you to change the Privacy setting (shared versus private), License Type (several Creative Commons and a generic paid file license are available), and file Type (free versus paid). These are shown in detail in Figure 9. Click Save to save your settings. Figure 8: Quick sharing your file, and the File Actions button Figure 9: Setting the file type, privacy, and license type for your file. Sell your Biomedical 3D Printing File and Generate Income If you would like to sell your file and charge a fee for each download, you may do so by making your file a Paid File. If you have a specialized model that there is some demand for, you can generate income by selling your file in the marketplace. From the Edit Details page under File Actions, as shown in Figure 8, scroll down until you see Type. Choose Paid for the Type. Choose the price you wish to sell your file for in the Price field. This is in US dollars. Buyers will use PayPal to purchase the file where they can pay with Paypal funds or credit card. Make sure that the privacy setting is set to Shared. If you list your file for sale but keep it private and invisible to members, you won't sell anything. Finally, make sure you choose an appropriate license for users who will download your file. The General Paid File License is appropriate and most instances, but you have the option to include a customized license if you wish. This is shown in Figure 10. Figure 10: Configuring settings to sell your file The General Paid File License contains provisions appropriate for most sellers. It tells the purchaser of your file that they can download your file and create a single 3D print, but they can't resell your file or make more than one print without paying you additional license fees. All purchasers must agree to the license prior to download. If you wish to have your own customized license terms, you can select customized license and specify your terms in the description of the file. Organize your file by moving it to a new category If you share your file, you should move the file into an appropriate file category to allow people to find it easily. This is quite simple to do. From the file page, select File Actions and choose the Move item, as shown in Figure 11. You will be able to choose any of the file categories. Choose the one that best fits your particular file. Figure 11: Moving your file to a new category. That's it! Now you can share your amazing 3D printable medical models with the world.