Etymology

Derived from the appearance of irregularly shaped stone paving, known as “crazy paving,” which resembles the radiologic pattern of intersecting linear opacities over ground-glass opacity.

AKA

  • None

What is it?

The crazy paving sign is a radiologic finding characterized by ground-glass opacity with superimposed thickened interlobular and intralobular septa, creating a pattern similar to irregular stone paving.

Characterized by

  • Ground-glass opacity (GGO) combined with thickened interlobular and intralobular septa.
  • The result is a polygonal or reticular pattern resembling a mosaic of irregular paving stones.
  • Typically visualized on CT.

Anatomically affecting

  • Lung parenchyma, primarily involving the alveoli and interstitium.

Causes include

  • Most Common Causes:
    • Pulmonary alveolar proteinosis (PAP): Classic association of the crazy paving sign.
  • Other Causes include:
    • Infection: Pneumocystis jirovecii pneumonia (PJP), viral pneumonia (e.g., COVID-19, influenza).
    • Inflammation: Sarcoidosis, nonspecific interstitial pneumonia (NSIP), organizing pneumonia.
    • Neoplasm: Bronchoalveolar carcinoma (adenocarcinoma in situ).
    • Mechanical: Pulmonary edema.
    • Trauma: Pulmonary contusion.
    • Metabolic: Lipoid pneumonia.
    • Circulatory: Chronic pulmonary venous congestion.
    • Idiopathic: Idiopathic pulmonary fibrosis (IPF) in early stages.

Inflammation Immune Mediated
Methotrexate

b12389-lungs-crazy-paving-upper-lung-fields-RA-methotrexate-peumonitis-89F-CXR-CT.jpg

Methotrexate-Induced Pneumonitis
An 89-year-old female with a known diagnosis of rheumatoid arthritis presents with shortness of breath and cough.
Findings
(a) Frontal chest radiograph reveals a diffuse interstitial pattern, setting the stage for further CT analysis.
(b, e) Axial CT through the upper lobes (magnified in e) demonstrates a crazy paving pattern, characterized by ground-glass opacities and interlobular septal thickening, a hallmark of alveolar and interstitial involvement.
(c) Sagittal CT reconstruction highlights the crazy paving pattern, with diffuse involvement across the lungs.
(d) Coronal CT shows ground-glass changes extending into the middle lobe and lingula, reflecting more widespread involvement.
(f) Axial CT through the lower lobes demonstrates no significant sparing, with diffuse changes present throughout the lungs.
Comment
Methotrexate-induced pneumonitis typically presents with bilateral, diffuse involvement across the lungs. While imaging findings can occasionally appear more prominent in one region (e.g., upper or lower lobes), the overall pattern generally lacks a strong regional preference. In some cases, involvement of the lower lung fields may be observed, further emphasizing the diffuse nature of the disease.
The characteristic crazy paving pattern, as seen here, reflects a combination of alveolar damage and interstitial involvement. Other findings may include consolidation, mosaic attenuation, or airspace nodules.
Differential diagnosis includes other drug-induced pneumonitis, infections (e.g., pneumocystis pneumonia), and interstitial lung diseases such as hypersensitivity pneumonitis. Early recognition of these imaging features is critical, particularly in patients on methotrexate therapy, to prevent further progression and complications.
Ashley Davidoff MD TheCommonVein.net (b12389)

Amiodarone Toxicity

CT Ground Glass Changes and Crazy Paving
75-year-old male with cardiomyopathy atrial fibrillation and treatment with amiodarone and a RUL infiltrate thought to be related to amiodarone therapy.
There was no clinical evidence nor radiological evidence of heart failure.
CT scan shows ground glass changes with multicentric crazy paving appearance that was thought to be related to amiodarone toxicity.
Ashley Davidoff MD TheCommonVein.net
278 Lu 32470

Infection

Possible PCP

HIV and Diffuse ground Glass
36 year old female with HIV presents with dyspnea. CT scan on presentation (upper images) show extensive disease with crazy paving pattern. Following adjunctive steroid therapy, the patient showed early mild improvement with persistence of crazy paving, peripheral sparing and regions of improved aeration (middle panel). Continued improvement continued subsequently with improved aeration, minimal residual crazy paving and bronchovascular infiltrate ( lower panel).
Ashley Davidoff MD TheCommonVein.net 30755c

Neoplasm Mechanical

Trauma

Acinar Nodules and Crazy Paving  Post Trauma
Coronal CT following trauma and resuscitative attempts in a 37 year old female shows a focus of crazy paving in the left upper lobe most likely secondary to contusion.  In addition  2-5mm  nodules in both the upper and lower lobes.  In this clinical context. these findings are consistent with hemorrhagic foci of ground glass opacities acinar shadows and crazy paving  following trauma.  
Ashley Davidoff MD TheCommonVein.net 137272  301Lu

Metabolic
Toxins Inhalational

CT 1month Later Prominent Upper Lobe and Superior Aspects of the Lower Lobe Ground Glass Parenchymal Changes
55-year-old male with substance use disorder presents with progressive and now more severe dyspnea. Sagittal CT through the right lung field shows ground glass changes in the upper mid and superior segment of the lower lobe. The fissures of the areas of involved lung are focally thickened. There is subpleural sparing
Progressive inhalational pneumonitis from smoking or cocaine inhalation was suspected. DIP and hypersensitivity pneumonitis remained in the differential diagnosis. (Crack Lung)
Ashley Davidoff MD TheCommonVein.net 251Lu 135940

Circulatory Inherited Iatrogenic

Idiopathic
DIP – Idiopathic but Inflammatory

CT Desquamative Interstitial Pneumonia (DIP) Diffuse Ground Glass Changes Lower Lung Fields and Interlobular Septal Thickening (Crazy Paving)
51-year-old female smoker with a history of COPD asthma and pulmonary hypertension presents with progressive dyspnea. Coronal CT through the posterior lungs shows diffuse ground glass changes in the lower lobes with interlobular septal thickening
Pathology confirmed a diagnosis of DIP
Ashley Davidoff MD TheCommonVein.net 252Lu 135997

Infiltrative

Pathophysiology

The crazy paving sign reflects:

  • Ground-glass opacity due to alveolar filling (e.g., fluid, protein, cells) or interstitial thickening.
  • Thickening of interlobular and intralobular septa caused by edema, fibrosis, or cellular infiltration.
  • The combination of these processes produces the characteristic reticulogranular pattern.

Histopathology

  • Alveolar filling processes (e.g., proteinaceous material in PAP).
  • Interstitial thickening due to edema, inflammation, or fibrosis.

Imaging

Applied Anatomy
  • Parts: Involves alveoli and interstitium within secondary pulmonary lobules.
  • Size: Variable extent, depending on the underlying condition.
  • Shape: Polygonal or mosaic pattern of intersecting lines.
  • Position: Diffuse or focal, often bilateral but can be asymmetric.
  • Character: Ground-glass opacity with superimposed thickened septa.
  • Time: Can be acute, subacute, or chronic, depending on etiology.
CXR
  • May demonstrate diffuse or patchy hazy opacities.
  • Subtle reticulonodular patterns may occasionally be visible.
  • Often non-specific and less sensitive compared to CT.
CT
  • Key Modality:
    • Ground-glass opacity (GGO) with superimposed interlobular and intralobular septal thickening.
    • The pattern is polygonal, resembling irregular stone paving.
    • Distribution can be diffuse, patchy, or focal depending on the underlying cause.
    • In pulmonary alveolar proteinosis (PAP):
      • Bilateral, symmetric involvement, typically in the mid to lower lung zones.
    • In Pneumocystis jirovecii pneumonia (PJP):
      • Diffuse, bilateral crazy paving pattern with a predilection for perihilar regions.
    • Contrast-enhanced CT may help assess associated vascular changes or superimposed complications.
MRI
  • Rarely used but may show hyperintense signals corresponding to GGO on T2-weighted imaging.
PET-CT
  • Variable metabolic activity depending on the etiology:
    • Increased uptake in active infections, malignancies, or inflammation.
    • Lower activity in pulmonary alveolar proteinosis.
Other
  • Not applicable.

Differential Diagnosis

  • Pulmonary alveolar proteinosis (PAP): Classic and most common association.
  • Pneumocystis jirovecii pneumonia (PJP): Diffuse, bilateral GGO with crazy paving.
  • Organizing pneumonia: Patchy consolidation with ground-glass opacity.
  • Pulmonary edema: Septal thickening secondary to fluid accumulation.
  • Lipoid pneumonia: Fatty alveolar material causing ground-glass opacity.
  • Diffuse alveolar hemorrhage (DAH): Alveolar filling with associated septal thickening.
  • NSIP: Reticular pattern with ground-glass opacity.

Recommendations

  • Further Imaging:
    • Contrast-enhanced chest CT to assess underlying vascular, neoplastic, or inflammatory processes.
    • Prone imaging may help differentiate dependent atelectasis from true disease involvement.
  • Laboratory Correlation:
    • Bronchoalveolar lavage (BAL) for infectious or proteinaceous causes.
    • Serum markers for inflammatory or autoimmune etiologies (e.g., ACE, ANA).
    • Fungal and viral serology in infectious conditions.
  • Biopsy: Consider transbronchial or surgical lung biopsy in uncertain cases.

Key Points and Pearls

  • The crazy paving sign is not disease-specific but strongly associated with pulmonary alveolar proteinosis (PAP).
  • Careful evaluation of distribution, associated findings, and clinical context is key to narrowing the differential diagnosis.
  • A combination of alveolar and interstitial processes produces the characteristic radiologic pattern.
  • Infectious causes, particularly PJP, are important considerations in immunocompromised patients.

 

  • characterized by
    • scattered or diffuse
    • ground-glass attenuation with
    • superimposed interlobular septal thickening and
    • intralobular lines
Alveolitis
Diagram shows inflammation (red ) in the walls of the alveoli with thickening of the interlobular septa (maroon) . The increased density in the interalveolar septa and interlobular septa results in a ground glass opacity with and crazy paving appearance on CT scan
Ashley Davidoff TheCommonVein.net
lungs-0736a
Crazy Paving ARDS
Ashley Davidoff TheCommonVein.net ARDS-crazy-paving

 

  • Causes
    • Infection
      • Pneumocystis carinii pneumonia
        • severely immunocompromised patient
    • Inflammatory
      • Sarcoidosis
      • NSIP
      • Organizing Pneumonia
    • Neoplasm
      • Mucinous bronchioloalveolar carcinoma
    • Circulatory
      • Hemorrhage
    •  Idiopathic
      • Alveolar proteinosis
    • Inhalational
      • Lipoid pneumonia
    • Systemic Disease
The Secondary Lobule
This image is a panoramic view of the lung showing almost rectangular secondary lobules surrounded by interlobular septa (cream borders) The distal bronchioles (teal) and pulmonary arteriole (royal blue are shown in the centre of a lobule in the right lower corner. The branches of these two structures are shown in the secondary lobule with the acinar airways shown in teal and the presumed course artistically inferred in royal blue. Within the interlobular septa (light pink) remnants of the pulmonary venules (red – inferred) and lymphatics (yellow inferred) course going in the opposite direction to the arteriole and the airways. 
Keywords:
lung pulmonary alveoli alveolus secondary lobule interlobular septa vein lymphatic histology interstitium interstitial normal
Courtesy of: Armando Fraire, M.D. Ashley Davidoff TheCommonVein.net

Thickened Interlobular Septa due to Inflammation – Acute Eosinophilic Pneumonia

Interlobular Septal Infiltration with Eosinophils and Inflammatory Exudate – Thickening of the Interlobular Septa – Crazy Paving Kerley B lines
The diagram shows the thickened septum surrounding the secondary lobule due to an inflammatory process, cellular infiltrate and congestion of the venules and lymphatics in the septum (a) .  An anatomic specimen of a secondary lobule from a patient with thickened interlobular septa is shown in c and overlaid in d.  CT of the lungs in a patient with acute eosinophillic pneumonia shows thickened interlobular septa and centrilobular nodules and the thickened septa are overlaid in red (e).
Ashley Davidoff MD The CommonVein.net 
lungs-0761
Anthracotic Lung
Anthracosis – Note the accumulation of carbon particles within the lymphatics along the interlobular septa, outlining the secondary lobules. The carbon particles are inhaled from an anthracotic urban environment.                                                                                                     Courtesy Ashley Davidoff MD. TheCommonVein.net  32291                                                     key words   lung interlobular septum septa secondary lobule pulmonary lobule intertstitium interstitial gross pathology carbon
Infection
P carinii pneumonia in a 32-year-old man with acquired immunodeficiency syndrome. (a) High-resolution CT scan shows areas of ground-glass attenuation with intralobular lines. (b) Photomicrograph (original magnification, 400; Grocott stain) of a specimen obtained with bronchoalveolar lavage shows alveolar exudates that contain cystic forms of P carinii (arrows).
Rossi, S.E et al ?Crazy-Paving? Pattern at Thin-Section CT of the Lungs: RadiologicPathologic Overview Radiographics  Volume 23 – Number 6,  2003

Inflammatory Diseases

ARDS

59M-ARDS-crazy-paving-01.jpg

Crazy Paving ARDS
Ashley Davidoff
TheCommonVein.net
Crazy Paving ARDS
Ashley Davidoff
TheCommonVein.net

77F with Aspiration Pneumonia ARDS and Crazy Paving

CHF – Alveolar Edema
CT scan shows Diffuse ground glass pattern with thickening of the interlobular septa and manifesting as crazy paving pattern
Ashley Davidoff MD

 

Crazy-paving sign. Axial CT of the chest shows thickening of the intralobular and interlobular septa with a superimposed background of ground-glass opacity in a patient with pulmonary alveolar proteinosis.
Source
Signs in Thoracic Imaging
Journal of Thoracic Imaging 21(1):76-90, March 2006.
CRAZY PAVING IN ILD
Crazy paving in ILD is a CT feature of interstitial lung disease and is characterised by diffuse ground glass caused by a combination of interlobular septal and intralobular septal thickening resulting well demarcated patchy densities in the lungs.
Idiopathic

57-year-old female with progressive dyspnea.

CRAZY PAVING
Ashley Davidoff MD
CRAZY PAVIN
Ashley Davidoff MD
CRAZY PAVING
Ashley Davidoff MD
CRAZY PAVING
Ashley Davidoff MD
CRAZY PAVING
Ashley Davidoff MD
CRAZY PAVING
Ashley Davidoff MD
CRAZY PAVING – NORMAL SIZED HEART
Ashley Davidoff MD
CRAZY PAVING – NORMAL SIZED HEART
Ashley Davidoff MD

 

References and Links