See Like You Have Never Seen Before
Part IV—Remember the Dime, Remember the Questions

Introduction

When looking at an image or an area around us, we have the feeling that we see everything in detail, but this is a clever trick developed by the brain to create an image from intermittent and incomplete visual signals. This last lecture of the series will review the mechanisms of visual search, put everything together and show you how to examine and interrogate radiographs… and see like you have never seen before.

Visual search

We all know that radiographs should be searched thoroughly, but how complete is a "complete" search? Studies have shown that large areas of radiographs are unexplored, even when interpreted by experienced radiologists. For the most part, this is not the result of carelessness but the consequence of complex visual mechanisms, many of which are not under our conscious control.

The area of acute vision is a lot smaller than we recognize. The fovea, the small central area of the retina densely packed with visual receptors, is the only structure of the eye capable of detailed vision. During interpretation of a radiograph or any other image, including our tri-dimensional world, eye movements temporarily fix the fovea on different locations to obtain detailed information. The indistinct information gathered by the peripheral vision is primarily used to guide the eye movements towards areas of motion or high contrast via oculomotor reflexes. They can also occasionally activates additional percepts.

As we all know from experience, a radiographic abnormality is easier to detect if it stands out from the background (high contrast). The explanation is simple: the abnormality is more likely to be detected by the peripheral vision and attract visual attention. On the other hand, if the lesion is too subtle to attract visual attention, it can be very difficult to find.

We rarely realize it, but our eyes constantly move from fixation to fixation in a succession of jumps. Each fixation lasts about one third of a second. Because vision is blurred during eye movements, we effectively see only when the eye is still, which can be as little as 30% of the time. We are unaware of this intermittent input of information because the process is efficiently smoothed in our visual perception centers. This smoothing of the visual input along with the filling of the peripheral field by mental images is dangerous in radiology: it gives us the impression of seeing in detail a large area at one glance, when the actual area of acute vision is only about the size of a dime at a distance of 60 cm.

The apparently simple act of reading (what you are doing effortlessly right now) displays the effectiveness and complexity of the filling mechanisms. As an experienced reader, you probably make three or four jumps per line. As you read, you have the impression of seeing all characters clearly, but you actually see in detail only six to twelve per line.

To convince yourself, try the following experiment: fix your eyes on any word of this page for a few seconds, and notice how most of the page turns into a blur. Before long you see clearly only two or three letters. This very small surface of the page is the only one that is actually seen in detail at any given time. You might be able to identify one or two short words on either side of the fixation, but this identification is based on inference from word length and grammatical construction rather than from seeing the actual letters. Note that as soon as you start reading again, you have the impression of seeing every letter of every word again.

The extrapolation and smoothing procedures giving us the impression of seeing everything in detail are performed by our brain that fills-in the missing information using pre-existing knowledge of the English language. The faster we read, the longer the jumps and the more information is filled-in by the mind. This explains why we have such a hard time finding typos in a familiar text. Struggling with a foreign language (spoken or written) really makes one appreciate the influence of previous knowledge on perception.

The same phenomenon occurs with radiographs: the more familiar we are with the images, the more comfortable we become and the quicker we make sense out of them; but at the same time, the more information is filled-in by the mind.

Another interesting aspect of the eye jumps is that they are not completely under conscious control: we cannot change the length or direction of a jump once it is initiated. Therefore, we cannot be certain about what we actually see and what our brain fills-in. The best we can do to control a search pattern is to concentrate on a certain area; the details of the search are always left to an unconscious part of our visual system.

Remember the dime

Knowing that we see in detail only a surface the size of a dime, that it is impossible to accurately control where the "dime" will rest on the image, that we see only one third of the time we are actually "looking", and that it is impossible to know how much we actually see and how much information our brain fills-in makes it easier to understand how lesions can be missed. And how important it is to use a systematic approach that takes these limitations into account.

A systematic search for lesions must aim at keeping the distance between eye jumps at or below the size of a dime, particularly in areas that are more likely to be affected by lesions. This is more easily said than done.

Remember the questions

We have a tendency to forget that the only information we can gather from radiographs about any structure or tissue is its location, size, shape, and relative opacity. Never more… and often less.

We also forget that the information about location, size, and shape is often incomplete and always distorted by geometric magnification (caused by the divergence of the x-ray beam and by the position of the object relative to the plane of the film).

Because every conclusion is deduced from that information—however limited it is—you should always ask yourself the following question when you look at each structure and tissue: "ARE LOCATION, SIZE, SHAPE, AND OPACITY NORMAL?" If the answer is yes, you go to the next structure, following a systematic approach.

After knowing the five basic radiographic opacities, the most important aspect of radiographic interpretation is to follow a systematic approach. When you read radiographs, you should always examine it the same way, look at the different structures exactly in the same order, and ask yourself exactly the same questions exactly in the same order.

There is too much information in a radiograph
to be assimilated all at once

You have to break the information in smaller parts that you can handle. Only this way can you be sure that you looked at everything when you are done, and be confident in your diagnosis, especially when your diagnosis is "normal".

Here is my systematic approach when I read thoracic radiographs. It is just an example; there are hundreds of good systematic approaches. If you are not comfortable with mine, modify it or even change it completely. What's important is that you find one that you like and that you use it all the time.

So here is my personal systematic approach to thoracic radiographs. To facilitate things, I divide the thorax in four parts:

• Thoracic wall
• Pleural space
• Mediastinum
• Lungs

I always start with the thoracic wall and look at all structures always in the same order asking myself if there is anything abnormal about location, size, shape, and relative opacity of these structures.

I also look in areas where normal structures cannot normally be seen but can sometimes be seen if abnormal in size, location, or relative opacity.

1) Thoracic Wall:

Look at the following structures, always in the same order

• Skin (all around the patient)
• Subcutaneous tissues (all around the patient)
• Thoracic inlet
• Cervical esophagus
• Cervical trachea
• Diaphragm
• Cranial abdomen
  • Liver
  • Stomach
  • Spleen
  • Large and small bowel
• Ribs (every one of them)
• Sternum (every sternebra)
• Vertebral column
  • every dorsal spinous proces
  • every articular facet
  • every vertebral body
  • every intervertebral disk space
  • every intervertebral foramen
  • vertebral canal

That's all. Notice that it did not require an advanced degree in anatomy. And at the end of this first part, you can say with confidence: "The thoracic wall is normal" or "The thoracic wall is abnormal." If you are not sure if something is normal or not, compare with illustrations of an anatomy book, a radiographic atlas, or an anatomy specimen (particularly useful for skeletal radiography).

2) Pleural Space:

Then I examine the pleural space. The pleural space is the easiest part of the examination: It is a virtual space between the surface of the lungs and the thoracic wall that cannot normally be seen on radiographs. But if there is fluid or air in it, the pleural space will increase in thickness and displace the surface of the lung away from the thoracic wall. If there is enough fluid or air, it can become visible between lung lobes wthin the interlobar fissures. The opacity of the pleural space will be that of fluid (same as intercostal muscles, heart, diaphragm, and liver) if there is pleural effusion, or lower than both thoracic wall and lungs (not very much though) if there is pneumothorax. Therefore, there are only two questions you should ask yourself when you examine the pleural space:

• Is there retraction of the lung from the thoracic wall and diaphragm?
• Can I see interlobar fissure lines? If so, are they wider than normal? (Should be the thickness of a sheet of paper.)

If the answer of either one of these questions is yes, determine the opacity (air or fluid) of the pleural space.

As you can see, the systematic examination of the pleural space is short and simple. Now, answer the following questions:

3) Mediastinum

      Now it is time to look at the mediastinum. Again, I look at all the following structures, always in the same order, asking myself if there is anything abnormal about location, size, shape, and relative opacity. I also look for structures that cannot normally be seen but can sometimes become visible if abnormal in size, location, or relative opacity.

• Trachea
• Cranial thoracic esophagus
• Longus colli muscle
• Major cranial thoracic arteries and veins
• Mediastinal lymph nodes
• Thymus
• Cranial vena cava
• Retrosternal lymph node
• Heart
  • Left atrium
  • Right atrium
  • Left ventricule
  • Right ventricule
• Aorta
• Main pulmonary artery
• Tracheobronchial lymph nodes
• Caudal vena cava
• Caudal thoracic esophagus
• Aorta

Here again, the anatomy is relatively simple. The only difficulty is in determining what can be seen and what cannot. But if you think in relative radiographic opacities, you should not have too many difficulties. Go back to the list and underline the structures that can be seen on normal radiographs. The other ones can be seen only if there are significant changes in their size, shape, location, or opacity.

4) Lungs

A lot of people have problems with this last part of the thoracic examination. I suspect it is because few use a systematic evaluation. I am not saying that lung radiographs are not confusing. They are. I am saying that a systematic evaluation is the only way to work around the confusion.

As done for the previous parts of the thorax, I look at all the structures, always in the same order, but instead of asking myself if there is anything abnormal about location, size, shape, and opacity, I ask myself five simple questions that cover just about everything that can be seen about the lungs on radiographs. Those questions are:

• Can I see the blood vessels crisp and clear?
• Are veins and arteries roughly equal in size and tapered?
• Do I see air bronchograms?
• Do I see thickened airway walls (donuts)?
• Do I see nodules?

Notice that there is only two possible answers for every question: Yes or No. Of course, there is always the possibility of a "maybe", but usually you can lean more toward one or the other, and this is what you should do: We are paid to make these choices. If you are not sure, it is probably normal, or you should verify with other tests. Therefore, if the answers are Yes, Yes, No, No, and No, the radiographic diagnosis is "normal" and you can be confident about it.

Therefore when I examine the lungs, I run my eyes over the entire lung field, one lobe at a time, on both views, and ask myself the five questions.

• Right cranial lobe: Question 1, 2, 3, 4, and 5
• Left cranial lobe: Question 1, 2, 3, 4, and 5
• Right middle lobe: Question 1, 2, 3, 4, and 5.
• Left middle lobe: Question 1, 2, 3, 4, and 5.
• Right caudal lobe: Question 1, 2, 3, 4, and 5.
• Left caudal lobe: Question 1, 2, 3, 4, and 5.
• Accessory lobe: Question 1, 2, 3, 4, and 5.

The only thing you need to memorize is that in the artery-bronchus-vein triad, the artery is dorsal on the lateral view and lateral on the dorsal view. It would be so easy to remember if it would be the opposite...

Believe it or not

This is all you need to know to be able to interpret thoracic radiographs. But you need to know it well and stick to it. The rest is a matter of becoming more familiar with radiographic images.

If you ask radiographs questions they can answer—preferably by "yes" or "no"—radiographs will start talking to you. And as you know, it is a lot more fun than just staring at them in silence.

Radiology: Art or Science?

Radiology is often said to be more art than science. But what is art? It usually means the exercise of some complex subconscious mechanisms that cannot be explained easily. To be honest, I believe that when we cannot explain how we come up with a diagnosis, the complex subconscious mechanisms involved in the process are usually what straightforward people call guessing. Obviously, this type of art should be kept to a minimum when reading radiographs.

In spite of its reputation, I believe that radiology should be and can be a science. Perhaps even more so than other branches of medicine because its fundamental principles are based on invariable physical laws. Unfortunately, those physical laws are rarely emphasized, rapidly forgotten, and sporadically applied.

It Is Simply a Matter of
Using a Systematic Approach and
Asking The Right Questions

I think that most of the confusion--and errors--in radiology comes from our looking for answers that can not be found on radiographs. In other words, when we ask the wrong questions.

To test this hypothesis, I will use a different didactic method. I will refrain from enumerating radiographic lesions or try to astound you with the subtleties of the “art”. Instead, I will suggest a systematic approach and a few simple questions to which straightforward answers can (usually) be obtained when interrogating radiographs.

I hope that by using a systematic approach and these questions, you will increase the amount of useful information you get from radiographs while decreasing stress and confusion.

After mastering the basic physical principles (not covered here), the most important aspect of radiographic interpretation is to follow a systematic approach. You should always examine radiographs the same way, look at all structures exactly in the same order, and ask yourself exactly the same questions preferably in the same order. Why?

Because there is too much visual information
on radiographs to be assimilated all at once!

Whatever our IQ, there is only so much our eyes and brain can handle at any moment. There is an incredible amount of visual information on radiographs. You have to break it in smaller parts otherwise you will invariably miss something. If you don’t break it in smaller parts, you can never be sure that you looked at everything when you are done—or be confident in your diagnosis, particularly when your diagnosis is "normal".

But the physical information
is surprisingly minimal

Although the amount of visual information is enormous, the amount of physical information is rather small: it is limited to location, size, shape, and opacity.
    Therefore, you should focus on that information and ask yourself the following question when you look at each structure and tissue: "Are location, size, shape, and opacity normal?"
     If the answer is "yes", forget about it and go to the next structure, always following a systematic approach (described later). If the answer is "no", think about what can cause the change(s) and generate a list of differential diagnoses.

Develop a systematic approach
and use it all the time!

The next pages describe the systematic approach I use when I read abdominal radiographs. It is just an example; there are hundreds of good systematic approaches. If you are not comfortable with this one, modify it or even change it completely. What's important is that you find one that you like and that you use it all the time.

Break down your analysis
in small parts you can handle

Here is the systematic approach I use to analyse abdominal radiographs. First, I divide the abdomen in four parts:

1. Abdominal wall
2. Peritoneal space
3. Gastrointestinal tract
4. Urogenital tract

I always start with the abdominal wall and look at all the structures, always in the same order, asking myself if there is anything abnormal about location, size, shape, and relative opacity. It is a good idea to examine routinely areas where normal structures are usually invisible because they can occasionally become visible when abnormal in size, shape, location, or relative opacity.

I look at the following structures, always in the same order:

• Skin (all around the patient)
• Subcutaneous tissues (all around the patient)
• Diaphragm
• Caudal thoracic structures
  • Lungs
  • Caudal vena cava
  • Esophagus
  • Heart
• Ribs (every single one)
• Vertebral column
  • every dorsal spinous process
  • every articular facet
  • every vertebral body
  • every intervertebral disk space
  • every intervertebral foramen
  • vertebral canal
• Sublumbar space
  • sublumbar muscles
  • sublumbar fat
  • sublumbar lymph nodes
  • major blood vessels
• Pelvis
  • Skeletal structures
  • Soft tissues
• Pelvic limbs
  • Skeletal structures
  • Soft tissues

Note that it did not require an advanced degree in anatomy. And if you do it right, at the end of this first part of the examination you can say with confidence : "The abdominal wall is normal" or "the abdominal wall is abnormal."

If you are not sure if a structure is normal or not, compare with illustrations from an anatomy book, a radiographic atlas, or an anatomy specimen (particularly useful for skeletal radiography).

The peritoneal space is the fastest part of the examination but can be tricky if the quality of the radiograph is suboptimal.

The peritoneal space is the space between the serosal surface of all abdominal viscera and the serosal surface of the abdominal wall. Contrary to the pleural space, it is usually not a virtual space because it contains fat (primarily omental fat). This intraperitoneal fat separates the water-opacity viscera from the water-opacity abdominal wall. It allows visualization of the serosal surfaces because it produces fat/tissue interfaces.

Young animals and thin or emaciated older animals do not have large amounts of intraperitoneal fat and serosal surfaces can be difficult to see or even completely invisible (because the water-opacity viscera are surrounded by water-opacity abdominal walls and everything blends together on the radiograph).

If there is progressive accumulation of peritoneal fluid (peritoneal effusion), the fluid infiltrates between the layers of fat and the overall opacity of the peritoneal space changes gradually (as the amount of fluid increases) from fat-opacity to water-opacity. The serosal surfaces of the viscera become surrounded by water-opacity material and the fat-water interfaces disappear. You can no longer see serosal surfaces. Therefore, the first of the four questions you should ask yourself is:

• Can I see serosal surfaces crisp and clear?

The answer should be yes or no. Never “yabutt...” If the answer is yes, there is no fluid in the peritoneal space and you can go to question #3. If the answer is no, ask yourself question #2.

• Can I see them at all?

Again, no “yabutt”. If the answer is "no", you can conclude there is a moderate to large amount of peritoneal fluid if the abdomen is distended, or that there is a lack of abdominal fat if the abdomen appears normal or reduced in size. An exception (what did you expect?) is that very young dogs and cats frequently have a distended abdomen with very little intra-abdominal fat; therefore it is normal to lose visualization of the serosal surfaces in these young patients. Note that moderate to large amounts of peritoneal fluid often hide underlying abdominal diseases (ex: tumors, organomegaly, etc...).

If the answer is "yes, I can see them a little bit”, there are four possibilities. They are, in order of frequency:

• Poor radiographic technique
• Small amount of peritoneal fluid
• Fibrinous peritonitis
• Serosal metastases.

If a poor radiographic technique (most commonly motion blurring and scattered radiation) is suspected, additional views may be taken. Abdominocentesis or ultrasonography are often necessary to confirm a radiographic diagnosis of peritoneal effusion.

There is another possible radiographic anomaly associated with the peritoneal space: the pneumoperitoneum. Small amounts of air are usually invisible or very difficult to detect on standard projections (lateral and ventrodorsal) because air floats over the heavy abdominal viscera and the air-tissue interfaces are perpendicular to the x-ray beam (an interface must be parallel to the x-ray beam to be prominent on a radiograph because our eyes are not good at all to detect gradual changes in opacity).

If there is enough air, however, it can produce interfaces parallel to the x-ray beam (vertical) by infiltrating between bowel loops or between the liver, stomach, and diaphragm. Because the normal fat-tissue interfaces of the peritoneal space are replaced by air-tissue interfaces, the serosal surfaces become very prominent when a severe pneumoperitoneum is present. If air is present between the liver/stomach and diaphragm, both surfaces of the diaphragm (thoracic and abdominal) become visible (normally only the thoracic surface is visible).

Therefore, question #3 and #4 are:

• Can I see both surfaces of the diaphragm?

• Are serosal surfaces more prominent than normal?

Remember again that small amounts of air are invisible on projections taken with a vertical x-ray beam because the air-tissue interfaces are perpendicular to the x-ray beam. Therefore, if a gastrointestinal perforation is suspected clinically, the only way you can confirm or rule out your tentative diagnosis is to take a horizontal beam radiograph, preferably with the patient in lateral recumbency, to look carefully for a air-tissue interface at the upper portion of the serosal surface of the abdominal wall.

     Summary: Examination of the peritoneal space is performed by
                        asking four questions:

• Can I see serosal surfaces crisp and clear?
• Can I see them at all?
• Can I see both surfaces of the diaphragm?
• Are serosal surfaces more prominent than normal?

Now it is time to look at the abdominal viscera. I like to start with the gastrointestinal tract and associated glandular structures (liver and pancreas). Again, I look at all the following structures, always in the same order, asking myself if there is anything abnormal about location, size, shape, and relative opacity. I also look for structures that cannot normally be seen (followed by an asterisk in the list) but that sometimes become visible if abnormal in size, location, or relative opacity. To facilitate the process, I simply follow the progression of an imaginary bolus going through the esophagus, stomach, small bowel, and large bowel.

Follow an imaginary bolus

• Thoracic esophagus*
• Abdominal esophagus (very short)*
• Fundus of the stomach
• Body of the stomach
• Pyloric antrum
• Pylorus
• Duodenum
• Jejunum
• Ileum
• Caecum
• Ascending colon
• Transverse colon
• Descending colon
• Rectum
• GI-associated structures
  • Liver
  • Gall bladder
  • Pancreas

Here again, the anatomy is relatively simple. The only difficulty is in determining what is normal and what is not. Because the opacity, shape, and size of the gastrointestinal content is extremely variable--from air, water, a mixture of both, to mineral or a mixture of all of the above--the normal radiographic appearance of the stomach and intestines changes constantly. For example, if a loop of bowel (or a part of the stomach) is filled with homogeneous fluid, it is totally invisible unless surrounded by intraperitoneal fat. However, the same segment becomes very prominent if filled with air or mineral opacities (ex: bone fragments).

Because the shape and opacity of the gastrointestinal tract are so inconsistent, the only reliable information we can obtain about the stomach and intestines from radiographs is location and size. The pertinent questions therefore are:

• Is there displacement of the stomach or bowel?

• Is there distension of the stomach or bowel?

   Here we are not talking about subtle changes, we are talking about "holy s...!" changes. There are so many normal variations in the location and size of the gastrointestinal tract that subtle changes always have a questionable significance.

There are two last questions:

• Is there a radiopaque foreign body?

Gastrointestinal foreign bodies are common in small animals and their clinical significance should be questioned if they are not associated with radiographic signs of intestinal obstruction ("holy s...! distension) or pneumoperitoneum (what are they?...).

• Is there a mass associated with the stomach or intestines?

Gastrointestinal masses are difficult to identify because they are superimposed over bowel loops but they are occasionally seen.

Summary: Examination of the GI tract is performed by following an imaginary bolus and asking the following four questions:

• Is there displacement of the stomach or bowel?
• Is there distension of the stomach or bowel?
• Is there a radiopaque foreign body?
• Is there a mass associated with the stomach or intestines?

4) Genitourinary Tract:

Again, I look for the following structures, always in the same order, asking myself if there is anything abnormal about location, size, shape, and relative opacity. This time, I follow the progression of urine through the kidney parenchyma, renal pelvis, ureter, bladder, and urethra. Then, I follow the genital tract starting with the gonads. In the following list, the structures that cannot normally be seen are followed by an asterisk.

Follow the flow of urine

• Urinary tract
  • Kidneys
  • Renal pelvis*
  • ureters*
  • Bladder
  • Urethra* in males
    • Prostatic
    • Membranous
    • Cavernous (penile)
    • Os penis

Follow the Ova

• Female genital tract
  • Ovaries*
  • Oviduct*
  • Uterine horns*
  • Uterine body*
  • Cervix*
  • Vagina*
  • Vulva

Follow the Spermatozoa

• Male genital tract
  • Testicles
  • Ductus deferens*
  • Prostate
  • Urethra
    • Prostatic
    • Membranous
    • Cavernous
  • Os penis
  • Penis
  • Prepuce

I do not use specific questions for the genitourinary tract. As I review all the structures of the list, I ask myself if there is anything wrong about size, shape, location, and opacity. And then I go back to the urinary tract and look carefully for calculi from the renal pelves to the tip of the penis.

What do we do when we identify an anomaly?

Not very many abdominal diseases produce characteristic radiographic changes allowing us to give an immediate final diagnosis. Most commonly, radiographic findings are used to elaborate a list of differential diagnoses that is later shortened in light of the history, physical examination, and laboratory results. For example, finding an enlarged kidney on a radiograph should lead to a list similar to this one:

• Hydronephrosis
• Primary renal tumor
• Metastatic renal tumor
• Renal cyst(s)
• Renal abscess or granuloma
• Renal hematoma
• Renal pseudocyst

In some cases, the shape might suggest one diagnosis over the others, but most of the time further investigations (lab work, ultrasound, ultrasound-guided biopsies) are necessary to pinpoint the problem.

The generation of a list of differential diagnoses is a process that involves your entire medical knowledge and will not be covered here. The only point I want to make is the following: Always try to find at first as many diagnoses as possible to explain an abnormal radiographic finding because, due to well documented psychological mechanisms, it is always easier to eliminate diagnoses from the list than to add new ones later.

I hope this reading technique will be helpful in increasing your radiographic interpretation skills. It certainly made all the difference in the world for me. Remember that practice makes better. Good luck!

Conclusion

Considering the pitfalls of our visual and perceptual systems, it is all the more important to use a systematic approach designed to minimize perceptual errors. A thorough geographic systematic approach followed by a pathophysiological approach linked to questions that can be answered by radiographs should significantly increase your radiographic diagnostic yield and minimize errors. Remember the dime and remember the questions.