Medical imaging & cross-sectional anatomy

• Recent Changes

• Aims & Objectives

• Organisation & Timing

• Preparation

• Practical Instructions

• Computer tomography

• Guided tours

• Revision Checklist

• Examination Questions

• Self-assessment tests

Until this year we offered two alternative webservers for this material: a conventional Faculty of Biological Sciences server and "image webserver" hosted by ISS. Image webserver provided much higher resolution images using streaming ECW (Enhanced Compressed Wavelet) technology with "pan and zoom" facilities. This software was originally designed for the rapid distribution of terrabyte satellite imagery but we were able to adapt it for medical teaching. Unfortunately the ISS server has been replaced, and we cannot afford the latest version of the imaging software that will run on newer hardware. As a result, this year we only have one conventional server available which provides basic access to the high resolution data.

This has required a substantial re-write of the practical instructions. Please make sure that you are using the latest 2011 version.

1. Appreciate the three-dimensional spatial relationships of the gastrointestinal tract, associated tissues, endocrine and exocrine glands.

2. Recognise the principal anatomical features revealed in frozen sections and diagnostic images of the gastrointestinal tract.

3. Understand (at an elementary level) the physical basis and limitations of diagnostic imaging techniques.

OBJECTIVES

On successful completion of the computer exercises and the dissection classes you will be able to:

1. Correctly identify the following organs and tissues in frozen sections, given the approximate position and orientation of the section: jaws, teeth, tongue, salivary glands, pharynx, oesophagus, stomach (fundus, antrum, pylorus), spleen, duodenum, jejunum / ileum, ileocaecal valve, appendix, caecum, ascending colon, hepatic flexure, transverse colon, splenic flexure, descending colon, sigmoid colon, rectum, liver, gall bladder, bile ducts, pancreas, pancreatic ducts, sphincter of Oddi / ampulla of Vater, pituitary, thyroid, adrenal, descending aorta, celiac trunk, superior mesenteric artery, inferior mesenteric artery, inferior vena cava, hepatic portal vein, splenic vein, superior mesenteric vein, psoas major, rectus abdominis, diaphragm, abdominal fat. Know the approximate location and functions of the vagus nerves and lymphatic drainage, which are difficult to see.

2. Be aware of the anatomical relationships between the GI tract and endocrine glands with the following structures that are studied in other ICUs: iliac arteries and veins, renal arteries and veins, kidney, ureter, bladder, urethra, seminal vesicles, prostate, uterus, vagina, ovary, heart, lungs, mediastinum, thymus, ribs, intercostal muscles, pelvis, vertebral bodies, spinal cord, inter-vertebral disks, hypothalamus, optic chiasma.

3. Correctly identify the following organs and tissues in diagnostic images, given the approximate position and orientation of the image, and the nature of the imaging technique: oesophagus, stomach, spleen, duodenum, jejunum / ileum, caecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, liver, gall bladder, gallstones, bile ducts, pancreas, kidney, ureter, bladder, pituitary, adrenal gland, descending aorta, inferior vena cava, hepatic portal vein, ribs, pelvis, vertebral bodies, spinal cord, diaphragm, psoas major, rectus abdominis.

4. Correctly distinguish between the following imaging techniques: plain X-rays, CT scans, MRI scans, PET scans and ultrasound, including the elementary use of positive and negative contrast media.

These classes will be circussed with other work sessions: the first group of students will meet at 2pm on Tuesday 24 May 2011 in the Medical Lecture Theatre for a briefing and then proceed to the Fourman computer cluster. The second group will meet on Thursday 26 May at the conclusion of the 9am lecture on the regulation of body weight.

• Dean & Herbener Cross sectional human anatomy Lippincott; 2000.

• Spitzer & Whitlock Atlas of the Visible Human Male Jones & Bartlett; 1998.

By all means refer to these sources if you get stuck, but you won't learn very much if you simply copy down the answers without studying the material. Print out or sketch some of the images, colour them in and add your own labels to the most important structures.

Plain X-rays are the oldest diagnostic imaging technique. The clinical applications were obvious and the first recorded medical and surgical uses followed within weeks of Röntgen's original paper in December 1895. Photographic films and fluorescent screens were used from the start, twenty years before the physical nature of the radiation was properly understood. There was no easy way to focus X-rays (as in a conventional camera) and sharp images were obtained only because the radiation came from a point source. Plain X-rays are essentially a projection technique, where the shadows cast by intervening objects are superimposed on the screen.

X-ray absorption is roughly proportional to atomic weight, so the strong shadows cast by the calcium phosphate in bones are easily distinguished from the weaker absorption of the soft tissues. Dense foreign bodies such as bullets show up well, but the faint contrast between blood vessels and viscera is barely adequate for diagnostic use. The pictures can be improved by the use of artificial contrast media: insoluble barium sulphate outlines the lumen of the gut, and non-metabolised organic iodine compounds reveal intravascular spaces. Most gases have a lower X-ray absorption than flesh, so air or CO2 insufflation can be used to show spaces where gases can be safely tolerated.

Computer tomography is a newer X-ray imaging technique developed since 1970. A single X-ray source illuminates a thin slice through the patient. Multiple X-ray detectors arranged in an arc on the opposite side of the body simultaneously measure the absorption along numerous optical paths with both radial and tangential components. The entire source and detector ensemble is rotated around the patient, so that the X-ray absorption can be swiftly determined at every conceivable angle through the slice. The computer uses this huge amount of information to construct a cross-sectional image.

CT scans exploit the same information as conventional X-rays, but the definition is better because more detailed information is acquired and the computer corrects for other incidental structures that happen to get in the way. Positive and negative contrast media can be used with CT as with conventional X-rays, and yield a considerable improvement in image quality. Spiral CT moves the patient smoothly through the detector while the equipment rotates around them, swiftly acquiring a complete set of serial sections free from movement artefacts.

Positron emission tomography (PET scans) are computationally similar to CT. Radioisotopes that decay initially through emission of a positron generate secondary pairs of g-ray photons when the positron immediately annihilates an electron. These photons are emitted in random orientations but travel in opposite directions along the same straight line. If both photons are "simultaneously" detected by high-speed sensors on opposite sides of the body, then the original radioactive atom must have been located on the straight line joining the two sensors. The minute difference in arrival times reveals the exact position. Combining the results from hundreds of separate disintegrations provides a three dimensional reconstruction of the radioactive source. This technique is best known for its images of increased blood flow to metabolically active areas of the brain during thought processes.

Other radionuclide imaging techniques include bone scans, and VQ "scints" - used to compare pulmonary ventilation and perfusion in the diagnosis of pulmonary embolism and other respiratory diseases. The first image is recorded using a g-ray camera while the subject inhales a radioactive gas. Radioactive microspheres are subsequently injected into a vein so they lodge within the pulmonary arterioles and a second picture is taken. Comparison of the images generated by the two isotopes reveals areas where ventilation and perfusion fail to match, as they should do in a healthy subject.

Magnetic Resonance Imaging (MRI scans) are becoming a major competitor for CT, although they are presently more expensive and only available in specialist centres. These images are formed in a fundamentally different way by monitoring the orientation of hydrogen nuclei aligned in a strong magnetic field. The behaviour of the nuclei is affected by their local environment, so that hydrogen atoms in a freely-tumbling water molecule (for example) can be distinguished from those held rigid in a collagen sheet.

Re-orientation of the nuclei proceeds at different speeds in directions parallel and perpendicular to the magnetic field. These relaxation times are referred to as T1 and T2 in MRI parlance and vary considerably between different tissues. It is possible to emphasise one or other component by changing the radiofrequency stimulation pattern applied to the tissue, and this is exploited to produce (for example) a T2-weighted image. The computer constructs slices through the subject, which can be in any desired orientation.

Tissues do not necessarily appear the same using different imaging techniques. The following is a simplified guide:

Note: diagnosis of bleeding from MRI images is a complex business, requiring considerable knowledge and skill.

Ultrasound is best known for foetal imaging, but can also provide pictures of many other structures, especially the heart and the biliary tree. It is cheap, non-invasive and free from radiation risks, but the images are often more difficult to interpret than those from the techniques listed above. It is the acoustic equivalent of radar scanning. Ultrasonic vibrations from a probe applied to the patient's skin (or within a body cavity) reflect mainly from the surfaces between internal structures. The delay before an echo is received gives the radial distance from the surface to the probe. Radiolucent objects such as gallstones show up well on ultrasound, when they cast characteristic acoustic shadows on objects that are further from the probe. When the imaged material is moving relative to the probe (e.g. arterial or venous blood) the reflected note is slightly Doppler shifted in frequency, and this can be used to reveal three-dimensional flow patterns within the heart and major vessels.

Orientation: Note that all sections are viewed from the feet towards the head, in the standard orientation for radiology images, so the subject's right is on your left. Do not expect to find organs always in exactly the same place. The visible human sections were cut from a cadaver frozen in a horizontal position, but in life the gut is a motile structure. When standing upright with the lungs inflated some organs could be several centimetres lower in the abdomen, in addition to the normal anatomical variations between individuals.

There is a printed hand-out for this class. If you lose yours you can download another one here.

The BMB web server is at   http://www.bmb.leeds.ac.uk/teaching/icu3/practic/comp/index.htm It has frozen sections and CT scans.

Don't follow these links without reading the detailed instructions below. You can click the greyscale images, or the tables of anatomical features to enter the BMB website at the appropriate section number.

The website is accessible off-campus and will remain open for the foreseeable future, so that you can return for self-assessment and revision purposes. There are two datasets available, frozen sections of a male cadaver and computer tomography of a female patient. In the future we hope to include magnetic resonance imaging techniques. For MRI, where it is easy to change the image orientation, there will be a choice of views.

There are navigation buttons on screen to move towards the subject's head or the feet, or jump to a specified section number. Try to relate the cross sectional images to your histology and dissection work. Do the easy bits first - some small blood vessels are difficult to spot. There is no need to complete the workbook today (although we hope that you will make a reasonable stab at it) because you can return to this website subsequently for revision purposes. Don't try to learn the section numbers - these vary from person to person, and even from minute to minute as the viscera slide over each other in life. Try to grasp the anatomical relationships between the various structures: the section numbers are only to help you find them again.

We suggest starting with frozen male section 1515 that is illustrated below. Identify the major landmarks first: the liver and the spinal column are difficult to miss! Click in the image below to enter the Visible Human website, and use the browser "Back" button to return to these instructions. This particular section has gone through the collagenous intervertebral disk (marked "ivd") but if you move up and down a few sections using the "HEAD" and "FEET" buttons you can see the bony vertebral bodies. The "MORE" button will take you directly to the higher resolution dataset. Remember that you are looking from the subject's feet, so his stomach is on your right. There are some major blood vessels in this area – you should be able to identify the artery "a" and vein "v" by looking through the adjacent sections to see where they connect.

This section is taken between the 10^th and 11^th thoracic vertebrae (T10 and T11). To relate this to your own body, note that there is a thin piece of sternum visible, and there is only a single bone in the arms, so we are looking well above the elbow joint. The white areas alongside the sternum are the cartilagenous ends to the ribs.

Note the rugae inside the stomach and the muscular, distensible oesophagus. Find the section where the oesophagus opens into the stomach, and trace the oesophagus towards the mouth. There is a section missing here, because the cutting equipment could not accommodate the entire frozen cadaver, which was initially subdivided with a saw.

There is a second navigation image further down the web page which shows longitudinal sections of the same subject. If you click at the appropriate level in this navigation image it is possible to jump directly to the corresponding transverse section. Alternatively, scroll through the individual sections, or use the "JUMP" button to move directly to section number 1555.

Section 1555 shows the pylorus opening into the duodenum. The tail of the pancreas "p" and the gall bladder "gb" are both visible, and you should be able to move through the neighbouring sections in order to follow and identify two major veins marked V1 and V2. Click on the "MORE" button to switch to the high resolution images and click elsewhere to return to the low resolution view. On the high resolution images it is easier to trace the common bile duct from the gall bladder down through the posterior sections to the ampulla of Vater / sphincter of Oddi, and see the junctions with the hepatic bile ducts and the pancreatic duct. Observe that the pancreas and duodenum are retroperitoneal structures. Other features of note are the descending aorta, splenic veins, diaphragm and the loops of large and small bowel. Notice the convoluted internal surface of the small bowel and the smooth internal surface of the colon. It is rarely possible to visualise the surface of the mesentery, but there is a large amount of mesenteric fat.

Click in the image above to enter the Visible Human website, and use the browser "Back" button to return to these instructions. This section is at the level of the 12th thoracic vertebra. Note the position of the xiphoid process "x", and feel for the end of your own xiphoid process to establish the position in space.

Some of the blood vessels have collapsed post mortem and the gut lumen is often distended with gas. Empty spaces were filled with frozen blue gelatine to facilitate sectioning and are usually coloured blue in the computer images so they can be readily distinguished from tissue.

Identify the diaphragm (not labelled) and trace it as it domes upwards over the liver. You are viewing this muscle in cross-section, so look for a closed loop rather than a sheet. Find the places where the oesophagus, descending aorta and inferior vena cava pass through the diaphragm into the thorax. It is possible to find an image where the diaphragm has a "figure 8" profile, showing liver and stomach in the middle of the section, with the heart and lungs around the outside.

Moving towards the feet, study the complex anatomy around the head of the pancreas (sections 1570-1620) and on these sections identify first the adrenals as conical "nightcaps" and then the kidneys, slightly posterior. First find the splenic veins and subsequently the renal vessels further towards the feet. For orientation purposes, note the elbow joint around 1600, and the most posterior part of the 12^th ribs about 1660. This is around the level of the second lumbar vertebra, L2. Observe the hepatic and splenic flexures, and the transverse colon posterior to the stomach. Several hours elapsed post mortem before this cadaver was frozen and the colon is greatly distended with gas.

Continuing towards the feet, note the psoas major muscle posterior to the kidneys, but in a confusingly similar location relative to the spine. Try to avoid the obvious mistake! Other important features are the ileo-caecal junction around section 1742. Notice the subject's navel on these sections, for orientation, and the pelvic bones. (These are the iliac blades, that you will study in year three. The sacro-iliac joint is clearly visible around section 1800.) The caecum ends about 1840. Note that there is no appendix in the visible male subject.

It is easiest to navigate the lower GI tract starting from the rectum on section 1880. Although this is year two material and not part of ICU3, there are some beautiful sections through the seminal vesicles (s.v.) prostate and urethra between 1880 and 1950, and these will allow you appreciate why a rectal examination is an important diagnostic procedure in the ageing male. Click in the image below to enter the Visible Human website, and use the browser "Back" button to return to these instructions.

Proceeding from 1880 towards the head, note the sigmoid colon, suspended from a mesentery, between 1866 and 1820. This is a convoluted structure that may present a challenge for the endoscopist, but sections 1824 and 1852 reveal how it is all connected together.

The salivary glands can be difficult to distinguish from the surrounding tissues. The parotid glands appear on sections 1150 to 1210, the submandibular (=submaxillary) glands from 1196 to 1235 and the sub-lingual glands from 1201 to 1220. All three glands are marked on the image of section 1205 below.

In addition to the anatomy of the GI tract, you should be aware of the locations of the major endocrine organs that regulate human energy supplies. You have already identified the pancreas and the adrenal glands, and you should remember that the stomach and the duodenal wall also produce several important peptide messengers. Now move to section 1107 which shows the optic chiasma where the two optic nerves partially cross over to the opposite sides of the brain. This is an important landmark within the skull. Note the eyeballs and the extra-ocular muscles. The hypothalamus is the tiny volume of brain (~ 4ml in total) immediately dorsal and anterior to the optic chiasma. It lies on the midline, on either side of the slit-like third ventricle, and controls most of the endocrine system and the autonomic nervous system. If you move slowly through the next five sections towards the feet, you can distinguish a thin strand of neural tissue (the pituitary stalk) dorsal to the optic chiasma, which connects the hypothalamus to the bulbous pituitary gland below. The posterior pituitary (neurohypophysis) is derived in the embryo from neural tissue and secretes vasopressin and oxytocin. The anterior pituitary (adenohypophysis) is derived from oral ectoderm in Rathke’s pouch. It secretes growth hormone, ACTH, TSH, LH, FSH, prolactin and MSH under the control of inhibiting and releasing factors made in the hypothalamus. These chemical messengers are delivered to the anterior pituitary via the bloodstream and the hypophyseal portal system.

The pituitary regulates the activity of the thyroid gland (which can be seen closely applied to the trachea and oesophagus in sections 1250 to 1310) and thereby controls the basal metabolic rate. Thyroid problems are the second most common endocrine disorder, after type 2 diabetes. They are much more common in women than in men and usually have an auto-immune origin. The thymus gland plays an important role in the maturation of the immune system but is greatly reduced in the adult. It is studied both in this ICU and in other parts of the course. Thymic tissue (or some fatty remnants) can be seen ventral to the great blood vessels in the visible male sections 1350 to 1408.

The CT sections are 1cm thick compared with the frozen sections which are 1mm thick, therefore the section numbering does not match. As with other X-ray images, air is black and bones are white. Note the increased diameter of the major blood vessels under normal operating pressures in a living subject. Click the image below to jump to the corresponding section in the website.

Work through the revision checklist below, examining the frozen sections and CT images of the male and female subjects. The clickable links will take you directly to the relevant sections. Many structures are only visible in one mode. At present we don't have a full MRI sequence and this list will be updated as new images become available. Each specimen has its own section numbering: these sections differ considerably in thickness and the section numbers do not match.

Rather than staring blankly at a complex image, it is easier to master this material if you follow key structures through several consecutive sections. Remember that you can type in the required section numbers, or double click the head and feet buttons to jump ten sections at a time. Ten sections corresponds one centimeter on the frozen male cadaver. As well as following your target anatomy, note the other structures that are situated nearby.

Don't forget that if you scroll down from your current transverse section, you will find a clickable sagittal view that you can use to quickly navigate to the desired level.

1. Diaphragm

2. Arteries

3. Veins

4. Bile ducts

5. Sigmoid colon

6. Caecum

7. Endocrine

Diaphragm: The diaphragm is a twin domed structure that flattens during inspiration, so in cross sections it appears as a series of concentric rings on sections from 1455 to 1580, with some strands of muscle visible down to 1645. It is penetrated at different levels by the descending aorta, inferior vena cava and the oesophagus. Make sure that you appreciate where all these structures are located. Note the relationship with the rib cage. In the thorax it is easy to find "figure of 8" sections where the heart and lungs are visible above the diaphragm, and the digestive organs below.

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Arterial tree: The major landmark is the descending aorta, which has partially collapsed on the frozen male cadaver, making it smaller and flatter than it appears in life. (Compare the cryosections with the CT scans.) The voids were refilled with frozen blue gelatine to facilitate sectioning. Observe where the aorta crosses the diaphragm around section 1582 and immediately gives off the celiac trunk. Some of these branches curve upwards into the dome of the diaphragm, so to follow them you must move towards the head. It is easy to find the superior mesenteric artery around section 1601, but the inferior mesenteric vessels are difficult to locate. The renal arteries can be seen around 1616, and the aorta divides into the iliac arteries below 1722.

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Venous system: The major landmark is the inferior vena cava, partially buried within the liver tissue before it crosses the diaphragm around section 1480. Note that this is about 10cm above the crossing point of the decending aorta. Follow the vena cava towards the feet, noting the renal veins around section 1616 and the division into the left and right common iliac veins near section 1740. The vena cava is located to the right of the midline, while the aorta is slightly towards the subject's left.

Having first located the systemic venous drainage, it is much easier to identify the portal system. The key section is 1580 which shows the junction between the superior mesenteric and splenic veins to form the hepatic portal vein. Moving towards the head you can trace the splenic vein draining the spleen and the portal vein flowing towards the liver, while moving from 1580 towards the feet reveals various branches of the superior mesenteric vein draining the gut.

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Biliary tree: It is easy to identify the biliary tree, because it has been stained yellow by the bile. The gall bladder is the most obvious component. If you follow the neck of the gall bladder to section 1555 it will lead you to the common bile duct. Moving towards the head reveals the common hepatic bile duct near the portal vein. The common hepatic duct divides into left and right branches near section 1542.

Moving from section 1555 towards the feet, it is possible to trace the common bile duct through the pancreas until it joins the main pancreatic duct and empties into the duodenum at the Ampulla of Vater / Sphincter of Oddi around section 1615. The accessory pancreatic duct is not clearly distinguished on these frozen sections.

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Sigmoid colon: This convoluted region may present a challenge for the endoscopist because the bowel is suspended from a mesentery and is able to adopt a considerable range of shapes. Even on our frozen specimen it is far from easy to decide how the various parts are connected together. It is easiest to start from the anus in section 1950, and to proceed towards the head through the rectum (1940 - 1850), noting the anal sphincters and other voluntary muscles that normally maintain faecal continence. Note also the relationships with the urinary bladder, urethra, seminal vesicles and prostate gland (which are studied in detail in other ICUs) and appreciate why a rectal examination should be important in the ageing male.

The other end of this labyrinth is around section 1800 on the subject's ventral left side, where the descending colon passes into the pelvic area. Between the descending colon and the rectum there is an amazing number of twists and turns as the sigmoid colon is folded into the limited space anterior to the bladder. By this stage the dehydrated faeces is forming into distinct stools. You will need to back track repeatedly through these sections to see how it is all joined together.

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Caecum: When we introduced the cross-sectional anatomy topic we used Spitzer & Whitlock's Atlas of the Visible Human Male which firmly identifies the ileo-caecal junction at section 1740. We based an examination question on this section. Imagine our consternation when Dean & Herbener Cross sectional human anatomy place this feature at section 1805, 65mm towards the feet. Which authority is correct? You can decide for yourself by examining the high resolution images, noting that this subject has undergone an appendicectomy, and that the cut end of the appendix has been sewn inside the caecum.

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Endocrine system: Your tour of the human gastrointestinal tract will not be complete until you have examined some of the major endocrine glands that regulate energy intake and expenditure. Both parts of the pituitary gland are visible on section 1113, and it is easy to see why a major surgical route to the pituitary is via the nose. If you move upwards from this point it is possible to distinguish the thin pituitary stalk that connects the pituitary to the hypothalamus passing dorsally to the optic chiasma, which is exceptionally clear on section 1107.

The thyroid gland is located in the neck, approximately level with the collarbones (clavicle) in section 1290. It is quite a large and highly vascular gland, even in healthy people, and entends over ~3.5cm from 1272 to 1307. It may become greatly swollen in iodine deficiency goitre, and the appearance changes in various autoimmune thyroid conditions.

The adrenal glands are visible in sections 1555 to 1595 as conical "nightcaps" anterior to both kidneys. The left and right glands are at slightly different levels. The layered structure of the adrenals is visible in the high-resolution pictures, but histological sections are necessary to distinguish between the various cell types. Histological preparations are also required to see the entero-endocrine cells in the pancreatic islet tissues and the duodenum, but it is easy to identify our largest hormone-secreting tissue, which is the abdominal fat.

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Revision Checklist

This is is a clickable list of key structures that will immediately jump to a clear example. Most of these sections are labelled, but in a few cases we expect you to identify the items yourself. Don't forget to trace the structures through the neigbouring sections as well as the example provided. There are ten self-assessment cross sectional anatomy tests towards the end of this page so that you can test yourselves on this material.

Please complete the anonymous feedback form on the Image Web Server. This will help us to secure more funds for medical teaching. Click on the "HELP" button and select the 'feedback' tab.

There are various possible styles of extended matching question, and some typical examples follow There are additional self-assessment cross sectional anatomy tests below. You should revise this work in conjunction with your dissection of the abdomen: the two approaches are complementary and will reinforce one another. Concentrate on the obvious features of the digestive tract rather than minor details of related organ systems.

Question 1: Examine the photograph of a transverse section through a frozen male abdomen at the level of the 5th lumbar vertebra. Some of the viscera were distended with gas which appears white in the picture.

Mark the EMQ card where the line numbers superimposed on the photograph correspond most closely with one of the structures coded "a" - "p" in the following table:

Question 2: The diagram below shows the relations of the pancreas. Particular structural features have been identified by the letters (A) to (M). Match the labelled features against the structures in the list.

structures options 1. Ampulla of Vater A B C D E F G H I J K L M 2. Common bile duct A B C D E F G H I J K L M 3. Haustra A B C D E F G H I J K L M 4. Jejunum A B C D E F G H I J K L M 5. Kidney A B C D E F G H I J K L M 6. Superior mesenteric vein A B C D E F G H I J K L M 7. Splenic artery A B C D E F G H I J K L M 8. Superior mesenteric artery A B C D E F G H I J K L M 9. Ureter A B C D E F G H I J K L M 10. Hepatic artery A B C D E F G H I J K L M 11. Adrenal gland A B C D E F G H I J K L M 12. Left gastric artery A B C D E F G H I J K L M 13. Taenia coli A B C D E F G H I J K L M

feedback options: marks only error list commentary

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SELF-ASSESSMENT TESTS

This link takes you into a series of ten self-assessment tests on frozen sections and CT scans. The last test returns to this site. Run them as often as you need. Your performance is not being recorded.

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