Pitfalls & Artifacts in F-18 FDG PET/CT Imaging

Pitfalls and Artifacts in F-18 FDG PET/CT Imaging

Jiraporn Sriprapaporn, M.D.

Faculty of Medicine Siriraj Hospital

Mahidol University

Pitfalls and Artifacts in PET/CT Imaging

Physiologic

Normal physiologic distribution (uptake & excretion)

Normal physiologic variants

Pathologic, nonmalignant

FDG avid benign pathologic conditions

Artifacts

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Physiologic Uptake of 18F-FDG

Head and neck

Brain

Extraocular muscles

Vocal cord

Tonsils

Salivary glands

GI tract

Esophagus

Gastric

Bowel

Liver-spleen

Chest

Heart uptake

Breast

Thymus

Genitourinary

Kidneys

Bladder

Uterus

Muscles

Bone marrow

Physiologic uptake is normally seen in the brain, heart,

liver, spleen, gastrointestinal tract, urinary collecting system (including the bladder), and bone marrow Jiraporn_spp

Normal PET-CT Scan

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Physiologic 18F-FDG Distribution

Head and Neck

Brain cortex: high uptake because glucose is the only source of energy in the brain.

Tonsils, base of the tongue: low-to-moderate uptake - associated with the lymphatic tissue in Waldeyer’s ring.

Salivary glands (parotid, submandibular, and sublingual glands): variable Asymmetric due to positioning, post radiation, inflammation

Anterior part of the floor of the mouth due to the genioglossus muscle: moderate uptake.

Muscles of mastication may be prominent if chewing. Extraocular muscles, muscles of the oral cavity, and laryngeal

muscles: variable uptake. Larynx: mild inverted V shape uptake but may be prominent if

excessive talking Focal unilateral uptake within the larynx could be due to

muscle overuse as in the case of contralateral vocal cord paralysis

Mohei M. Abouzied, et al. JNMT05

FDG Uptake in Extraocular Muscles (EOM) Transverse FDG PET scan (a) and fused PET/CT image (b) show bilateral marked uptake in the

medial and lateral rectus muscles of the head.

Shammas A et al. Radiographics 2009;29:1467-1486

©2009 by Radiological Society of North America

Kostakoglu L et al. Radiographics 2004;24:1411-1431

Figure 7b. Physiologic laryngeal uptake (arytenoid muscles) Larynx


Abnormal

Thyroid

Thyroid: absent to mild uptake

Diffuse increased uptake: thyroiditis or Graves’ disease

Focal uptake: autonomously functioning thyroid nodules and thyroid malignancies. Patients with focal thyroid uptake should

be further evaluated because of a higher risk of malignancy ! (27-64%)


Thyroid

FIGURE 3. CT (left) and fused PET/CT (right) images of 56-y-old female patient with NSCLS showed abnormal focal 18F-FDG uptake in right thyroid gland (SUVmax = 5.3). That lesion had low CT attenuation (76 HU) without calcification.

Papillary carcinoma was confirmed histopathologically.

Choi JY, et al. JNM2006

http://jnm.snmjournals.org/content/vol47/issue4/images/large/609fig3.jpeg

Breasts

Normal: mild uptake

Increased uptake in lactating breasts


http://tech.snmjournals.org/content/vol33/issue3/images/large/zl80030502840030.jpeg

Myocardium

Myocardial uptake: variable even in the same patient imaged on different occasions, usually in LV, occasionally RV During the fasting state, the

myocardium depends on fatty acids to produce energy (21) should not expect high myocardial uptake.

In the postprandial state, the myocardial uptake could be enhanced significantly




Figure 17. Physiologic diaphragmatic uptake

GI Tract

The origin of 18F-FDG uptake in the digestive tract is unknown; possible causes are active smooth muscle, metabolically active mucosa, swallowed secretions, or colonic microbial uptake (22).

Esophageal activity: mild linear uptake anterior to the spine and is best seen in the sagittal plane.

mostly due to swallowed saliva and partially due to smooth muscle metabolism.

Prominent esophageal uptake is abnormal eg. esophagitis.

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Normal esophageal activity

GI Tract

Gastric activity: Curvilinear homogeneous mild uptake corresponding to the gastric wall is commonly identified.

Colonic activity is variable, ranging from faint heterogeneous activity, to mild focal, segmental, or diffuse activity.

Uptake in the cecum and right colon is usually higher than the rest of the colon (more lymphocyte cells very glucose avid.

Focal intense activity in the colon should be further evaluated with colonoscopy.

By reviewing the sagittal and the rotating images, one can, in most cases, eliminate any pathologic uptake.

Liver uptake is faint and homogeneous.


Caecal Uptake




Figure 19. Physiologic gastric uptake



Figure 20. Gastritis

Genitourinary

Because FDG is filtered through the glomeruli, without reabsorption unlike glucose, one should see the activity within the collecting system, ureters, and urinary bladder.

Horseshoe kidney


Genitourinary

Increased 18F-FDG uptake has been reported in

the normal uterus during menstruation (23,24),

a follicular ovarian cyst (25), and

the ovary with an inflammatory reaction during ovulation (26).

Nishizawa et al. concluded that in women of reproductive age, 18F-FDG imaging should preferably be done within a week before or a few days after the menstrual flow phase to avoid any misinterpretation of pelvic 18F-FDG PET images (27).


Endometrial FDG uptake in a premenopausal woman

The endometrium and the inflammatory changes within the ovary during ovulation

Gorospe L, et al. Nucl Med Commun. 2005 Aug;26(8):671-87.

Muscular Activity

Vigorous exercise in the days just before a scan can cause intense uptake in the associated skeletal muscles.

Stress-induced muscle tension as is often seen in the trapezius and paraspinal muscles.

Hyperventilation may induce uptake in the diaphragm as well.

Muscle uptake is typically symmetric, mild-to-moderate linear activity (Figs. 17 and 18).

The use of insulin to adjust the serum glucose level immediately before injection of 18F-FDG can result in 18F-FDG accumulation in skeletal muscle.

Benzodiazepines may be used to decrease paraspinal and posterior cervical muscle uptake in tense patients.

Intercostal muscles in coughing Pt Mohei M. Abouzied, et al. JNMT05



Physiologic Variants: Brown fat

18F-FDG uptake can be seen in the adipose tissue of the neck, supraclavicular regions, around the large vessels in the mediastinum, the axillae, the perinephric regions, and in the intercostal spaces along the thoracic spine

This was found in 3.7% of patients undergoing 18F-FDG PET (28).

Uptake in the neck adipose tissue is typically bilateral and symmetric, intense, and more often multifocal than linear.


BAT= Brown Adipose Tissue

Uptake in supraclavicular area fat ("USA-Fat").

Brown Fat Uptake



MIP FDG PET image shows the typical distribution of uptake in brown adipose tissue in pediatric patients

Thymus Uptake

Thymus uptake is a normal finding in children and young adults, appearing as an upside down letter V.

Also, thymic hyperplasia after chemotherapy is a normal variant in the adult population as well (25,30)


Bone marrow uptake

BM uptake is generally diffuse low-grade activity, < liver activity and mostly seen in vertebral bodies.

Focal activity within bone marrow is always suspicious for an abnormality.

Uniform diffuse increased BM activity can be seen with BM recovery after chemotherapy,

which usually resolves by 1 mo after therapy (31).

in patients taking hematopoietic growth factors (HGF), granulocyte-macrophage colony-stimulating factor (GM-CSF), granulocyte colony-stimulating factor (G-CSF), and erythropoietin (31,32).

waiting for 2–4 wk after BM stimulation to perform 18F-FDG study is advised.

Mohei M. Abouzied, et al. JNMT05 Gorospe L, et al. 2005

BM Uptake before and after CMT

Blake MA, et al

Radiographics 2006

Splenic Uptake

Normally, spleen has faint FDG uptake.

Increased splenic FDG uptake

after G-CSF since spleen is an active site for the extramedullary hematopoiesis

other hematologic diseases such as thalassemia, which can cause extramedullary hematopoiesis,

infection


Benign Lesions

Benign lesions with increased FDG uptake are found

in more than 25% of the PET/CTstudies performed in

patients with proven or suspected malignancy, with

inflammation being the most common cause (73%).

[Metser U, et al. AJR 2007]

Benign Tumors and Tumor-Like Conditions

Nonossifying fibroma

Enchondroma

Eosinophilic granuloma

Paget’s disease

Fibrous dysplasia

Gaucher’s disease

Spur

Exostosis

Schmorl’s node

Avascular necrosis

Metser U, et al. SNM 2007

Metser U, et al. AJR 2007

Benign lesions with increased FDG uptake are found in > 25% of the PET/CT studies performed in patients with proven or suspected malignancy, with inflammation being the most common cause.

The causes for benign uptake of FDG were inflammatory processes (n = 154, 73.3%), benign tumors (n = 23, 11%), hematoma or seroma (n = 17, 8.1%), fracture (n = 7, 3.3%), fat necrosis (n = 3, 1.4%), and others (n = 6, 2.9%).

Lesion characterization on the CT portion of the PET/CT study increases the specificity of PET/CT reporting, especially for lesions with moderate to marked uptake.

Benign Conditions: Recent surgery

Abdominal Surgery Sternotomy for bypass surgery





Benign Conditions

s/p breast biopsy

Aortic uptake-unstable plaque formation




FDG uptake at the fractured rib with traumatic Hx 2 weeks ago


Copyright © 2008 by the American Roentgen Ray Society

Metser, U. et al. Am. J. Roentgenol. 2007;189:1203-1210

Diffuse FDG uptake in left shoulder is suggestive of synovitis.



FDG uptake in small soft-tissue mass with fat-attenuation center suggestive of fat necrosis

http://www.ajronline.org/content/vol189/issue5/images/large/11_07_2083_03e.jpeg

Benign Tumors

Thyroid adenoma Adrenal adenoma Nonossifying fibroma Enchondroma

Metser U, et al. AJR 2007

Neurofibroma Colon adenoma Uterine fibroid Papilloma-breast Langerhans cell histiocytosis



Moderate uptake of FDG in a left adrenal adenoma that was stable for > 2 years.

Adrenal adenoma has usually no uptake, but some may show some uptake.

http://www.ajronline.org/content/vol189/issue5/images/large/11_07_2083_01e.jpeg

18F-FDG PET in Characterizing Adrenal Lesions

Detected on CT or MRI. By Yun M, et al. JNM 2001

Dec;42(12):1795-9

A retrospective analysis on 50 adrenal lesions in 41 patients with suspected or known malignancy.

All malignant lesions are positive on PET. (uptake >liver) Most lesions (13/18) showed uptake >> liver. 5/13 lesions (2 metastatic NETs, 2 early metastases, and 1

necrotic metastasis), FDG uptake was equal or slightly > liver. 32 benign lesions, mostly < liver activity

2 lesions with uptake equal to or slightly higher than liver activity,

3 with uptake < liver, but > background, 27 uptake = background.

FDG PET for characterization of adrenal lesions showed a sensitivity of 100%, a specificity of 94%, and an accuracy of 96%.

Conclusion: FDG PET showed excellent diagnostic performance in differentiating adrenal lesions

Adrenal activity >> liver activity malignant, 72% sensitivity

1 Adrenal Adenoma vs 2 Lymphomatous adrenal gland involvement

Elaini AB, Radiographics 2007

1 2



Neurofibroma involving C6–C7 nerve root with mild FDG uptake

http://www.ajronline.org/content/vol189/issue5/images/large/11_07_2083_01h.jpeg



Abnormal FDG uptake in polypoid lesion (arrows) along right rectal wall revealed tubulovillous adenoma

http://www.ajronline.org/content/vol189/issue5/images/large/11_07_2083_05a.jpeg

FDG uptake by a uterine fibroid at fundus


Artifacts in PET/CT

Definition of the Artifact

False image caused by artificial reasons not by physiological or pathological disease .

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Factors of Artifact

Radiotracer-related

Patient-related

Motion artifact misregistration between

PET & CT images

Instrument-related

Incorrect attenuation correction

Truncation artifact

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Radiotracer-related Artifacts

Improper administration of radiotracer

Extravascular injection

Contamination of radiotracer

Urine contamination*

Scatter artifact due to intense radiotracer accumulation.

Bladder activity

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Scatter artifact from intense bladder activity [IAEA 2014]

Patient-related Artifacts

• Motion artifact

• High metallic artifacts

• External eg. accessories

• Internal eg. surgical clips

Curvilinear cold artifact due to

respiratory motion between PET & CT [JNMT 2005]

Metal ring on left breast (arrow) produces streaking artifacts and high CT numbers (A), resulting in

falsely increased radiotracer uptake on PET images with CT attenuation correction (B), whereas PET image without attenuation correction shows only background activity. [JNMT 2005]

Artifact due to high density material-CMT Port

CMT port (a) produces artifact shown in AC PET (b) but not on noAC PET (c) images. [IAEA 2014]

Artifact due to patient motion between PET & CT scanning (Motion Artifact)

Voluntary motion – It commonly occurs

in the head and limbs misregistration

Involuntary motion - Respiratory motion

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Kapoor V et al. Radiographics 2004;24:523-543


Figure 26 Misregistration

Respiratory Motion: Hepatic dome lesion*

A. Lesion at dome of liver is mislocalized to right lung base (arrow) because of respiratory motion.

B. Image without attenuation correction shows that all lesions are confined to liver.

Sureshbabu W, JNMT 2005

Instrumentation Artifacts

Artifact related to incorrect attenuation correction

Metal Artifact

Metallic implants higher radiodensity

overcorrection of PET image

Contrast Artifact

A radiodensity < 400-500 HU should not produce

any artifact.

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Figure 28 Attenuation correction artifact. (a) AC coronal fused FDG PET-CT image shows a focus of intense hypermetabolism in the right supraclavicular region due to pacemaker





Figure 27 Attenuation correction artifact due to contrast material in the left axillary vein high-attenuation)

overcorrection by the attenuation correction software

Instrumentation Artifacts

Reconstruction artifact - Truncation

artifact due to smaller FOV of CT

component as compared to PET

CT attenuation correction

reconstruction algorithm does not

account for any attenuation of the CT

x-rays by the tissues outside the FOV.

CT Based Attenuation Correction

Procedures of Correction

I=I0*e -µx

I

0=I*e µx

To obtain CT Image

µ≈ (CT/1000)*(µw-µa)+ µw≈ CT*S

S= µ(511Kev) /µ(70Kev)

Reasons of over estimated

Wrong scale factor of CT

Scale factor is a ratio between CT number and µ

Scale factor related to energy and materials

According to the interaction of materials, Scale Factor

is smaller for high energy and dense materials, so high

energy and high CT number should have smaller scale

factor

Using high Scale Factor for 511 keV correction , high

CT number of metal or high density contrast produce

over attenuation correction artifacts

Truncation Artifacts

Truncation artifacts in PET/CT are due to the difference in size of the field of view between the CT (50 cm) and PET (70 cm) tomographs.

These artifacts are frequently seen in large patients or patients scanned with arms down.

It is crucial that technologists carefully position patients at the center of the field of view and with arms above head to reduce truncation artifacts.

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Conclusion

Artifacts have great impact on the image judgment

and diagnosis

Artifact recognition is extremely important in making

accurate diagnosis .

Most of the artifacts of PET/CT come from

radiotracer, instrument and patients themselves .

In order to reduce and /or eliminate artifacts good

QA/ QC program should be carried out frequently.

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Teaching Points

1. Uptake of F-18 FDG can also be seen in the

normal tissue of human.

2. Recognizing various physiological F-18 FDG

focuses is essentially important .

3. Effective methods and measures should be

taken to prevent and eliminate patient-related

artifacts

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References

1. IAEA: PET/CT Atlas on quality control and image artefacts; IAEA

Human Health Series No. 27;2014.

[www-pub.iaea.org/MTCD/Publications/PDF/Pub1642web-16821314.pdf ]

1. Metser U, Miller E, Lerman H, Even-Sapir E. Benign nonphysiologic

lesions with increased 18F-FDG uptake on PET/CT:characterization

and incidence. Am J Roentgenol. 2007 Nov;189(5):1203-10.

2. Metser U, Even-Sapir E. Increased (18)F-fluorodeoxyglucose

uptake in benign, nonphysiologic lesions found on whole-body

positron emission tomography/computed tomography (PET/CT):

accumulated data from four years of experience with PET/CT.

Semin Nucl Med. 2007 May;37(3):206-22.

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References

5. Blake MA, Singh A, Setty BN, Slattery J, Kalra M, Maher MM, et al.

Pearls and pitfalls in interpretation of abdominal and pelvic PET-CT.

Radiographics. 2006 Sep-Oct;26(5):1335-53.

6. Bogsrud TV, Lowe V. Normal variants and pitfalls in whole body

PET imaging with F18-FDG. Appl Radiol 2006;35:16–30.

7. Sureshbabu W, Mawlawi O. PET/CT imaging artifacts. J Nucl Med

Technol. 2005 Sep;33(3):156-61; quiz 163-4.

http://tech.snmjournals.org/cgi/content/full/33/3/156

8. Abouzied MM, Crawford ES, Nabi HA. 18F-FDG imaging: pitfalls and

artifacts. J Nucl Med Technol. 2005 Sep;33(3):145-55; quiz 162-3.

http://tech.snmjournals.org/cgi/content/full/33/3/145

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References

9. Al-Hawary MM, Francis IR, Korobkin M. Non-invasive evaluation of

the incidentally detected indeterminate adrenal mass. Best Pract

Res Clin Endocrinol Metab. 2005 Jun;19(2):277-92. [sci direct]

10. Yun M, Kim W, Alnafisi N, Lacorte L, Jang S, Alavi A. 18F-FDG PET

in characterizing adrenal lesions detected on CT or MRI. J Nucl

Med. 2001 Dec;42(12):1795-9.

11. Cook GJ, Fogelman I, Maisey MN. Normal physiological and benign

pathological variants of 18-fluoro-2-deoxyglucose positron-emission

tomography scanning: potential for error in interpretation. Semin

Nucl Med. 1996 Oct;26(4):308-14.

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Quiz

1. When is PET/CT most appropriate for diagnosing cancer of unknown primary (CUP)?

a) All cases b) Metastasis in inguinal lymph nodes c) Visceral metastasis d) Lung metastasis e) Cervical metastasis

2. Which is the most common site of CUP? a) Pancreas b) Ovary c) Head and neck d) Lung e) Stomach

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Quiz

3. Which is the advantage of the acquisition of contrast-enhanced PET/CT images of the neck region dedicated images for head and neck tumor patients?

a) Improved primary tumor detection b) Improved accuracy of size measurement c) Improved the detection of small lung metastasis d) Improved the detection of lymph node metastasis

4. Which is the most important limitation in the use of PET and PET/CT after radiotherapy for head and neck cancer?

a) Post-surgical and radiotherapy artifacts b) False-positive results due to inflammatory changes c) False negative results due to radiotherapy stunning d) There are no limitation since this is the preferential setting.

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Quiz

5. Which of the following indications can be defined as the most appropriate for F18-FDG PET/CT imaging?

a) TB

b) FUO

c) Osteomyelitis

d) Spondilodiscitis

e) Orthopedic prosthesis infection

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Quiz

6. How can we increase the specificity of F18-FDG PET/CT for diagnosing infection?

a) Delayed PET/CT acquisition to differentiate infection from inflammatory and neoplastic F-18 FDG uptake.

b) Specificity is not an issue.

c) It is not possible to overcome low specificity with F-18 FDG.

d) Pre-scan selection of patients.

e) Early acquisition.

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Answers

1. E

2. D

3. D

4. B

5. B

6. C

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Health & Medicine

Pitfalls & Artifacts in F-18 FDG PET/CT Imaging