Principles Of Ct Scan Abdomen

PRINCIPLES OF CT SCAN ABDOMEN GUIDE- PROF.DR P. K. PANDEY(M.S) PROFESSOR,DEPT. OF SURGERY

PRESENTED BYPRAKASH CHINNANNAVAR

HISTORY OF CT SCAN 1917-J RADON,WORKING WITH GRAVITATIONAL THEORY  1956-BRACEWELL WORKING IN RADIOASTRONOMY  1961-OLDENDORF UNDERSTOOD CONCEPT OF CT  1972-G.N.HOUNSFIELD PUT A CT SYSTEM TOGETHER. 

HISTORY OF CT SCAN 

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CT was originally proposed and used as an extension of the basic X-ray: doctors wanted to see inside the head, but standard X-ray techniques could not penetrate the dense skull while distinguishing soft tissues The first scanners could only do one slice at a time and each slice took 4 minutes to complete By 1976, whole body scanners were developed Today’s machines can acquire a slice in less than half a second (.1 second for the GE top-end multislice LightSpeed Pro16 with Xtream) Advancements still occurring through new technology

COMPUTED TOMOGRPHY  BASIC PRINCIPLE of CT is that the internal structure of an object can be reconstructed from multiple projections of that object.  The ray projections are formed by scanning a thin cross section of the body with a narrow xray beam and measuring the transmitted radiation with a sensitive radiation detector.  The detector adds up the energy of all the transmitted photon. The numerical data from multiple ray sums are than computer processed to reconstruct an image.

CT SCANNER

It is having 3 component– 1. X-ray tube 2. collimator 3. Detector –two types of detectors used in CT scanner Scintillation crystals  Xenon gas ionisation chambers 

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various designs of CT scanner1. First generation (translate –rotate ,one detector )

2. Second gen. (translate – rotate ,two detector)

Third gen. (rotate – rotate )

Fourth gen. (rotate –fixed)

IMAGE RECONSTRUCTION

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In CT cross section of body is divided into many tiny blocks and each is assigned a number proportional to the degree that the block attenuate the x-ray beam. - The linear attenuation coefficient (µ) is used to quantitate attenuation and is determined by composition and thickness of ‘voxel’ ,along with quality of the beam . - CT numbers are derived by comparing linear attenuation coefficient of a pixel with that of water and are described in HOUNSFIELD UNIT .

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CT density scale :Water – ‘0’ HU Air - ‘-1000’ HU Calcification – ‘+1000’ HU Fat - ‘-100’ HU Hemorrhage – ’60-70’ HU Bone appears white; gases and liquids are black; tissues are gray

Improvements over X-Rays 

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Provides 3D images and cross-sectional views instead of basic 2D images CT Scans can show soft tissue as well as bone, allowing physicians to detect problems such as cancerous tumors Extremely helpful in determining organ anatomy, especially following trauma Can determine tissue density difference of less than 1% while Xrays can allow determine tissue density difference of 5%

Circa 1975, in the early days of the CT scan.

A present-day scan, showing a six-fold increase in detail

History of CT Scan (continued) Specifications First CT (circa 1970)

Modern CT Scanner (2001)

Time to acquire one CT image

4-5 minutes

0.5 seconds

Pixel size

3 mm x 3 mm

0.5 mm x 0.5 mm

Number of pixels in an image

64,000

256,000

Artifacts

Aliasing Artifact or Streaks

These appear as dark lines which radiate away from sharp corners. It occurs because it is impossible for the scanner to 'sample' or take enough projections of the object, which is usually metallic. It can also occur when an insufficient X-ray tube current is selected, and insufficient penetration of the x-ray occurs. These artifacts are also closely tied to motion during a scan. .

Ring Artifact Probably the most common mechanical artifact, the image of one or many 'rings' appears within an image. This is usually due to a detector fault

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Noise Artifact

This appears as graining on the image and is caused by a low signal to noise ratio. This occurs more commonly when a thin slice thickness is used. It can also occur when the power supplied to the X-ray tube is insufficient to penetrate the anatomy. 

Motion Artifact This is seen as blurring and/or streaking which is caused by movement of the object being imaged.

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Beam Hardening This can give a 'cupped appearance'. It occurs when there is more attenuation in the center of the object than around the edge. This is easily corrected by filtration and software.

HELICAL/SPIRAL SCANNING

CONTRAST MATERIAL 

1. Ionic monomeric contrast media (high-osmolar contrast media, HOCM), e.g. amidotrizoate, iothalamate, ioxithalamate

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2. Ionic dimeric contrast media (low-osmolar contrast media, LOCM), e.g. ioxaglate

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3. Nonionic monomeric contrast media (low-osmolar contrast media, LOCM), e.g. iohexol, iopentol, ioxitol, iomeprol, ioversol, iopromide, iobitridol, iopamidol 4. Nonionic dimeric contrast media (iso-osmolar contrast media, IOCM), e.g. iotrolan, iodixanol

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Risks of CT Scan Low-risk procedure  Main risk is allergic reaction to contrast dye, often mild and resulting in itching, hives, or a rash  More radiation than modern standard XRay, but still minimal amount  Only radiation risk significant only for pregnant women 

Which CT Scanner is best? 

Axial v. Helical scanners  Axial scanners

Longer time to scan  Danger in misregistration of scanner 

 Helical scanners

Quicker scan time  Images for overlapping slices can be generated  More complicated image reconstruction 

Single-slice vs. Multi-slice detectors 

Single-slice detectors  Slow exam times

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Multi-slice detectors  Much quicker exam times  Up to 4 slices in 0.5 seconds

What typically gets scanned brain and spinal abnormalities  brain tumors and strokes  sinusitis  aortic aneurysms and other blood vessels  hemorrhage  chest infections  diseases of organs such as the liver, kidneys, and lymph nodes in the abdomen. 

CONTRAINDICATIONS FOR CT SCAN -CONTRAST ALLERGY -CLAUSTROPHOBIA -PREGNANCY

CT APPLICATIONS

LIVER

LIVER

HEPATIC PARENCHYMA 

ATTENUATION VALUE AROUND 45-65

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HEPATICSPLENIC ATTENUATION DIFFERENCE

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AFTER IV CONTRAST ADMINISTRATION THIS ATTENUATION DIFFERENCE REVERSES

PRINCIPLES OF HEPATIC CONTRAST ENHANCEMENT 

TO INCREASE ATTENUATION VALUE DIFFERENCE BETWEEN LIVER LESIONS AND NORMAL HEPATIC PARENCHYMA

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ATTENUATION VALUE DEPENDS ON HISTOLOGY,VASCULARITY,NECROSIS,CALCIFICATION,HEMO RRAGE.

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USUALLY HEPATIC NEOPLASMS HAVE LOWER ATTENUATION VALUE THAN NORMAL LIVER EXCEPT IN DIFFUSE FATTY LIVER WHERE THEY ARE HYERATTENUATING

HEPATIC CONTRAST ENHANCEMENT VASCULAR  REDISTRIBUTION  EQUILIBRIUM 

HEPATIC CYST

BILIARY HAMARTOMAS

LIVER HEMANGIOMAS

FOCAL NODULAR HYPERPLASIA

HEPATOCELLULAR ADENOMA

HEPATOCELLULAR CARCINOMA

FIBROLEMELLAR HEPATOCELLULAR CARCINOMA

METASTASIS TO LIVER

PYOGENIC LIVER ABSCESS

HYDATID CYST OF LIVER

BILARY SYSTEM

PRINCIPLES OF BILIARY TRACT IMAGING USG IS INITIAL IMAGING STUDY  CT DETECTS BILIARY DILATATION,BILIARY WALL THICKENING,DUCTAL STONES,PANCREATIC MASS, ADENOPATHY 

PORCELAIN GALL BLADDER

GALL BLADDER CARCINOMA

CAROLI’S DISEASE

CHOLEDOCOLITHIASIS

MIRIZZI SYNDROME

INTAHEPATIC CHOLANGIOCARCINOMA

SPLEEN

SPLENIC CYSTS

NON HODGKIN’S LYMPHOMA

HODGKIN’S LYMPHOMA

PANCREAS

PANCREATIC CARCINOMA

CHRONIC PANCREATITIS

MUCINOUS CYSTIC NEOPLASM

GASTRINOMA

ACUTE PANCREATITIS

CHRONIC PANCREATITIS

PSEUDOPANCREATIC CYST

INFECTED PANCREATIC NECROSIS

RETROPERITONEUM

RETROPERITONEAL HEMATOMA

IDEOPATHIC RETROPERITONEAL FIBROSIS

RETROPERITONEAL SARCOMA

PSOAS ABSCESS FROM INFECTED URINOMA

HORSESHOE KIDNEY

POLYCYSTIC KIDNEY DISEASE

RENAL CELL CARCINOMA

RENAL TUBERCULOSIS

NORMAL ADRENAL GLAND

ADRENAL ADENOMA

ADRENAL CARCINOMA

PHEOCHROMOCYTOMA

GASTROINTESTINAL TRACT

NORMAL ESOPHAGUS

ESOPHAGEAL ADENOCARCINOMA

ESOPHAGEAL PERFORATION

GASTRIC WALL

GIST

GASTRIC ADENOCARCINOMA

GASTRIC LYMPHOMA

SMALL BOWEL LYMPHOMA

COLON CARCINOMA

MULTISEGMENTAL CROHN COLITIS

ULCERATIVE COLITIS

CT COLONOGRAPHY

CT IN BLUNT TRAUMA ABDOMEN METHOD OF CHOICE INITIAL EVALUATION IN HEMODYNAMICALLY STABLE AND UNSTABLE TRAUMA  ACCURACY IN DIAGNOSIS IS 97%  ALSO EVALUATE EXTRA-ABDOMINAL INJURIES  ROLE OF MDCT. 

INDICATIONS 

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IN THE PAST-INDICATED FOR ONLY HEMODYNAMICALLY STABLE PATIENTS AND ASSOCIATED INJURIES RECENTLY INCREASINGLY USED IN UNSTABLE PATIENTS ADVANTAGE OF CT OVER DPL ADVANTAGE OF CT OVER FAST. CT LARGELY REPLACED RADIONUCLEOTIDE SCINTIGRAPHY,ANGIOGRAPHY AND USG.

HEMOPERITONEUM

SPLENIC LACERATION

LIVER LACERATION AND HEMATOMA

PANCREATIC LACERATION

RENAL LACERATION

INTERVENTIONAL CT Therapeutic and diagnostic  Image guided procedures can also be done under fluoroscopic, ulrasonographic and MRI guided  CT provides precise, three dimensional localisation  Can used in presence of open wounds,dressings,ostomies etc 

DIAGNOSTIC INTERVENTIONAL CT PERCUTANEOUS BIOPSY OF LIVER, PANCREAS, ADRENAL GLAND,RETROPERITONEUM,PARATHY ROID  COMPLICATIONS ARE INFREQUENT AND GENERALLY MINOR. 

THERAPEUTIC INTERVENTIONAL CT DRAINAGE PROCEDURES  PERCUTANEOUS NEPHROSTOMY  CECOSTOMY  NEUROLYSIS  TUMOR ABLATION-CHEMICAL -THERMAL 

NEUROLYSIS

WHAT’S NEW IN CT? VIRTUAL ENDOSCOPY -IMAGING PERFORMED VIA CT TO PRODUCE THREE DIMENSIONAL IMAGES -USED TO DIAGNOSE DIVERTICULOSIS,POLYPS AND MALIGNANCY -PROCEDURE -OPTICAL COLONOSCOPY HAS BECOME GOLD STANDARD IN SCREENING OF COLORECTAL CANCERS. 

ADVANTAGES-MORE COMFORTABLE -NO SEDATION -MORE DETAILED EVALUATION -TAKES LESSER TIME

DISADVANTAGES-EXPOSURE TO RADIATION -SMALLER LESIONS NOT PICKED UP -BIOPSY CAN’T BE TAKEN

COMPOSITE CT-PET SCAN

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PROVIDES BOTH FUNCTIONAL AND ANATOMIC IN SINGLE IMAGING STUDY.

In the Future      

Still a larger role for ct over next decade MRI provided further stimulus Multislice CT imagers take center stage Image manipulation will become easier Computer assisted diagnosis will become more common Still be need for understanding of principles of CT

Conclusions 

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CT is not very exciting from a physics point of view (… didn’t you think the Saha chapter on CT was facinating?) However, it is the most popular “modern” imaging technique: available at over 30,000 world locations, including over 6,000 health care centers in the US (many with multiple CT machines) New uses of CT are constantly being developed. Recently, smaller CT setups are being used in the OR to evaluate surgeries as they progress. Better computer techniques will also enhance the value of CT studies.

Thanks