How to Read a Treasure Map – Part 1

My treasure? Why, it’s right where I left it… It’s yours if you can find it… But you’ll have to search the whole world!

Gol D. Roger – One Piece

Hi guys… Here we are again… I hope you’ve missed us!!!

We have been a little busy managing a few other things… And it’s from them that we bring you this new post, opening a brand-new section about imaging…

As promised during the Young ESTES workshop in Ljubljana last May 2023, we report hereafter the Surgical Pizza guidelines on how to interpret computed tomography (CT) scan images!!!

Yeah… We know… You are not radiologists, and this is not a website for radiologists either…

Anyhow…

Why Do Surgeons Need to Know How to Read CT Images?

Today… or, better, tonight, you are working in a small peripheral hospital. You are on call from home, and no surgeon is in-hospital… Only the emergency medicine doctors are attending the Emergency Room.

It’s about 1 AM when an 82-year-old man is brought to the ER from the Nursing Facility where he lives because of mild abdominal pain, vomiting, and an altered state of consciousness.

His past medical history includes hypertension, mild cognitive impairment, chronic renal failure, previous stroke, ischemic heart failure, past left inguinal repair, and previous aortic-bifemoral and femoral-femoral bypasses. He’s on aspirin, nitroglycerine, furosemide, pantoprazole, and statin.

On admission, the abdomen is not particularly distended, not tender, slightly painful, and a non-reducible and painful mass is present in the left groin.

The ER doctor asks for the following:

• Lab exams: WCC 15’640/mm³; Hb 13.6 g/dL; CRP 87 mg/L; Creatinine 1.39 mg/dL; Lactates normal;
• Chest and abdominal x-rays: nodule in the right lower lobe, mild small bowel distension, coprostasis.

At this point, the ER doctor requested a CT scan that was performed without IV contrast because of the “altered” renal function. The CT report says, “no free fluid, no free air, small bowel distension, coprostasis involving the whole colon and rectum, regular outcomes of previous vascular procedures, left testis ascended inside the inguinal canal with associated hydrocele, 3.5 cm nodule in the right lower pulmonary lobe”.

This is the moment when the ER doctor calls you in the middle of the night… and he explains everything to you…

And our question now is: What would you do?

Well, you decide to take a look at the CT images from home (you have access to the hospital network via VPN).

When you look at them, you notice that something is wrong with the CT report: that mass doesn’t seem like an ascended testis at all. So, you decide to go to the hospital to visit the patient yourself.

When you arrive and you see the patient, even from a distance, your suspicion is confirmed: there is no way that the mass is an ascended testis; it’s an incarcerated hernia for sure.

However, just to be 100% positive, you perform a point-of-care ultrasound, confirming your hypothesis.

What you do afterward is basic: activate the OR and operate on the patient.

So, what has this clinical case taught us?…

…That all surgeons (maybe it’s better to say “clinicians”) need to know how to read CT scan images. The main reasons are:

• Surgeons have a better knowledge of the patient’s clinical status. In fact, radiologists usually have no precise clinical information about the patient they are scanning, and they may not consider possible clinical scenarios.
• Surgeons may determine details that may change the management. For instance, if the radiologist reports the “presence of pneumoperitoneum”, it is important to know how much air is inside the abdominal cavity and where it is. This information is critical to deciding whether to go to the OR or manage the patient conservatively.
• Surgeons may recognize important anatomical features relevant during surgery. It is important to know which anatomical structures are related to the pathological process (e.g. the vascular involvement by cephalopancreatic cancer) or if the patient has any anatomical anomaly (e.g. variation in hepatic arterial anatomy)
• Radiologists are humans… and sometimes they make mistakes.

All You Ever Wanted to Know About CT Scan (but You Were Afraid to Ask)

Hereafter, in a very schematic way, we are going to state the basic information you need to know about CT scans…

1. What is a CT Scan?

Physically, a CT scan is a rotating tube releasing X-rays. The X-rays go through the tissues of the patient and are therefore attenuated. On the other side of the machine, there are detectors in a gantry that measure this attenuation. All this information is used in reconstruction algorithms to produce tomographic (cross-sectional) images (virtual “slices”) of the patient’s body.

2. What about the Planes?

The CT scan images can be looked at in three different planes:

• Axial – This is the “original” reconstruction, basically how the data are collected. This plane divides the body “up-down”.
• Coronal – This plane divides the body “front-back”… Imagine how the crown is placed on the new king’s head during coronation…
• Sagittal – This plane divides the body “left-right”. Imagine how Sagittarius, the archer, strikes an apple with an arrow, splitting it into two halves.

3. And the Hounds… thing?

The Hounsfield unit (HU) determines the grade of gray displayed in each pixel, and it depends on the attenuation coefficient of each tissue.

This unit is the result of the need for an easy system/scale to communicate the scale of grays, and it depends on the formula:

In the following image, you can see the values in Hounsfield of the most common body tissues. We can notice that most tissues have similar values, ranging from 0 to 80 HU.

This can lead to a challenge of accurately reading the image. So, in order to try and solve this problem, the radiologist invented the “window” concept.

4. What About Windows?

The “window” concept is a way to study in more detail images with a similar shade of gray and similar HU. This concept is to increase contrast among these tissues with similar HU, so that certain parts of the spectrum are enlarged.

Let’s make a couple of examples:

• Soft tissue setting with a window width of 400 HU and a window level of 40 HU means that all structures above +240 HU and under -160 HU are projected as white and black, respectively.
• Lung setting with a window width of 1500 HU and window level of -650 HU means that all structures above +100 HU and under -1400 HU are projected as white and black, respectively.
• Bone setting with a window width of 2000 HU and level of +400 HU means that all structures above +1400 HU and under -600 HU are projected as white and black, respectively.

5. Contrast Medium?!… Is it alchemy?

The contrast medium is a drug, iodine-based, used to enhance the visibility of CT scan images. It may be administered in different ways: intravenously, orally, through the rectum, etc…

Since it is iodine-based, it may cause allergic reactions. Moreover, its excretion is via the renal system, so patients with a glomerular filtration rate (GFR) <30 mL/h, who are not under dialysis, require preparation with 0.9% NaCl.

Given intravenously, the contrast medium arrives rapidly at the heart, passes to the arteries, then enters the portal circulation and later on the venous circulation, reaches the kidneys, and, lastly, is expelled by the ureters and the bladder. This path gives different CT scan phases, corresponding to registrations at different time frames:

• t0 – Not enhanced
• 15 seconds after contrast injection – Early arterial
• 30 sec – Late arterial
• 70 sec – Late portal-hepatic
• 100 sec – Nephrogenic
• 6 minutes – Delayed

The use of the contrast medium, the way of administration, and which phase to register are all depending on what we are looking for, and which is our clinical suspicion.

Please remember that a non-contrast CT scan will give you very little and specific information, extremely useless in an emergency setting: if you need an urgent CT scan make it with CT contrast, no matter the patient’s kidney status!

Ok… That’s enough for today…

Next time we are going to see how to properly read CT scan images from the clinician’s point of view…

Have a great life!!!

References

1. Murphy A. Hounsfield unit. Radiopaedia. Accessed on September 10, 2023. Available at [https://radiopaedia.org/articles/hounsfield-unit?lang=us].
2. European Society of Urogenital Radiology. ESUR Guidelines on Contrast Agents. Accessed on September 10, 2023. Available at [https://www.esur.org/esur-guidelines-on-contrast-agents/].
3. Clarke JR, et al. Time to laparotomy for intra-abdominal bleeding from trauma does affect survival for delays up to 90 minutes. J Trauma 2002;52:420-5.
4. Ordonez CA, et al. Computed tomography in hemodynamically unstable severely injured blunt and penetrating trauma patients. J Trauma Acute Care Surg 2016;80:597-602.
5. Hecker A, et al. Intra-abdominal sepsis: new definitions and current clinical standards. Langenbeck’s Archives of Surgery 2019;404:257-71.

How to Cite This Post

Bellio G, Sandano M, Mastronardi M, Biloslavo A, Marrano E. How to Read a Treasure Map – Part 1. Surgical Pizza. Published on October 1, 2023. Accessed on February 16, 2025. Available at [https://surgicalpizza.org/imaging/how-to-read-a-treasure-map-part-1/].