Radiological anatomy and medical imaging

X ray

Radiography is the imaging method which uses x-rays or electromagnetic waves. These waves pass through the person’s body, with some rays being absorbed by the tissues and others reaching the radiographic film behind. Thus creating a 2 dimensional (flat) image called a radiograph. Dense tissues (bone) will absorb most of the rays and come out on the radiograph as white, while air doesn’t block any rays and comes out as black. Other tissues are somewhere in between of this grayscale. 

These rules translate into a basic radiographic language; 

• Density or opacity refers to bright (white) areas of the graph. E.g. humerus bone.
• Lucency refers to dark (black) areas of the graph. E.g. air in the lungs.

X-ray continues to be a highly utilized medical imaging modality as it offers high spatial resolution and enables comprehensive visualization of structures which can be hard to perceive in axial (cross sectional) perspectives. Radiography is most often used in chest x-rays (cxr), abdominal and skeletal x-rays.

CT (CAT)

Computed tomography (CT), earlier referred to as computed axial tomography (CAT), is another non-invasive imaging procedure. CT works by using x-rays too but the machine is more advanced. It rotates around a stationary person creating multiple cross-sectional images, which can then be rendered into a 3D image. This gives us a cross-sectional slice of the specific body region. As CT uses x-rays, the image also depends on tissue density. Density is expressed in Hounsfield unit (HUs), which spans from +1000 for bones (bright), 0 for water (gray), to -1000 for air (dark). Every tissue in the body has its normal density familiar to radiologists. If the density is altered, we express that by using basic CT terminology; hyperdense, hypodense or isodense when compared to some other structure. 

The advantages of CT over x-ray radiography are that it enables a three-dimensional insight into the body, giving a more accurate presentation of the area of interest. There are many CT techniques, such as single slice CT (SSCT), spiral CT and multi slice CT (MSCT). These techniques offer variations in the “slice” thickness and the radiation dose used to create the image. CT machines can also switch between the “bone window” and “soft tissue window”, depending on which structures we want to observe. Furthermore, CT imaging can be combined with radiological contrasts which act as visualisation aids. 

It is important to know how to orientate with CT scans. For axial scans, imagine as if you’re looking at the person through their feet (viewing the CT slice from below) while both of you are facing opposite directions. Then you can orientate by using RALP abbreviation for 9, 12, 3 and 6 o’clock positions on the slice; 

• 9 - right aspect of the patient
• 12 - anterior aspect
• 3 - left aspect
• 6 - posterior aspect

MRI

MRI is an imaging modality that, besides anatomy, can also show physiological processes in the body (functional MRI - fMRI). MRI works by using magnetic fields and radiofrequency pulses to excite protons (hydrogen ions) in our body. Excited hydrogen ions emit signals toward the MRI scanner which, based on the intensity of the signal, creates a gray-scale image. As we’re made mostly of fat and water, there’s plenty of hydrogen to detect.

The density of these protons in our tissues is related to signal magnitude, i.e. increased density = increased signal. High signal intensity is shown as white, intermediate signal intensity as gray and low signal intensity as black. When a structure is brighter than it should be, we say it’s hyperintense. If it’s darker, then it’s hypointense. Proton density is increased in some types of lesions; edema, infection, inflammation, demyelination, hemorrhage, some tumors and cysts, and decreased in other types of lesions; scar tissue, calcification, some tumours, capsule and membrane formation. Note to newbies, we use the word density for CTs and intensity for MRIs, you definitely don’t want to mix these terms up in your exams! 

MRI uses no radiation, can combine with contrasts and any plane of the body can be imaged. Although this seems like the perfect imaging method, it does have some disadvantages. MRI scanning takes longer than CT, and it can be uncomfortable for some people as the machine is very loud and requires the person to be placed in a narrow tube (problematic for people with claustrophobia). In addition, MRI is absolutely contraindicated for people with metal implants, due to the intense magnetic field it creates. With all of its properties, unless contraindicated,  MRI is the best technique for soft tissue imaging.

Dive into MRI world with our MRI study guide with quizzes and test questions.

MRI offers several modalities between which the radiographers can switch depending on what structure they want to focus on. The basic MRI methods are:

• T1w - T1 weighted image best shows structures made of mainly fat (fluids are dark / black; fat is bright / white).
• T2w - T2 weighted image presents structures made of both water and fat (fat and fluids are bright).
• PD - Proton density is handy for examination of muscles and bones. 
• FLAIR - Fluid-attenuated inversion recovery best shows the brain. It is useful for identifying central nervous system disease, such as cerebrovascular insults, multiple sclerosis and meningitis. 
• DWI - Diffusion weighted imaging detects distribution of fluids (extra- and intra- cellular) within tissues. As the balance between fluid compartments is altered in some conditions (infarctions, tumors), DWI is useful for both structural and functional soft tissue assessment.
• Flow sensitive - examines the flow of body fluids but without using contrast agents. This method examines if everything is okay with the cerebrospinal fluid flow and blood flow through the vessels. 

MRI is mostly used for neuroimaging (NMRI), musculoskeletal, gastrointestinal and cardiovascular system assessments.

Ultrasonography

Ultrasonography uses high frequency sound waves emitted from a transducer through a person's skin. These sounds echo from the contours of the inner body structures bouncing back to the transducer, which then translates them into a pixelated image displayed on the connected monitor. The density of the tissues here defines how echogenic they are, meaning what amount of sound will they resonate back (echo) or pass through themselves.

Very solid tissues (bones) are hyperechoic and are shown as white, loose structures are hypoechoic and shown gray, while fluid is anechoic and is shown as black. Ultrasound shows processes in real time, which is why it is useful for immediate assessment of certain structures. It has many applications, such as tracking pregnancy progress (obstetric ultrasound), pathology screening (e.g. breast cancer) and examining the content of hollow organs (e.g. gallbladder). Ultrasonography adjusted for examining blood flow through arteries and veins is called Doppler ultrasonography, of which transcranial ultrasonography and carotid ultrasonography are nice examples. The former examines brain blood flow and the latter examines flow through the carotid arteries.

Nuclear medicine imaging

Nuclear medicine imaging is used to visualize the function rather than the structure of specific body parts. A radiopharmaceutical (radioactive pharmaceutical) is administered to the patient (intravenously) and images are created of the passage, accumulation or excretion of that product. This provides information on the function of the organ/s in question. One common nuclear medicine imaging technique is positron emission tomography (PET) scan. PET can be used for functional examination of basically every body system - skeletal, cardiovascular, nervous, endocrine. We’ll present two common examinations as examples;

• Brain PET - administration of ¹⁸FDG (radioactive fluorodeoxyglucose) which uses glucose analogue and distributes throughout the brain for assessment of brain activity. Useful for detecting hypo- and hyper-functional zones of the cerebral cortex, and thus for diagnostics of conditions such as epilepsy, dementia, Alzheimer’s and Parkinson’s disease
• Myocardial perfusion - administration of ⁸²Rb (radioactive rubidium) for detecting myocardial infarction or coronary ischaemic disease.

Radiological contrasts

Contrast agents are substances that specifically interact with imaging tools, increasing the visual contrast of the body structures under examination. Contrasts absorb radiation (x-ray, CT), have the ability to magnetize (MRI) or alter the spread of ultrasounds (ultrasonography). Nuclear medicine (PET) uses radionuclides or radiopharmaceuticals that emit radiation towards the imaging machine. Common contrast materials include iodine, barium and gadolinium based products. They can be swallowed, injected into a blood vessel or taken by enema. 

Brain MRI

On a brain MRI, we review the anatomy of the cerebral cortex (gray matter), white matter, cerebrospinal fluid (CSF), ventricles, cisterns and skull bones. Remember that, broadly speaking, in a T1w MRI fluids are dark and fat is bright, while in a T2w MRI both fat and fluids are bright. So;

• On T1w; cortex is gray, white matter is light gray, CSF is black and bone marrow within bones is white. 
• On T2w; cortex is light gray, white matter is dark gray, CSF is white, and bone marrow is light gray

At the caudate nucleus level, shown in the image above, the main structures to view are the skull bones, cortical gyri, ventricles, subcortical structures and lobes of the brain (frontal, temporal, occipital and insular). First, see the outer circle of white, this is the bone marrow of the skull bones surrounding the brain. Moving internally, the black space between the skull bones and brain is an area occupied by muscles, paranasal sinuses and meningeal spaces.

Next, take a look at the outer surface of the brain, this thin white layer is the cortical gyri. Notice how they are tightly packed yet distinguishable. Then look for the third ventricle, it is the slit-like white structure located in the centre of the brain. Just anterolateral are the lateral ventricles with their normal appearance of horns. The choroid plexus also shows as hyperintense on a T2w MRI. The subcortical structures (basal ganglia and thalamus) are located on each side of the third ventricle. Notice how dark gray they appear. Lastly, use your neuroanatomy knowledge to locate the brain lobes in the MRI; frontal, temporal, occipital and insular.
 
Back up your imaging abilities with our cross section video tutorials, MRI quizzes and dozens of cross sections and brain MRI labeled diagrams.

Head CT

A head CT is another method that allows us to view the brain anatomy. Let’s start by describing head anatomy in grayscale CT language. Black is everything filled with air only, which in our head are the paranasal sinuses and mastoid cells. Everything with calcium, meaning bones, are shown as white. Fluids (blood and CSF) and soft tissues (e.g. brain, eyes, muscles) all appear as various shades of gray. 

To know what is what, first you should remember their anatomical location in order to know where to expect them; and second, remember the order of darkness typical for CT scans: air > water > white matter > gray matter > blood > bone.

First, notice the bright white shapes in the image. These are the bones of our neurocranium. In our scan we can clearly see the frontal, zygomatic, sphenoid, mandible, temporal and occipital bones. Focus on the cavities of these bones. Our scan shows the frontal sinus, ethmoidal cells and mastoid cells. As these are filled with air, they are seen as pure black. Besides these structures, in this scan we also see the eyeballs and extraocular muscles (medial and lateral rectus muscles). These appear isodense and symmetrical to their contralateral counterparts, just as we would expect in a normal CT.
 
The brain tissue is overall gray, with gray matter (cerebral cortex and deep nuclei) being slightly lighter than the inner white matter. Paradox, right? And that’s exactly how you’ll remember it. Subarachnoid cisterns and brain ventricles are normally filled with CSF, so they appear dark (hypodense) in a normal head CT.

Learn more about CT imaging of the head with our quizzes, cross sections and labeled CT diagrams.

Neck CT

In anatomy classes, you’ve learned all about the important structures of the neck, such as the vertebrae, upper respiratory and digestive tracts, glands, blood vessels and nerves. Now you can apply that knowledge to a normal neck CT.

While examining a neck CT, locate the neck structures by following the three-colour-pattern; black, white, gray.
 
Let’s start with black. The only black signal we should see here is the air inside the trachea, seen as the dark black circle at the anterior aspect of the image. The only white signal should come from the cervical vertebra, which is clearly seen as the only bright hyperdense structure in our image. It clearly displays the familiar vertebral shape with a central vertebral canal (gray). The rest of the neck contents are soft tissues, which all appear as various shades of gray. This includes the organs, connective tissues and muscles of the neck.

Directly posterior to the trachea is the muscular tube of the esophagus, while the lobes of the thyroid appear on each side of the trachea. The paired carotid sheath wraps common and internal carotid arteries, internal jugular vein, vagus nerve (CN X) and deep cervical lymph nodes. Applying our head and neck anatomy knowledge, we’d expect to see these vessels of the carotid sheath located laterally to the lobes of the thyroid gland, having regular rounded lumens. The remaining structures are the muscles of the neck; look at the image and see if you can locate the sternocleidomastoid, scalenes, sternohyoid, sternothyroid, levator scapulae and erector spinae muscles.

Master the head and neck imaging anatomy with more neck CT scans and quizzes.

Chest x ray (cxr)

The easiest way to read a chest x-ray (CXR) is by following the ABCD rule, which stands for Airways, Breathing, Cardiac and Diaphragm.

Breathing means examining the trachea, lungs and pleura. If you look closely, you will see the air filled trachea located in the midsagittal plane, anterior to the vertebrae and overlapping with their shadow. Follow the trachea to the carina, where it divides into the left and right principal bronchi. Principal bronchi then enter the lung hilum with pulmonary arteries, veins and lymph nodes. The lymph nodes surrounding the hilar are not typically seen in healthy people, while the vessels and bronchi continue branching further in the lung parenchyma. You can see this as a threaded opacity projecting into the lungs from the hila. If it weren't for the tracheobronchial shadow, the lungs would show entirely black due to being filled with air. You should pay attention to pleura only if you can see it, as in normal cxr pleura is not visible. 

Cardiac refers to the heart silhouette, of which we see left and right margins. The right margin features the two convexities; the lower comes from the right atrium, and the upper comes from the ascending aorta. The left margin shows two convexities separated by a concavity. The upper convexity comes from the aortic knob which is the spot where the aortic arch continues as the descending aorta. The lower convexity is from the left ventricle. The concavity comes from the pulmonary trunk and left pulmonary artery.

When looking at the diaphragm we first notice that the right hemidiaphragm is slightly higher than the left, due to it being pushed up by the underlying liver. The respective angles where the diaphragmatic density merges with the ribs and heart are called costodiaphragmatic and cardiophrenic angles. Normally, these angles are acute and empty. Lastly, look at the bony framework of the thorax. Identify the clavicles, scapula and sternum, and try to count the ribs and vertebrae.

Overview the thorax anatomy with many chest cross section images and learn more about chest x-ray with labeled radiographs, articles and quizzes.

Chest CT

Chest CT is another thorax imaging modality with particular usefulness for showing the lung interstitium, as well as the sizes and diameters of organs and vessels.

On different CT levels we can see different anatomical landmarks, such as sternoclavicular joint on T1, brachiocephalic trunk on T3 or aortic arch on T4. In the image below we can see the brachiocephalic trunk which tells us that we have a T3 level CT scan to review. 

In a CT of the thorax the majority of the image is black. We already know that air is black on CT, so can deduce that this represents the air filled lung tissue. The other air filled structure is the trachea. It can be seen in the centre of the image, having a definite circular shape.
 
The white seen in our image is the bones. Trace each of the bones of the thoracic cage, applying your anatomy knowledge as you go. Can you see the T3 vertebra, ribs, sternum and clavicle?
 
Gray is the colour of the soft tissue and organs. Adjacent to the trachea, we can see the heart and great vessels (ascending and descending aorta, superior vena cava and pulmonary trunk). Notice how the great vessels appear circular in shape. External to the rib cage we can see the gray of the thoracic musculature and subcutaneous tissues.

Shoulder MRI

MRI is the method of choice for examining joints, as it gives a high resolution presentation of musculoskeletal structures. Here we have an axial PD scan of the shoulder. In this modality, bones show as white, muscles show as dark gray, while tendons and ligaments show as black.

The outer rim of white in our image is the skin and subcutaneous tissues, while the white circle in the centre of our image is the humerus. Notice also the bright coracoid process and scapula. The remaining soft tissues show as gray to black. In a shoulder MRI scan, these soft tissue elements are combined into two functional groups; static stabilizers of the joint (glenoid labrum, fibrous capsule, glenohumeral and coracohumeral ligaments) and dynamic stabilizers of the joint (rotator cuff and surrounding muscles).
 
Static stabilizers of the shoulder joint are the fibrous capsule, glenoid labrum and ligaments. The glenoid capsule bounds the glenoid cavity, seen here as a black space surrounding the humerus. Next, the fibrocartilaginous rim of the glenoid cavity, the glenoid labrum, shows as a black triangular space at the margins of the glenohumeral junction. The ligaments of the shoulder joint, glenohumeral, coracohumeral and transverse humeral, stabilize the joint by preventing dislocations of the humeral head. The former two attach between the glenoid labrum and humerus, while the latter covers the intertubercular sulcus of the humerus. All the ligaments are shown as pure black stripes extending in transverse plane. 
 
Dynamic stabilizers are the rotator cuff, biceps brachii and triceps brachii muscles. They reinforce the fibrous capsule of the joint during movements. Bellies and tendons of the rotator cuff muscles are normally seen converging towards the glenohumeral joint. Note that the biceps tendon should insert at the 12 o’clock position, so if you see it somewhere else, you might be looking at shoulder impingement injury. The tendons of these muscles are susceptible to tearing, and in that case you would see hyperintense (white) signal coming from their location.
 
Find out more about normal shoulder MRI here.

Wrist MRI

In a wrist T1w MRI; bones are white, vessels are bright gray, muscles are dark gray, tendons, ligaments and nerves are black. To best examine a wrist MRI, divide the process into bones, ligaments, carpal tunnel and tendons.

For bones, note the cube-like bright structures forming an arch across the centre of our image. These are the carpal bones. From medial to lateral, identify the pisiform, triquetral, hamate, capitate and scaphoid. Next take a look at the ligaments, these are shown as the gray tissue filling the spaces between the bones.  

Nestled under the curvature of the carpal arch is the carpal tunnel. Note the gray tissue of the flexor retinaculum, the large medium gray circle of the median nerve, and the dark black circles for all the flexor tendons. Test yourself and see if you can name all of the flexor tendons seen in this MRI. The flexor retinaculum separates the carpal tunnel from the ulnar canal, which transports the ulnar nerve and artery. You can see these structures as two adjacent medium gray circles on the ulnar side of the ventral hand.

Extensor tendons traverse through the dorsal side of the hand, covered by the extensor retinaculum. These take the same form as their palmar counterparts; dark black circles for the tendons and gray circles for the blood vessels. Also gray are the dorsal ligaments and connective tissues of the hand. Lastly bellies of the intrinsic muscles of the hand are seen in a usual wrist MRI. In our scan, we see the dark gray muscle belly of the adductor digiti minimi muscle, surrounded by hyperintense fat tissue.