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Learn Radiology
Medical images are more than shades of black, white, and gray. What we see on an image reflects the physical properties, anatomy, and pathophysiology of the tissues being examined.
Learning radiology is more than memorizing patterns. Imaging findings can be better understood by connecting what appears on an image with the anatomy, pathology, and pathophysiology occurring within the patient.
This section is designed to build that connection. Explore how disease processes alter normal anatomy and physiology, how imaging physics allows us to visualize those changes, and why they produce characteristic radiologic findings.
Start with the fundamentals of medical imaging, learn the language radiologists use to describe what they see, and then apply those concepts to real imaging findings.
Go beyond recognizing imaging findings. Understand why they occur.
Fundamentals of Medical Imaging
Every medical image begins with an interaction between energy and the human body. Different imaging modalities use different forms of energy and physical principles to create images, which is why the same tissue or disease process can look very different depending on how it is imaged.
Understanding these basic principles is the first step toward understanding why medical images look the way they do. Let's explore the physics behind the major imaging modalities.
I hope to expand this section in the future as I gain a deeper understanding of imaging physics during radiology residency.
How Does an X-ray Image Form?
X-rays are a form of electromagnetic radiation. During an X-ray examination, X-ray photons are produced by an X-ray tube and directed through the body toward a detector.
As X-rays travel through the body, they interact with different tissues. Some X-ray photons pass through the body, while others are absorbed or scattered. The reduction in the number of X-ray photons as they travel through matter is called attenuation.
The amount of attenuation depends on the properties of the tissue the X-rays encounter. The X-rays that pass through the body reach the detector and contribute to the final image. In general, tissues that attenuate more X-rays appear whiter, while tissues that allow more X-rays to pass through appear darker.
The Five Basic Radiographic Densities
From darkest to brightest:
AIR → FAT → SOFT TISSUE/FLUID → BONE → METAL
Black → → → → White
These differences in appearance result from differences in how strongly each material attenuates the X-ray beam.
Why Does Air Look Black?
Air causes very little attenuation of the X-ray beam. Most X-ray photons pass through air and reach the detector, causing air-filled structures, such as the lungs, to appear relatively dark.
Why Does Bone Look White?
Bone attenuates substantially more X-rays than air and most soft tissues. Because fewer X-ray photons pass through bone and reach the detector, bone appears relatively white on a radiograph.
Why Do Soft Tissues Look Gray?
Soft tissues attenuate more X-rays than air but less than bone. As a result, they generally appear as varying shades of gray. Differences between soft tissues can be difficult to distinguish on conventional radiographs because many have relatively similar X-ray attenuation.
Putting It All Together
The brightness of structures on an X-ray is therefore not arbitrary. It reflects how X-rays interact with the tissues they encounter on their way to the detector.
When looking at a radiograph, ask yourself:
What is the X-ray beam passing through, and how much of that beam is being attenuated before it reaches the detector?
That question provides the foundation for understanding why many findings appear the way they do on X-ray.
How Does a CT Image Form?
Computed tomography (CT) uses X-rays to create cross-sectional images of the body. During a CT scan, an X-ray tube rotates around the patient while detectors measure the X-rays that pass through the body from many different angles.
Different tissues attenuate different amounts of the X-ray beam, just as they do on a conventional X-ray. A computer uses these measurements to reconstruct detailed cross-sectional images of the body.
Because CT measures differences in X-ray attenuation, tissues that attenuate more X-rays generally appear brighter, while tissues that attenuate fewer X-rays generally appear darker.
Hounsfield Units
CT represents X-ray attenuation using a scale called Hounsfield units (HU). Water is assigned a value of approximately 0 HU, with materials that attenuate less than water having negative values and materials that attenuate more than water having positive values.
A simplified scale looks like this:
AIR → FAT → WATER → SOFT TISSUE → BONE
−1000 HU → ~−100 HU → 0 HU → positive HU → hundreds to >1000 HU
These values are approximate and can vary depending on the tissue and how the image is acquired.
Hyperdense vs. Hypodense
Radiologists often describe structures on CT based on their relative attenuation compared with surrounding tissues.
Hyperdense (hyperattenuating) structures have greater X-ray attenuation and appear relatively brighter.
Hypodense (hypoattenuating) structures have lower X-ray attenuation and appear relatively darker.
These terms are relative, meaning that something is usually described as hyperdense or hypodense compared with another structure or expected appearance.
Why Does Bone Look White on CT?
Bone strongly attenuates X-rays and therefore has relatively high Hounsfield unit values. Because fewer X-ray photons pass through bone, it appears bright on CT.
Why Does Air Look Black on CT?
Air produces very little X-ray attenuation and has a Hounsfield unit value near −1000 HU. It therefore appears very dark on CT.
What Is Windowing?
The human eye cannot distinguish all of the attenuation values measured by CT at the same time. Windowing changes which range of Hounsfield units is displayed across the available shades of gray.
Different windows can therefore emphasize different tissues within the same CT dataset. For example, a lung window helps evaluate the lungs, while a bone window helps evaluate osseous structures and a soft-tissue window helps evaluate many of the body's soft tissues.
The underlying CT data have not changed but the way those data are being displayed has.
Putting It All Together
CT builds on the same fundamental principle as X-ray: different tissues attenuate X-rays by different amounts. The major difference is that CT measures attenuation from many angles and reconstructs that information into cross-sectional images.
When looking at a CT image, ask yourself:
How much does this structure attenuate X-rays compared with the tissues around it, and why might its attenuation have changed?
How Does an MRI Image Form?
Magnetic resonance imaging (MRI) uses a strong magnetic field and radiofrequency energy to create detailed images of the body. Unlike X-ray and CT, MRI does not use ionizing radiation.
The human body contains an abundance of hydrogen atoms, particularly in water and fat. The nuclei of hydrogen atoms contain a single proton, which behaves like a tiny magnet. MRI takes advantage of the behavior of these hydrogen protons within a strong magnetic field.
What Happens Inside the Magnetic Field?
Normally, hydrogen protons within the body are oriented in many different directions. When a patient is placed inside the MRI scanner's strong magnetic field, a slight excess of these protons align with the magnetic field, producing a net magnetization.
A radiofrequency (RF) pulse is then applied, transferring energy to the hydrogen protons and changing the orientation of this net magnetization.
When the RF pulse is turned off, the net magnetization returns toward its equilibrium state through processes known as relaxation. As this occurs, measurable radiofrequency signals are generated and detected by MRI receiver coils.
These signals are spatially encoded and processed by a computer to create an image.
Why Do Different Tissues Look Different?
Different tissues contain different molecular environments and therefore produce different MRI signal characteristics.
Importantly, there is no single appearance of a tissue on MRI. By changing how the MRI signal is acquired, we can emphasize different tissue properties and create different types of images called sequences or weightings.
This is why the same tissue can appear bright on one MRI sequence and dark on another.
T1-Weighted Imaging
T1-weighted images emphasize differences in T1 relaxation between tissues.
As a useful starting point, fat is typically bright on conventional T1-weighted images, while simple fluid such as cerebrospinal fluid (CSF) is typically dark.
T1-weighted imaging is particularly useful for evaluating anatomy and is also commonly used before and after administration of gadolinium-based contrast material.
T2-Weighted Imaging
T2-weighted images emphasize differences in T2 relaxation between tissues.
Fluid is typically bright on T2-weighted images. Because many disease processes increase tissue water content, T2-weighted imaging can help demonstrate edema, inflammation, and many other abnormalities.
A common memory aid is:
T1 → Fat tends to be bright
T2 → Water tends to be bright
This is a useful starting point, but MRI appearance depends on the specific sequence and imaging parameters being used.
MRI Is More Than T1 and T2
T1- and T2-weighted images are only the beginning. MRI includes many additional sequences and techniques that emphasize different tissue characteristics.
Examples include FLAIR, diffusion-weighted imaging (DWI), susceptibility-sensitive imaging, fat-suppressed sequences, and post-contrast imaging.
You don't need to understand all of these yet! The important concept is that MRI allows us to manipulate how an image is acquired to highlight different properties of tissues and disease processes.
Putting It All Together
MRI uses the behavior of hydrogen protons in a strong magnetic field to generate signals from the body. By changing how those signals are acquired and emphasized, MRI can produce images with very different tissue contrast.
When looking at an MRI, one of the first questions to ask is:
What sequence am I looking at, and what tissue properties is this sequence designed to emphasize?
That question is important because, unlike CT where attenuation provides a relatively intuitive scale, “bright” or “dark” on MRI has little meaning without knowing the sequence.
How Does an Ultrasound Image Form?
Ultrasound uses high-frequency sound waves to create images of structures within the body. A transducer sends sound waves into the body and then detects echoes that return after interacting with different tissues.
When sound waves encounter boundaries between tissues with different physical properties, some of the sound is reflected back toward the transducer while some continues deeper into the body. The returning echoes are detected by the transducer and used to construct an image.
In general:
Stronger returning echo → Brighter appearance
Weaker or absent returning echo → Darker appearance
Unlike X-ray and CT, ultrasound does not use ionizing radiation.
Why Do Different Tissues Look Different?
How much sound is reflected depends partly on a property called acoustic impedance, which is determined by the density of a tissue and the speed of sound traveling through it.
When sound encounters a boundary between tissues with different acoustic impedances, some of the sound is reflected. Greater differences in acoustic impedance generally produce stronger reflections.
This helps explain why boundaries between different tissues can be visible on ultrasound.
Describing Echogenicity
Radiologists describe the appearance of structures on ultrasound using their echogenicity, or their ability to produce returning echoes.
Hyperechoic → produces relatively stronger echoes and appears brighter
Hypoechoic → produces relatively weaker echoes and appears darker
Anechoic → produces essentially no internal echoes and appears black
Like many radiology terms, hyperechoic and hypoechoic are usually relative descriptions compared with another tissue or expected appearance.
Why Does Simple Fluid Look Black?
Simple fluid is typically anechoic because relatively little sound is reflected from within the fluid back toward the transducer. As a result, simple fluid-filled structures such as the urinary bladder or a simple cyst generally appear black on ultrasound.
Importantly, sound can travel efficiently through simple fluid. This leads to another useful ultrasound finding: posterior acoustic enhancement.
Posterior Acoustic Enhancement
When sound travels through a structure that attenuates the ultrasound beam relatively little, such as simple fluid, more sound reaches the tissues behind it than reaches tissues behind surrounding structures.
As a result, the area deep to the fluid-containing structure may appear brighter.
This phenomenon is called posterior acoustic enhancement and can help identify fluid-containing structures such as simple cysts.
Acoustic Shadowing
Some structures strongly attenuate the ultrasound beam through absorption, reflection, and/or scattering, allowing relatively little sound to reach the tissues behind them.
This produces a dark region called an acoustic shadow deep to the structure.
Acoustic shadowing is commonly seen behind structures such as calcifications, bone, and many gallstones.
So:
Fluid → often posterior enhancement
Bone/calcification/stones → often posterior shadowing
What Is Doppler Ultrasound?
Ultrasound can also be used to evaluate motion, particularly the movement of blood.
Doppler ultrasound takes advantage of changes in the frequency of reflected sound waves caused by moving structures, such as red blood cells. This information can be used to assess the presence, direction, and characteristics of blood flow.
Color Doppler can display information about blood flow over the grayscale ultrasound image, while spectral Doppler can display how blood-flow velocity changes over time.
Putting It All Together
Ultrasound images are created from the interactions between sound waves and tissues. The appearance of a structure depends on how sound is reflected, transmitted, and attenuated as it travels through the body.
When looking at an ultrasound image, ask yourself:
How are the sound waves interacting with this tissue, and why is that interaction causing the structure to appear bright, dark, or somewhere in between?
That question can help connect the underlying physics to findings such as echogenicity, posterior enhancement, and acoustic shadowing.
Coming soon!
Common Radiology Terms & What They Mean
Radiology has its own language. Radiologists use specific terms to describe how structures appear on medical images, the patterns they form, and how findings compare with surrounding tissues. Some terms are specific to a particular imaging modality, while others are used across radiology.
Use this glossary as a reference as you explore imaging findings throughout ExploreRadiology.
Explore Imaging Findings
Now that you have explored how medical images are created and learned some of the language radiologists use to describe what they see, it's time to put those concepts together.
In this section, explore common imaging findings and the disease processes behind them. Rather than simply memorizing what a finding looks like, we'll connect the underlying anatomy and pathophysiology with imaging physics to understand why it appears the way it does.
Imaging examples will be added throughout my intern year and radiology residency as ExploreRadiology continues to grow!
Pneumothorax
What Is It?
A pneumothorax occurs when air enters the pleural space between the visceral and parietal pleura. This can cause partial or complete collapse of the affected lung.
What's Happening?
Normally, the visceral and parietal pleura are closely opposed, and negative pressure within the pleural space helps keep the lung expanded.
When air enters the pleural space, this normal pressure relationship is disrupted. The elastic lung recoils inward, separating from the chest wall.
What Does It Look Like?
On an upright chest radiograph, a pneumothorax may appear as a thin visceral pleural line with increased lucency and an absence of normal lung markings peripheral to the line.
Why Does It Look That Way?
Remember that air attenuates very few X-rays and therefore appears dark on a radiograph.
In a pneumothorax, air collects in the pleural space outside of the lung. Because there is no aerated lung tissue within this space, there are no pulmonary vessels producing the normal lung markings.
This creates the characteristic appearance of a dark, lucent area without lung markings beyond the visceral pleural line.
Airspace Consolidation
What Is It?
Airspace consolidation occurs when air normally present within the alveoli is replaced by another material, such as fluid, pus, blood, or cells.
Pneumonia is one common cause of airspace consolidation, but consolidation itself is an imaging finding rather than a specific diagnosis.
What's Happening?
Normal alveoli are predominantly filled with air. In airspace disease, material accumulates within the alveoli and replaces some or all of that air.
What Does It Look Like?
Consolidation typically appears as an area of increased pulmonary opacity on a chest radiograph.
Air bronchograms may sometimes be visible when air-filled bronchi remain surrounded by opacified alveoli.
Why Does It Look That Way?
Air normally attenuates very few X-rays, which contributes to the relatively dark appearance of healthy lungs.
When alveolar air is replaced by fluid, pus, blood, or cells, the affected region attenuates more X-rays. Fewer X-rays reach the detector, causing the area to appear whiter or more opaque.
Pleural Effusion
What Is It?
A pleural effusion is an abnormal accumulation of fluid within the pleural space.
What's Happening?
Normally, only a small amount of fluid is present between the visceral and parietal pleura. With a pleural effusion, additional fluid accumulates within this space.
When a patient is upright, gravity causes the fluid to collect predominantly in the dependent portions of the pleural space.
What Does It Look Like?
On an upright chest radiograph, a pleural effusion may cause blunting of the costophrenic angle and a characteristic meniscus, with fluid appearing to curve upward along the lateral chest wall.
Larger effusions can produce more extensive opacification of the affected hemithorax.
Why Does It Look That Way?
Fluid attenuates more X-rays than air. When fluid replaces the normally air-containing appearance of the lower thorax, fewer X-rays reach the detector and the region appears more opaque.
The shape and distribution of the opacity are influenced by gravity and the anatomy of the pleural space.
Atelectasis
What Is It?
Atelectasis refers to incomplete expansion or collapse of lung tissue, resulting in loss of lung volume.
What's Happening?
Atelectasis can occur through several different mechanisms, including airway obstruction, compression of the lung, or inadequate expansion.
Regardless of the cause, the affected portion of lung contains less air and occupies less volume.
What Does It Look Like?
Atelectasis commonly produces an area of increased opacity accompanied by signs of volume loss.
Depending on its location and severity, signs of volume loss may include displacement of fissures, elevation of the diaphragm, crowding of vessels or ribs, and movement of the mediastinum toward the affected side.
Why Does It Look That Way?
As lung tissue loses air, its overall attenuation increases, causing the affected region to appear more opaque.
At the same time, the reduction in lung volume pulls nearby structures toward the area of collapse. This combination of increased opacity and volume loss helps distinguish atelectasis from many other causes of pulmonary opacity.
Pulmonary Edema
What Is It?
Pulmonary edema is the abnormal accumulation of fluid within the lung interstitium and/or alveoli.
What's Happening?
When fluid accumulates in the lungs, it may first collect within the pulmonary interstitium and, as the process progresses, enter the alveolar spaces.
Pulmonary edema can occur for different reasons, including elevated hydrostatic pressure from heart failure and increased permeability of the pulmonary capillaries.
What Does It Look Like?
The appearance depends on the cause and severity. Chest radiographs may demonstrate findings such as interstitial opacities, septal lines, perihilar or diffuse airspace opacities, and pleural effusions.
In cardiogenic pulmonary edema, enlargement of the cardiac silhouette and vascular redistribution may also be present.
Why Does It Look That Way?
Normal lungs contain large amounts of air and therefore appear relatively dark on radiographs.
As fluid accumulates within the interstitium and alveoli, the amount of material attenuating the X-ray beam increases. The affected portions of the lungs therefore become more opaque.
The distribution of that fluid and the structures it involves determine the particular pattern seen on the image.
Pneumoperitoneum
What Is It?
Pneumoperitoneum refers to the presence of free gas within the peritoneal cavity.
Although pneumoperitoneum is an abdominal finding, it is often identified on upright chest radiographs, where free intraperitoneal air may be particularly conspicuous beneath the diaphragm.
What's Happening?
Free intraperitoneal gas can occur when gas escapes from a hollow abdominal organ, such as with gastrointestinal perforation. It can also occur in non-emergent settings, including following abdominal surgery or certain procedures.
What Does It Look Like?
On an upright chest radiograph, free intraperitoneal gas may appear as a crescent-shaped lucency beneath the diaphragm, particularly beneath the right hemidiaphragm where the adjacent liver provides a relatively uniform soft-tissue background.
Why Does It Look That Way?
Free gas rises to the highest nondependent portion of the peritoneal cavity when the patient is upright and can collect beneath the diaphragm.
Because gas attenuates very few X-rays, it appears dark or lucent. The contrast between this dark gas and the relatively opaque diaphragm and upper abdominal organs makes small amounts of free air potentially visible on an upright radiograph.
Acute Ischemic Stroke
What Is It?
An acute ischemic stroke occurs when blood flow to part of the brain is interrupted, usually because an artery supplying that region becomes occluded. Without adequate blood flow, brain tissue becomes deprived of oxygen and nutrients, resulting in cellular injury.
What's Happening?
Loss of blood flow leads to failure of normal cellular energy production. As energy-dependent ion pumps fail, water shifts into cells, producing cytotoxic edema.
If blood flow is not restored, this process can progress to irreversible tissue injury and infarction.
What Does It Look Like?
The appearance of an ischemic stroke depends on the imaging modality and the time since onset.
On noncontrast CT, early findings can be subtle and may include loss of normal gray-white matter differentiation, focal swelling, and decreased attenuation within the affected brain tissue.
On diffusion-weighted MRI (DWI), acute ischemic injury typically demonstrates high signal on DWI with corresponding low signal on an ADC map, reflecting restricted diffusion.
Why Does It Look That Way?
As ischemic cells lose the ability to maintain normal ion gradients, water moves into cells and the movement of water molecules within the affected tissue becomes restricted.
This change can be detected very early with diffusion-weighted MRI.
As edema develops, the affected tissue also contains more water and becomes less attenuating on CT, contributing to its hypoattenuating appearance and loss of normal gray-white differentiation.
Intracranial Hemorrhage
What Is It?
Intracranial hemorrhage refers to bleeding within the skull. Blood may accumulate in different locations, including within the brain tissue itself or within spaces surrounding the brain.
Examples include intraparenchymal, subarachnoid, subdural, and epidural hemorrhage.
What's Happening?
When a blood vessel ruptures, blood escapes into a location where it normally should not be present. The location and amount of blood determine its effects on surrounding structures and help characterize the type of hemorrhage.
What Does It Look Like?
On a noncontrast CT, acute blood typically appears hyperattenuating, or brighter, relative to normal brain tissue.
The shape and distribution of the blood can provide important clues about where the hemorrhage is located. For example, extra-axial hemorrhages such as epidural and subdural hematomas can produce characteristic patterns.
Why Does It Look That Way?
Freshly clotted blood has relatively high attenuation compared with normal brain tissue, causing acute hemorrhage to appear bright on noncontrast CT.
The appearance of blood changes as it evolves over time, so hemorrhage does not always maintain the same imaging appearance.
Cerebral Edema
What Is It?
Cerebral edema is an abnormal increase in water content within the brain.
Different mechanisms can produce cerebral edema, including ischemia, tumors, infection, inflammation, and traumatic injury.
What's Happening?
Water accumulates within brain cells and/or the extracellular spaces of the brain, depending on the underlying mechanism.
As the amount of water within the tissue increases, the brain can swell. Severe swelling can increase intracranial pressure and displace adjacent structures.
What Does It Look Like?
On CT, cerebral edema generally appears as an area of decreased attenuation and may cause loss of normal gray-white matter differentiation.
Swelling may also produce mass effect, including narrowing of the cerebral sulci, compression of the ventricles, or displacement of other intracranial structures.
Why Does It Look That Way?
Water attenuates X-rays less than normal brain parenchyma. As the water content of affected tissue increases, its CT attenuation decreases, causing it to appear relatively darker.
At the same time, the increased tissue volume physically compresses and displaces nearby structures, producing the imaging findings of mass effect.
Hydrocephalus
What Is It?
Hydrocephalus is abnormal enlargement of the ventricular system related to an imbalance between the production, circulation, and absorption of cerebrospinal fluid (CSF).
What's Happening?
CSF normally circulates through the ventricular system and subarachnoid spaces before being absorbed.
If CSF flow is obstructed, absorption is impaired, or—much less commonly—CSF production is excessive, CSF can accumulate and cause enlargement of the ventricles.
What Does It Look Like?
CT or MRI may demonstrate enlargement of the ventricles. The pattern of ventricular enlargement can provide clues about the location and cause of impaired CSF circulation.
In some cases, increased intraventricular pressure causes CSF to move across the ventricular lining into the surrounding white matter, producing transependymal flow of CSF.
Why Does It Look That Way?
As CSF accumulates, increasing volume and pressure within the ventricular system can cause the ventricles to enlarge.
Because CSF has imaging characteristics similar to water, it appears dark on CT, dark on conventional T1-weighted MRI, and bright on conventional T2-weighted MRI.
Transependymal movement of CSF increases the water content of the surrounding white matter, producing additional characteristic imaging changes.
Small Bowel Obstruction
What Is It?
A small bowel obstruction (SBO) occurs when the normal passage of intestinal contents through the small bowel is partially or completely blocked.
Common causes include adhesions, hernias, and tumors.
What's Happening?
When the small bowel becomes obstructed, fluid and gas accumulate upstream from the site of obstruction. This causes the bowel proximal to the obstruction to dilate, while bowel distal to the obstruction may become decompressed.
Severe or prolonged obstruction can compromise bowel perfusion and lead to ischemia.
What Does It Look Like?
On CT, an SBO typically appears as dilated loops of small bowel proximal to a transition point, with relatively decompressed bowel distally.
The transition point is the location where dilated bowel changes to decompressed bowel and can help identify the site of obstruction.
Why Does It Look That Way?
The obstruction prevents normal forward movement of intestinal contents. Gas and fluid therefore accumulate within the bowel upstream from the blockage, causing those loops to dilate.
Beyond the obstruction, relatively little intestinal content passes through, leaving the distal bowel decompressed.
Acute Appendicitis
What Is It?
Acute appendicitis is inflammation of the appendix, most commonly associated with obstruction of the appendiceal lumen.
What's Happening?
Obstruction of the appendiceal lumen can lead to continued secretion of fluid, increasing intraluminal pressure and promoting bacterial overgrowth and inflammation.
As inflammation progresses, the appendix becomes enlarged and surrounding tissues may also become inflamed.
What Does It Look Like?
On CT, acute appendicitis may appear as a dilated, fluid-filled appendix with wall thickening and surrounding inflammatory change, including increased attenuation or stranding of the adjacent fat.
An appendicolith, a calcified deposit within the appendix, may sometimes be present.
Why Does It Look That Way?
Inflammation causes swelling and thickening of the appendiceal wall, while obstruction can cause the appendix to become distended with fluid.
Inflammation also extends into the surrounding fat. Normally, abdominal fat appears relatively dark on CT. When edema and inflammatory fluid infiltrate that fat, its attenuation increases, creating the streaky appearance commonly called fat stranding.
Acute Cholecystitis
What Is It?
Acute cholecystitis is inflammation of the gallbladder, most commonly caused by obstruction of the cystic duct by a gallstone.
What's Happening?
When the cystic duct becomes obstructed, bile cannot exit the gallbladder normally. Continued gallbladder secretion and inflammation lead to distention, wall edema, and surrounding inflammatory changes.
What Does It Look Like?
On ultrasound, findings may include gallstones, gallbladder distention, wall thickening, pericholecystic fluid, and a sonographic Murphy sign.
Gallstones commonly appear echogenic and may produce posterior acoustic shadowing.
Why Does It Look That Way?
Gallstones can strongly reflect and attenuate the ultrasound beam, allowing relatively little sound to reach the tissues behind them. This produces the characteristic dark acoustic shadow posterior to many gallstones.
Inflammation causes edema and thickening of the gallbladder wall and may produce fluid in the surrounding tissues.
Acute Pancreatitis
What Is It?
Acute pancreatitis is acute inflammation of the pancreas caused by premature activation of digestive enzymes and resulting pancreatic injury.
Gallstones and alcohol are among the common causes.
What's Happening?
Injury to the pancreas triggers an inflammatory response that can cause pancreatic edema and inflammation of the surrounding tissues.
More severe disease may result in pancreatic or peripancreatic necrosis and fluid collections.
What Does It Look Like?
On contrast-enhanced CT, acute pancreatitis may demonstrate enlargement or edema of the pancreas, surrounding fat stranding, and peripancreatic fluid.
Imaging findings vary considerably with the severity and type of pancreatitis, and imaging is not required to diagnose every case of acute pancreatitis.
Why Does It Look That Way?
Inflammation increases fluid within and around the pancreas. This can enlarge the gland and alter its normal appearance.
Inflammatory fluid and edema also extend into the normally low-attenuation fat surrounding the pancreas, increasing its attenuation and producing peripancreatic fat stranding.
Nephrolithiasis
What Is It?
Nephrolithiasis refers to the formation of stones, or calculi, within the kidneys. Stones can enter the ureter and obstruct the normal flow of urine.
What's Happening?
When a stone obstructs the ureter, urine can accumulate upstream from the obstruction. Increasing pressure can dilate the ureter and renal collecting system, producing hydroureter and hydronephrosis.
What Does It Look Like?
On noncontrast CT, most urinary calculi appear as hyperattenuating foci within the kidney or ureter.
When obstruction is present, imaging may also demonstrate dilation of the ureter and renal collecting system upstream from the stone.
Why Does It Look That Way?
Most urinary stones contain materials that attenuate X-rays substantially more than the surrounding soft tissues and urine, causing them to appear bright on CT.
When a stone obstructs urine flow, urine accumulates proximal to the obstruction and causes the collecting system to dilate.
Ascites
What Is It?
Ascites is the abnormal accumulation of fluid within the peritoneal cavity.
It can occur in many conditions, including portal hypertension, malignancy, infection, and disorders involving the heart, liver, or kidneys.
What's Happening?
Normally, only a small amount of fluid is present within the peritoneal cavity. When the balance of fluid production and absorption is disrupted, fluid can accumulate around the abdominal and pelvic organs.
What Does It Look Like?
On CT, ascitic fluid typically appears as low-attenuation material surrounding abdominal and pelvic structures and collecting within dependent spaces.
On ultrasound, simple ascitic fluid is typically anechoic and therefore appears dark or black.
Why Does It Look That Way?
Simple fluid has attenuation relatively close to water and therefore appears relatively dark on CT compared with most soft tissues.
On ultrasound, simple fluid produces very few internal echoes and therefore appears anechoic.
Its distribution is determined partly by gravity and the anatomy of the peritoneal spaces, causing fluid to collect in characteristic locations.
Fracture
What Is It?
A fracture is a break in the structural continuity of a bone. Fractures can occur from trauma, repetitive stress, or weakening of bone by an underlying disease process.
What's Happening?
When the force applied to a bone exceeds its ability to withstand that force, the bone can fail and fracture.
Fractures vary widely in their appearance and can be displaced or nondisplaced, complete or incomplete, and may extend into an adjacent joint.
What Does It Look Like?
On a radiograph, a fracture may appear as a linear lucency or disruption of the normal cortical contour of the bone. There may also be displacement, angulation, or separation of fracture fragments.
Some fractures are subtle or occult on initial radiographs and may require additional imaging such as CT or MRI.
Why Does It Look That Way?
Bone normally attenuates X-rays strongly and therefore appears relatively white on a radiograph.
When a fracture creates a gap or disruption within the bone, the X-ray beam may pass through the fracture plane with less attenuation than through the surrounding bone. This can produce a lucent fracture line and interruption of the normally smooth cortex.
Osteoarthritis
What Is It?
Osteoarthritis is a degenerative joint disease characterized by progressive changes involving articular cartilage and other structures within the joint.
What's Happening?
As articular cartilage is lost, the space between opposing bones decreases. The underlying bone responds to altered mechanical stress with changes such as subchondral sclerosis and osteophyte formation.
What Does It Look Like?
On radiographs, osteoarthritis commonly demonstrates:
Joint-space narrowing
Osteophytes
Subchondral sclerosis
Subchondral cystic change
The distribution of these findings varies depending on the joint involved.
Why Does It Look That Way?
Articular cartilage is not directly visualized well on conventional radiographs. Instead, the distance between opposing bones serves as an indirect representation of cartilage thickness. As cartilage is lost, the apparent joint space narrows.
Increased bone formation and remodeling produce osteophytes and subchondral sclerosis, which appear relatively radiopaque because bone strongly attenuates X-rays.
Joint Effusion
What Is It?
A joint effusion is an abnormal accumulation of fluid within a joint.
Joint effusions can occur with trauma, infection, inflammation, degenerative disease, and many other conditions.
What's Happening?
Inflammation, injury, bleeding, or other pathological processes can increase the amount of fluid within the joint capsule, causing the joint to become distended.
What Does It Look Like?
The appearance depends on the imaging modality and joint involved.
On radiographs, an effusion may produce distention or displacement of normal soft-tissue and fat-pad contours.
On ultrasound, simple joint fluid is typically anechoic or hypoechoic.
MRI can directly demonstrate increased fluid within the joint, which is typically bright on fluid-sensitive sequences.
Why Does It Look That Way?
The appearance of an effusion reflects the physical properties of fluid and therefore changes with the modality being used.
On ultrasound, simple fluid produces few internal echoes and appears dark. On fluid-sensitive MRI sequences, fluid typically produces high signal and appears bright.
This is a good example of how the same abnormality can look completely different depending on the imaging modality.
Bone Lesion
What Is It?
A bone lesion is an area of abnormal bone identified on imaging. Bone lesions can result from many different processes and may be benign or malignant.
The term lesion describes an abnormality and does not by itself represent a specific diagnosis.
What's Happening?
Different disease processes can alter normal bone formation or cause destruction of existing bone.
Radiologists evaluate characteristics such as the lesion's location, margins, pattern of bone destruction or formation, effect on the cortex, and associated periosteal reaction or soft-tissue component to help characterize it.
What Does It Look Like?
On radiographs and CT, a bone lesion may appear predominantly lucent, sclerotic, or mixed, depending on how the underlying process alters the bone.
Lucent lesions contain or produce relatively less mineralized bone and appear darker.
Sclerotic lesions contain or produce relatively more mineralized bone and appear whiter.
Why Does It Look That Way?
The amount of mineralized bone strongly influences X-ray attenuation.
When a process reduces or replaces mineralized bone, fewer X-rays are attenuated and the region may appear more lucent. When a process results in increased mineralization or bone formation, more X-rays are attenuated and the region may appear more sclerotic.
The imaging appearance therefore provides information about how the underlying process is affecting normal bone.
Tendon Tear
What Is It?
A tendon tear occurs when some or all of the fibers of a tendon are disrupted. Tears may be partial or complete and can result from acute injury or chronic degeneration.
What's Happening?
Normal tendons are composed primarily of tightly organized collagen fibers. Injury can disrupt this organized structure and may produce fluid, edema, or a gap between torn tendon fibers.
What Does It Look Like?
On MRI, normal tendons generally demonstrate low signal intensity on most conventional sequences. A tear may produce abnormal increased signal, fiber disruption, or a fluid-filled gap, depending on the type and severity of the injury.
On ultrasound, a tendon tear may appear as disruption of the normal fibrillar architecture, with a hypoechoic or anechoic defect in some tears.
Why Does It Look That Way?
The tightly organized collagen of a normal tendon produces characteristic low signal on MRI and a fibrillar appearance on ultrasound.
When the tendon is torn, its normal architecture is disrupted and may be replaced by fluid, edema, or hemorrhage. These changes alter the way the tissue interacts with MRI and ultrasound, producing the abnormal imaging appearance.
Breast Mass
What Is It?
A breast mass is a three-dimensional space-occupying finding that can be identified on one or more imaging modalities. Breast masses can result from many different processes and may be benign or malignant.
The term mass describes an imaging finding and does not by itself represent a specific diagnosis.
What’s Happening?
A mass forms when tissue develops into a localized three-dimensional abnormality that differs from the surrounding breast tissue.
Radiologists evaluate features such as a mass’s shape, margins, density, echogenicity, and internal characteristics to help determine how concerning it appears and whether additional evaluation is needed.
What Does It Look Like?
The appearance of a breast mass depends on the imaging modality.
On mammography, a mass may appear as a focal area of increased density with a particular shape and margin.
On ultrasound, a mass may be hypoechoic, isoechoic, or hyperechoic relative to surrounding tissue and can have a variety of shapes, margins, and internal characteristics.
Why Does It Look That Way?
Mammography uses X-rays, so the appearance of a mass partly reflects differences in X-ray attenuation between the mass and surrounding breast tissue.
Ultrasound instead depends on the interaction of sound waves with tissue. Differences in tissue composition and interfaces therefore produce differences in echogenicity.
This is another example of how the same abnormality can have different appearances depending on the imaging modality.
Breast Calcifications
What Are They?
Breast calcifications are deposits of calcium within breast tissue that appear as small radiopaque foci on mammography.
Calcifications are common and can occur with both benign and malignant processes.
What’s Happening?
Calcium can accumulate within breast tissue for many reasons, including normal aging, previous inflammation or injury, benign breast changes, and certain malignancies.
Radiologists evaluate characteristics such as the size, shape, morphology, and distribution of calcifications to help determine their significance.
What Do They Look Like?
On mammography, calcifications appear as small bright or white areas within the breast.
Some are relatively large and have characteristically benign appearances, while others are very small and may require careful evaluation of their morphology and distribution.
Why Do They Look That Way?
Calcium strongly attenuates X-rays compared with surrounding soft tissue.
As a result, fewer X-rays pass through calcifications to reach the detector, causing them to appear radiopaque or bright on mammography.
Breast Cyst
What Is It?
A breast cyst is a fluid-filled structure within the breast. Simple cysts are benign and are particularly well characterized with ultrasound.
What’s Happening?
A cyst forms when fluid accumulates within a confined space in the breast.
What Does It Look Like?
On ultrasound, a simple cyst typically appears:
Anechoic
Circumscribed
With posterior acoustic enhancement
Why Does It Look That Way?
Simple fluid produces essentially no internal echoes, causing the cyst to appear black or anechoic.
Sound also travels through simple fluid with relatively little attenuation. More sound therefore reaches the tissues behind the cyst, producing increased brightness known as posterior acoustic enhancement.
This is the same ultrasound physics you encountered earlier in Learn Radiology.
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